|Maine strawberry growers typically use the matted row system to grow their crop. Photo by Eric Sideman.|
by Eric Sideman, Ph.D.
As with any crop, producing strawberries organically entails a systems approach to the whole farm rather than just substituting approved organic materials for synthetic materials. Of course, many practices are the same in organic and conventional strawberry production, but the fundamental approach to soil husbandry and pest management may be quite different. Successful organic strawberry production depends on building a biologically active soil with good structure and reservoirs of nutrients, and ridding the site of weeds and soilborne pests BEFORE strawberries are planted. After that, managing a successful organic strawberry farm depends on crop rotation to distant fields, with cover crops maintaining soil health and fertility and preventing build-up of weeds and pests. Some conventional systems can maintain strawberry beds for five or more years, controlling weeds, diseases and insects with chemicals, but organic growers rely on crop rotation.
Following is a guide to farm-scale organic strawberry production of June-bearing varieties in a perennial matted row system. Strawberries grown in a matted row system typically are planted in early spