Seedling Production

Summer 2018
A concrete center aisle in a greenhouse is easy to clean but less expensive than an entire floor made of concrete. Photo by Stephanie Burnett
Unit heaters are less expensive than boilers but can require more maintenance. Photo by Stephanie Burnett

The 2018 Spring Growth Conference at MOFGA addressed seedling production – setting up production equipment (benches, containers, etc.), evaluating soil mixes, maintaining plant nutrition and avoiding disease problems.

Greenhouse and High Tunnel Setup

Stephanie Burnett, UMaine associate professor of horticulture, said to consider the cost of a structure, its durability, expected life, load tolerance, ease of construction and maintenance, and how you will heat, cool and vent the space.

High tunnels, said Burnett, typically have no heat or electricity, are cooled through ventilation (as with roll-up sides), and are not rated for snow, wind or other load. They cost $0.75 to $1.50 per square foot and extend the season for four to eight weeks in spring and two to five weeks in fall. They are fairly easy for farmers to erect.
Greenhouses, on the other hand, cost about $6 per square foot, depending on the structure, but they can be used all season; heat and electricity are standard (so they are more reliable in cold weather), they are cooled through ventilation and other options, and they are rated for load, so they are more permanent structures. They are not so easy to put up yourself.

Purchase the structure that fits your business best, Burnett said.

Inside the House

Burnett advised making benches a convenient height (usually waist-high) and width (typically 4 to 5 feet) to minimize strain on your body. OSHA recommends 28-inch-wide aisles.

You can make benches from wood (probably the cheapest option), although this may not be so durable and is difficult to maintain long term, supporting algae, for example. You can replace wooden benches periodically to overcome these problems. They can be as simple as pallets on top of concrete blocks.

Metal-topped benches last longer and can be custom built in-house or purchased. Burnett showed some made from chain link fencing and other mesh, metal materials. Metal benches are typically cleaner than wooden benches, she said. Many Maine growers get them from Bartlett Bench & Wire, Inc., in Howland, Maine.


You can grow crops directly in the ground, particularly in a high tunnel, Burnett continued. However, in a permanent structure such as a greenhouse, you may encounter problems in the soil over time that are difficult to manage, such as nematodes, weeds, fertility issues or soil-borne diseases.

Alternatively and inexpensively, the floor can be covered with a weed mat, which may also be covered with pea gravel or crushed stone. Weeds will probably come through the gravel or stone, and moving plant material can be challenging, since you can’t push a cart over gravel.
More expensive is a concrete floor sloping toward a drainage basin. Less expensive is to pour just concrete center aisles, where most traffic occurs. Concrete is easier to keep clean (e.g., with a flame weeder) than weed mats and gravel or stone.
The goal with heating, said Burnett, usually is to maintain the house at 65 to 75 F during the day and 10 to 15 F lower at night. Maintaining temperature is the most expensive operational cost after labor because fuel costs can be high. The greenhouse is essentially a giant solar collector, said Burnett; it heats up fast once the sun is out. Without proper control, even in winter, the inside temperature can become too high and stress plants.

Heat may come from boilers, unit heaters or root zone heating. You may have one boiler (and possibly a backup boiler), typically not located in the greenhouse. Heating costs $2.50 to $3.25 per square foot of greenhouse floor space, depending on the type of boiler. Most commonly, hot water is distributed from the boiler through pipes that run along the knee walls, under gutters (to help melt snow) or in the ground.
Boilers can use a variety of fuels, including natural gas, propane, fuel oil, waste kitchen oil, wood or biomass. Choose the type of fuel before you buy a boiler, said Burnett. They are durable and involve less maintenance than unit heaters.
Unit heaters are smaller and cheaper but require more maintenance. They incur costs of $0.50 to $1 per square foot of greenhouse floor space. A house will have multiple independent units, so if one stops working, others maintain the desired temperature. Unit heaters have a combustion system in the base and fans that blow hot air throughout the house. They must be vented at the top rear of the house into a smokestack that exits the house, since burning fuel creates CO (which is toxic to people) and ethylene (which can harm plants). Fewer fuel options exist with unit heaters. Fuel must burn fairly completely and not put a lot of soot into the house, such as number 2 fuel oil, natural gas, kerosene or liquid propane. They have a heating capacity of 20,000 to 320,000 Btus.
A third option, root zone heating, provides supplemental heat where needed – near the root zone – or it can be the entire source of heat in a high tunnel. You can buy or build a system and put it under a bench or in the ground. This is particularly helpful during propagation, said Burnett, decreasing the time for seedlings to germinate and enabling you to reduce the air temperature by 5 to 10 degrees. A 30,000 to 40,000 Btu water heater designed for a home will work for about 2,000 square feet of bench space. Connect 5/16 inch EPDM tubing (from Grainger, for example) to the heater and run it back and forth, 2 inches apart, on a mat or under the bench or in the ground. For in-ground use, you can sink PVC pipe a foot or so down.
To conserve fuel, use double, triple or quadruple glazing on the greenhouse. An energy curtain can reduce heat use by 40 percent by blocking heat from the upper layer of the house and reflecting it back down onto plants. Energy curtains cost $1.25 to $3 per square foot and tend to fit gothic arch houses better than Quonset-shaped houses.

Distribution pipes in the floor can move heat from a boiler to where the plant needs it. Also, insulate knee walls as much as possible without blocking light.

Horizontal air flow fans should be situated in a way that moves air around the house. Illustration by Doug Bailey


Greenhouses are natural solar collectors, gaining up to 300 to 350 Btus per square foot per hour from the sun. Extreme temperatures can delay flowering, make buds abort and weaken stems. Cool greenhouses and hoophouses through ventilation (with roll-up sides, for example) and shading – and/or horizontal air flow fans, used primarily for greenhouses. Ventilation removes hot air and replenishes CO2 and oxygen. Ridge vents are commonly used for natural ventilation. Air is heated in the greenhouse by light energy. Since hot air is lighter than cool air, it rises toward the top of the house and escapes through the ridge vent, while cooler air is sucked into the greenhouse through side vents – the “chimney effect.” Some greenhouses have only ridge or side vents, instead of both.

Internal or external shade cloth is inexpensive and reduces heat from the sun but also reduces light; such reduction is detrimental to crops such as tomatoes. Energy curtains can be used as shade cloth as well.

Horizontal air flow (HAF) fans are used inside greenhouses (but not hoophouses) to help circulate air, control humidity (thus discouraging diseases) and get rid of hot and cold pockets. They are mounted above the crop and positioned for sufficient air movement. HAF fans are strongly recommended in production of any flower crop. In a single-span house of 60 feet or less, one fan can be placed at each end of the house, on opposite sides, so that air blows around the house. A larger house should have two more fans, halfway down each side.

Greenhouse Soil Management for Organic Production

Bruce Hoskins of the UMaine Analytical Lab and Maine Soil Testing Service said that seedling starts and greenhouse bench production require different rooting and growing media and different management practices than outside crop production. Commercial, OMRI-approved organic growing media are available, or you can follow proven recipes to make your own. Seeding mixes are especially difficult to produce and manage, due to the small volume of soil and the high sensitivity of new seedlings to soil and environmental conditions.
A soil mix must have sufficient large and small pore spaces – large for drainage and aeration; small to hold sufficient water against the pull of gravity. Commercial soilless mixes, said Hoskins, are typically 80 to 90 percent total pore space by volume, while field and garden soil is normally 50 to 70 percent. At field capacity (when the soil is saturated and then drains by gravity for 24 hours), about 60 percent of the pore space should be filled with water (which is held by surface tension) and 20 to 50 percent with air, ideally.
Coarse materials create large pores. They include sphagnum peat (for large and small pores), coir fiber (large and small pores), coarse (#2) vermiculite, perlite, composted bark (which may required extra nitrogen in a mix), coarse sand and, out West, pumice (a volcanic material). Fine materials create small pores and should be limited to 25 to 30 percent of a mix by volume. They include compost, field or garden soil (heat sterilized or at least taken from an area without plant diseases), fine (#3) vermiculite, worm castings and black (highly humified) peat.
These materials vary in water-holding capacity relative to dry weight, with peat holding three to five times its weight in water at field capacity; coir, four to six; vermiculite, one and one-half to two; perlite, one; and composted bark, one and one-half. Finished compost varies greatly but probably holds five to 10 times its weight. The ideal complete mix will hold two to three times its dry weight in water.
The water content of a mix in a container is not uniform from top to bottom, said Hoskins, because gravity pulls more water from the top, creating something like a perched water table in the container. So what works well in a large container may not work in a small container. The smaller the container, the less variable the water content – so drainage is most critical in seedling flats or plugs.
To retain large pore spaces, avoid compaction: Don’t compress the mix in pots or cells, don’t stack flats or packs, and moisten a mix lightly before filling cell packs so that it doesn’t expand and get compressed once in the cell.
Different species grow best at different pH ranges, but typically 5 to 10 pounds of lime is added per cubic foot of mix – closer to the 5-pound rate if compost, with a pH of 7 to 8, is included, and closer to the 10-pound rate if the mix contains more than 50 percent peat or if field or garden soil is included. While the optimum pH range for a field soil is generally 6.0 to 7.0, it is between 5.4 and 6.0 for a soilless mix for maximum nutrient availability.    
The nitrogen source might be blood, soy or feather meal (all released relatively quickly), alfalfa meal or pelletized broiler manure (slower acting), or compost (exceedingly slow). Cooler soils result in slower N release. Phosphorus may be added as bone meal, bone char or rock phosphate, and potassium as KMag or sulfate of potash. Many blended natural fertilizers with multiple N-P-K analyses are on the market now, from Bradfield, Fertrell, Espoma and North Country Organics, for example.
Regarding mycorrhizal inoculants, Hoskins cited Aaron Englander master’s degree project in which any field soil he looked at had an abundance of propagules already; anything he applied was lost in that abundance. “If you’re not using field soil in your mix, then mycorrhizal inoculants are worth the cost,” said Hoskins, adding that most commercial mixes include them now.

Beneficial (antagonistic) bacteria or fungi compete with and suppress soil pathogens. They include the Trichoderma fungus and Bacillus subtilis and B. pumilis bacteria and are in most OMRI-approved mixes. They do not make your crops bullet-proof, said Hoskins. “You’ll still need to apply RootShield (Trichoderma) every couple of weeks.”

Hoskins provided two sample recipes for soilless mixes and directed Spring Growth participants to the publication “Potting Mixes for Certified Organic Production” at for several others.

Classic Cornell Mix Modified for Organic Production

1/2 cubic yard sphagnum peat moss
1/2 cubic yard perlite
10 lbs. bone meal
5 lbs. limestone
5 lbs. blood meal

You could mix in some greensand or potassium sulfate to add potassium.

Eliot Coleman’s Recipe
1. Mix equal parts of blood meal, rock phosphate and greensand.
2. To 1 cubic yard of soilless mix (peat moss and perlite or vermiculite), add 14 lbs. of mix from step 1.
3. Wet and allow the complete mix to incubate for a month or more before planting. Be sure to do this step before checking pH and nutrient levels.
Among the many OMRI-approved commercial mixes available are some products from Sun Gro, Premier Horticulture, Vermont Compost, Living Acres, GreenTree, Pro Mix and Fafard.

Problems with Planting Media
Common problems with container mixes include poor drainage, poor aeration and incomplete N mineralization. Poor drainage, the most common, results from excess small-pore material, compacting the mix and/or including compost. Poor aeration results in poor root function, because roots need oxygen; so poor aeration limits nutrient uptake.
When, in a saturated soil, mineralization of N from organic forms to plant-available nitrate (NO3-) stops at the ammonium (NH4+) state, NH4+ builds up, leading to a high pH (>7) and conversion of some NH4+ to free ammonia (NH3) gas, which is phytotoxic.

Mark Hutchinson of UMaine Cooperative Extension grew lettuce seedlings in commercial potting soil with 20, 30 or 50 percent compost by volume, or in 100 percent compost, as well as in a private blend medium. Growth was best with 20 and 30 percent compost; seeds didn’t germinate in 100 percent compost; and growth was severely stunted in 50 percent compost and in the private blend. Photo by Mark Hutchinson
Farmers can use portable meters or pens to test media pH and electrical conductivity. Photo by Bruce Hoskins

Hoskins showed a photo taken by Mark Hutchinson of UMaine Cooperative Extension of lettuce seedlings growing in commercial potting soil with 20, 30 or 50 percent compost by volume, or in 100 percent compost, as well as in a private blend medium. Growth was best with 20 and 30 percent compost; seeds didn’t germinate in 100 percent compost; and growth was severely stunted in 50 percent compost and in the private blend.

Nitrogen deficiency is common with compost-based mixes, said Hoskins, due to the slow release of N from compost. Mineralization is further slowed by cool or cloudy weather and cool soil. Growers can address this problem by transplanting into larger containers with fresh mix or through fertigation – watering in supplements such as fish hydrolysate (available from Fedco, Neptune’s Harvest and North Country Organics) at two-week intervals. Other OMRI-approved liquid formulations include a Nature’s Source 3-1-1 hydrolyzed soybean product and AgroThrive, hydrolyzed food waste.

Salt buildup is also common in greenhouse soil mixes. Finished compost commonly has a high electrical conductivity (EC) because normal nutrient release exceeds uptake. This can lead to water stress and wilting of seedlings and to root damage and/or death. To minimize salt buildup, water enough to regularly flush excess salts from the container. Some nutrient loss is unavoidable in this process, said Hoskins.

Hydrophobic soil mixes can be difficult to wet when they’re very dry. They can shrink when dry so that water runs down the outside of the mix instead of through it. This is most common with peat-based mixes; blending in some coir can make mixes less prone to shrinkage. Soaking an entire flat or container in water for 5 or 10 minutes can also help, as can mixing in an OMRI-approved wetting agent.

Evaluating mixes differs from evaluating field soil because of the high water retention and limited nutrient retention in mixes. You can monitor fertility by testing soil water. A lab analysis uses saturated media extract (adding water in a closed vessel just to the saturation point) to test pH, EC and water-soluble nutrient content. Farmers can use portable meters or pens to test pH and EC. To do so, place a clean saucer under a pot an hour after normal watering, pour in 50 to 100 cc water (2 to 4 ounces), collect the leachate from the saucer and test it with a pen.

Regarding the effects of irrigation water on media pH, Hoskins noted that different plants have different pH requirements. Iron-inefficient plants (petunia, pansy, snapdragon, calibrachoa, vinca) need a pH of 5.4 to 5.8 for adequate uptake, while iron-efficient plants (geranium, marigold, New Guinea impatiens) need a pH of 6.0 to 6.6 to avoid iron and manganese toxicity. Vegetable starts are somewhere in the middle of these pH requirements. Because well water in Maine often is highly alkaline, surface water from ponds, streams or shallow, dug wells is usually preferable for watering. Otherwise, an acidifier, such as an OMRI-approved citric acid, can be injected into irrigation water. Hoskins referenced the North Carolina State publication “Alkalinity Control for Irrigation Water Used in Greenhouses.”
In summary, Hoskins said to ensure that mixes have adequate large pore material for proper drainage; be careful not to compact a mix; always use containers with bottom drainage; allow the mix to dry nearly to the wilting point and then water through to leach excess salts; don’t water on cloudy days, if possible; and supplement nitrogen as necessary. As resources he recommended the NCSU Horticultural Substrates Lab (, and the UMass Greenhouse/Floriculture Program ( publication “Effects of growing media on water and nutrient management.”

Seedlings by Annie/One Drop Farm

Ann Mefferd said her Seedlings by Annie/One Drop Farm in Cornville, Maine, now focuses exclusively on seedlings as its commercial product, although she and her husband, Andrew, grow crops for themselves.

They track and plan sales of about 200 varieties on spreadsheets. They’re also trying to increase on-farm sales rather than adding more farmers’ markets so that they spend less time loading, transporting and unloading seedlings. Customers can come to the farm to shop and can preorder seedlings.

One spreadsheet lists the types of containers seedlings are in for various sales channels; number of pre-orders; number sold at the farmers’ market, on the farm, wholesale and elsewhere; total sales; and sale value per unit. Another tracks costs for soil, amendments, seed, pots and fuel; labor costs for seeding, filling, mixing, watering, moving and potting up; and gross profit per flat and per unit. Products that don’t meet a certain profit per unit are dropped. Mefferd now sells nightshades only in 4-inch pots, for example, as smaller containers weren’t profitable. She works hard on  messaging so that people are comfortable paying what she charges.

Spreadsheets showing seeding amounts, dates and how long each seeding operation takes enable Mefferd to schedule work for employees, who can easily see what to do. Mefferd also grows seedlings wholesale, for other farms; that information is on a separate sheet, arranged per customer.

Seeds are stored in a closet next to the coldest wall of the house. For best storage, the temperature (degrees F) plus relative humidity should not exceed 100, Mefferd said.

The Mefferds created a seeding/potting up/germinating station in their garage. Photo courtesy of Seedlings by Annie/One Drop Farm

Seeding and Germinating

While 200-cell seed-starting trays are popular, she’s moving toward starting the same amount of seed in open 1020 trays for the added soil volume. “If you have to wait a few days to get to them,” said Mefferd, “they won’t become a tiny, tight plug.” The farmers recycle flats and containers, removing debris and soaking them in a large tank of Sanidate 5.0 for 10 minutes.

Lacking greenhouse space to spare, the Mefferds created a seeding/potting up/germinating station in their garage. They mix Sustane fertilizer (especially for plants that will be in pots for any length of time) with Living Acres media in a cement mixer. A large wooden tray under the mixer holds six 1020s. They add mycorrhizae to the mix, wet it, dump it onto the six 1020s, spread it out, and they’ve filled six flats.

They don’t use a vacuum seeder because they’re not sowing large numbers of any one variety. Instead they use a Sow Easy, a squeeze mechanism, to sow individual pelleted seed or round brassica seed; and the Tapper for others. For very small seeds, they bought used dentistry tools. They’ve also developed dibblers, including full-flat dibblers, to help with seed placement.

They mark everything by variety and, when seeding several varieties in an open 1020, lay shish kabob skewers on top of the seeding mix to delineate varieties. They sanitize skewers before reuse.

To save energy early in the season, the Mefferds germinate seeds in a light cart that holds 16 flats. A fluorescent light on a timer is raised or lowered, depending on how much heat particular flats need. Flats of plants such as peppers that germinate best with warmer temperatures sit over heat mats. The Mefferds cover the trays with a sheet of greenhouse plastic close to the soil to maintain humidity. If plants sit on a light cart too long, they’re hardened off by moving them to the greenhouse for a number of days before they’re potted up.

For less than $200, the Mefferds made a germination chamber that holds 30 flats and sits next to their potting bench. It doesn’t have lights, and flats aren’t watered while they’re in the chamber, because the farmers know how long it takes for each variety to germinate. “If we pull brassicas out of there on day two,” said Mefferd, “within two hours … they start to come up.” They heat the chamber with a $15 space heater in an area under the box.

Growing On

Once seeds germinate, flats go to a 12- x 20-foot propagation house if they need to be potted up. Seedlings in the propagation house are heated with heat mats and a space heater on a thermostat. Row cover and domes cover the flats at night. A thermostat determines when exhaust vents open and close in the house.

Varieties that are direct-seeded and never potted up – greens, lettuce, brassicas, cucurbits, alliums, dill and cilantro and other fast-growing herbs – go directly into plastic retail containers and to the retail nursery. The Mefferds encourage customers to return containers, which they sanitize and reuse.

They pot slower-growing herbs and flowers and varieties that need to be warm into CowPots (made from composted manure). Mefferd noted that Jiffy now has an OMRI-approved pot. She tops off CowPots with soil mix because leaving a lip wicks water away from the mix, which can dry out when seedlings are small and vulnerable. The potting mix settles as it’s watered, so the top of the pot holds more water as the plants grow and need more water.

Plants are moved to the 20- x 50-foot retail nursery according to traffic flow (with the most popular plants in the back), plant type, container size (for even watering) and heat needs (nearness to heater).

Their hose retracts when not in use. It can hook into a Dosatron or a Syphonject for fertigation – which is rarely needed, since the Mefferds plan to have needed nutrition in the potting mix.

They turn on a unit heater in the nursery on April 4, keeping the house at 55 F at night so that plants are stouter. A vertical heat curtain divides the nursery so that only half need be heated early in the season, and ProtekNet keeps pests out when the sides are rolled up.

The Mefferds control weeds so that pests have no place to overwinter, they release beneficial insects, and they try to keep nectar sources (including a lot of alyssum) blooming in the house. They educate customers about beneficials that they might see.

The retail nursery is open every day but Sunday from mid-May through the second week of June. Customers see educational signage in the nursery and at checkout. Wholesale and preordered plants go in a separate 30- x 50-foot greenhouse. Plants are transported to the farmers’ market in a Ford Econoline van, which carries 43 flats, and a trailer made from an old Datsun pickup truck.
Green Spark Farm

Mary Ellen and Austin Chadd started Green Spark Farm in 2009. They’re cropping 4 acres, including 1 acre in tunnels, on rented land in Cape Elizabeth. The Chadds sell year-round at the Portland Farmers’ Market, a 12-minute drive from their farm.

“We’re known for our tomatoes and have a couple of large wholesale tomato accounts,” said Mary Ellen. In addition, they have spent the last six years increasing winter greens production and investing in infrastructure to improve year-round cash flow. They also sell seedlings – vegetable starts, herbs, specialty flowers and flowering perennials – and larger plants at the market and from their farm.

From Apartment to Greenhouse Growing

In 2009 they sold tomato plants started under grow lights, moved to a neighbor’s greenhouse and potted up in their Portland apartment. In 2010 they continued germinating seeds in their apartment and then moved plants to their 12- x 24-foot unheated structure. Double row cover and sheet plastic covered flats at night. Overflow seedlings went on pallets covered with row cover. That created mouse habitat, so they now put plants on a truck bed a couple of days before transplanting to the field.

In 2011, with NRCS funding help, they built a 30- x 36-foot house to grow winter greens. They used the smaller tunnel as a germination chamber. A 30 x 144 house went up in 2013, and another – also 30 x 144 – in 2016. Now they have lettuce and greens at the market into winter and spring.

Efficient Use of Space

They originally had three paths running the length of their greenhouse. For benches they used clamps to make sawhorses, put metal hangers on the cross pieces of the sawhorses, and slid 2x4s into the hangers. Trays sat on top of the 2x4s. They soon learned that concrete block bench legs would stick out less than sawhorses. They have looked at other designs for benches, including a 2×4 system with metal legs at Villageside Farm in Freedom and, at Snell Family Farm in Buxton, a more traditional metal top on a 2×4 rack sitting on one concrete block or one horizontal and one vertical block for a waist-high bench.

Chadd advised knowing how may flats you’ll plant, how many fit on a table and per house, and then plan for that much room plus room to expand. In the first year in their new tunnel, they ran out of space.

After seven seasons the Chadds hired mentors, including Seth Kroeck, who told them to turn their tables 90 degrees for a “peninsula layout,” with one central aisle running the length of the house and small paths down the sides. That enabled them to fit 150 more trays –30 percent more – in the house. Chadd cited the UMass Extension Greenhouse Crops and Floriculture Program publication “Increasing Production Space in Greenhouses,” which says that using rolling benches (which cost about $600 each) can create 81 percent growing area in a house, versus 68 percent with a peninsula layout and 59 percent with a longitudinal layout.  

In 2016 the Chadds put overflow flats onto tables in a four-season, 72- x 14-foot caterpillar that can take snow and wind. They attached the 2x4s on the sawhorses to the greenhouse tubing with plumber’s strapping and then ran lath over the sawhorses, the length of the house. Three 1020 trays fit across the lath on each side of the house, with each tray on two pieces of lath. The tunnel holds 500 trays – the same as their 30 x 36 house, but at much less expense. Hardier crops go in this cooler house, while tomatoes go in the 30 x 36 heated house.
Heating and Cooling

Chadd said they had used an unvented kerosene torpedo heater for a couple of seasons before learning that ethylene gas from the heater can delay plants and flowering and cause deformations. She had seen damage that looked like frost burn, which she attributed to ethylene damage. The solution, she said, is to get an L.B. White Tradesman 170 or a vented Modine propane heater.

Know and maintain the best germination temperature and light conditions for your crop, said Chadd, citing the Johnny’s Selected Seeds catalog and “The New Seed Starters Handbook” by Nancy Bubel as good resources. They heat with incandescent rope lights on tables for germination early in the season (see sidebar for cautions) and then with propane. A $38 Inkbird probe in the soil monitors and controls the temperature of the rope lights. Flats are covered when seeds are germinating, and seedlings are covered at night in late winter/early spring. In an unheated situation, they use two layers of Agribon AG-19 row cover plus one layer of plastic inside the greenhouse; outside they add a second layer of plastic.

They keep the house at 55 F until it’s full, then increase it to 60 F. Chadd read in the Johnny’s catalog that peppers exposed to temperatures of 55 for the first four weeks of their lives had more flower buds, leading to higher yields. In April they increase the heat to 65 F, even at night, to be sure they have plants to sell in May. They hope to invest in water-heated benching to save an estimated 30 percent in heating costs.

For cooling they open the house before 9 a.m., opening doors and sides (which roll up to 5 feet high) depending on the weather. Peak vents open manually or (this year) automatically, and four fans circulate air.


Clean floors (no wet soil or mud, no weeds) help with sanitation, as does air circulation with fans. Most of their floors are covered with landscape fabric from Nolt’s Greenhouse Supplies, which they order from Progressive Grower Agricultural Supply in Massachusetts to save on shipping. Their new 30- x 96-foot tunnel has a gravel floor rather than soil.


Regarding tagging, Chadd said, “If you’re selling plants, tag everything.” Use popsicle sticks, generic tags from John Henry & MasterTag ( or a pot tag printer. You can tag whole trays with masking tape on the end of each tray or buy waterproof horticultural tray labels from Retail Service Company in South Portland.

Watering and Fertilizing

For watering, Chadd prefers the Red Head nozzle for its gentle flow. She checks it regularly for clogs. Small-seeded plants in 288s are sub-irrigated in a tote with an inch of water in it. That soaking holds them for about three days. A fogger nozzle works for small-seeded plants, said Chadd, but bottom watering lasts longer. When using the Red Head nozzle, she waters back and forth over each tray, does the entire table of trays, and then does the whole table again, making sure to wet the corners and edges of trays. She lets the soil dry somewhat between watering and at night.

They fertilize weekly beginning in April, using a gallon watering can, and they drench trays at transplanting unless they were fertilized within five days. For established seedlings, they mix 1 gallon of water, 1/2 cup of fish/kelp and 1 teaspoon each of Blooming Blossoms Bioplin, Nutritech and Nutrifoliar.

Succession Planting and Growing On

Their EZ Seeder vacuum seeder from Johnny’s helps with succession planting and is used with pelleted lettuce, brassicas and spinach; it does not work with onion seeds. One  person can seed 50 trays per hour (versus 18 per hour by hand). They also use dibblers from Johnny’s made with marbles hot glued to metal trays.

Don’t pot up, if possible, said Chadd. She pots up solanaceous crops, flowers and herbs only once.  

For ergonomics they stack crates to adjust the workspace height.

They formerly transported two layers of 40 flats to the field in the bed of a pickup and on boards spanning the top of that bed. Now they use a larger flatbed truck with flats stacked on three levels (the bed and two shelves) and sitting in a trailer pulled by the truck. They transport flats to market in stacked bulb crates. They also use metal shelving from Uline in their truck.

For hardening off, plants previously went under row cover outside; now they go on the trailer, where they’re covered with Agribon Ag-19. Tender crops (cucumbers, squash, etc.) are also covered in the field. The Chadds irrigate immediately after transplanting.

Hiring Labor

Hiring greenhouse managers enables the Chadds to spend more time with their 3-year-old twins, improve their records and systems, expand their operation and attend trainings. Managers can make mistakes, though, leading to losses of crops or quality. When hiring greenhouse management, trust and respect are important, said Chadd, as are instructional spreadsheets and greenhouse rules about how to tag material, and planning ahead (with seeds, clipboard, marking pens, seeding and transplanting schedules).

Seed Storage

They store seeds in a cool, dark, dry area in jars or Ziploc bags that, in turn, are in plastic boxes. Packs of seed are sorted by family, by whether they’re direct seeded or transplanted and by seeding date.

Chadd recommended the Johnny’s Selected Seeds catalog; “The New Seed Starter’s Handbook” by Nancy Bubel (2018); “Ball Culture Guide” by Jim Nau (1993); UMass Amherst Cooperative Extension; and “Specialty Cut Flowers: The Production of Annuals, Perennials, Bulbs, and Woody Plants for Fresh and Dried Cut Flowers” by Allan Armitage (2008).

Seedling Diseases

Alicyn Smart, a doctor of plant medicine and assistant extension professor and plant pathologist who runs the UMaine disease diagnostic lab, talked about seedling diseases. Damping off, she said, causes most issues with seedlings, but many pathogens cause damping off, including Pythium (a water mold), Rhizoctonia, Alternaria, Sclerotinia, Botrytis and Fusarium.


Damping off can occur before or after seedling emergence. Symptoms may include failure to germinate; necrosis (browning) on leaf edges; or a wet or a thin, hardened stem lesion that is almost string-like and will flop over and cause the seedling to wilt. The disease may start in a central location and then radiate out among plants. It can grow relatively quickly.

Problems that resemble damping off include damage from applying excess fertilizer, which can cause leaf chlorosis (yellowing); hot water or heat stress, which can cause wilting; lack of water; and burning when an oil is applied under high temperatures. Some of these may cause root tips to die back as well.


To prevent damping off, be sure the floor is clean. Sweep the area and dispose of any plant residue. Practice sanitation. Keep hoses off the ground. Use OMRI-approved protectants (RootShield, Actinovate AG, BotryStop, MillStop, etc.) in or on the soil so that roots become coated with the mycelium as they grow; pathogens will have trouble penetrating this protective barrier. These products don’t last long and have specific storage conditions, so follow instructions closely; growers may want to make group orders so that they use these materials while they are still active.

Cultural practices that deter damping off include purchasing seed from a reputable company, germinating seeds under favorable conditions (as with bottom heat to hold temperatures at 70 to 75 F), encouraging seedlings to grow quickly, filling flats with pre-moistened material so that it doesn’t compact, avoiding sowing seeds too deeply or too densely, and providing adequate light. Keep greenhouse temperatures warm with low humidity; condensation on the plastic can drip, which can lead to damping off. Manage weeds. Avoid compaction by not stacking trays when you move them. Don’t stress plants with low temperatures. Don’t use excess water or fertilizer. (Dave Colson recommended watering before noon so that the soil surface isn’t wet during the cooler evening hours.)

Smart showed a photo of a seedling with its stem indented at the soil line due to Pythium. The Pythium pathogen mates with itself, so it doesn’t need another mate. It forms zoospores that, once water activates them, are released and travel through the soil. This is one reason water-related disease issues sometimes occur at the bottom of a hill. You can identify Pythium in a plant that is still alive by looking at its roots. You’ll see some necrosis, but you can also take two fingers and pull away at the root. You’ll remove the epidermis and be left with the cortex of the root. That tells you that you have a water mold (Pythium or Phytophthera).

Sclerotinia can be an issue on seedlings. If you use trays year after year, plastic is best. The sclerotia can get lodged in Styrofoam and can germinate, even after the Styrofoam is sterilized.

Botrytis can affect tomato, strawberry, sunflower and a range of other plants. Even if they make it through the seedling stage, they may be affected later.

Fusarium, a prevalent root pathogen, was the most common pathogen Smart received in her lab last year. She showed an affected watermelon plant and explained that such plants may wilt during the day and bounce back at night.


If you have damping off and want to know which pathogen is present and which may cause a problem later, Smart encourages you to send samples to her for a free diagnosis. (See for mailing instructions. The disease diagnostic lab is moving this summer, so check the website just before mailing for the right address.) Then get rid of the entire tray of seedlings. She showed a good sample, with the roots and soil in a Ziploc bag and the top, green tissue in a damp paper towel. Don’t send dead tissue, since the original pathogen may be outnumbered by subsequent pathogens. Try to send two representative plants, and list the symptoms. Send them on a Monday, ideally, and not on a Friday. Photos can help with disease identification. Diagnosis and treatment recommendations can take a week or two, depending on the pathogen.

Smart recommended the “IPM Images” link on for reliable images of pathogens and their damage.

Minimize Fire and Shock Hazard

Incandescent rope lighting, which is significantly cheaper than heat mats, was mentioned as a heat source for seedling production at the Spring Growth Conference – either under flats of plants with an air space between the lighting and the flats, or under row covers or in insulated boxes. Note that incandescent rope lighting is not rated for that purpose and may present a shock or fire hazard if improperly installed or attached. It may also short out and present a hazard to people contacting a wet floor. Be sure to allow sufficient space around rope lighting for heat to dissipate, and connect rope lighting to GFCI outlet. Heat mats should have a screen over them that is grounded in case of a short. Set up a smoke alarm in the greenhouse in case of fire. For other safety points, follow the instructions that come with the product and see

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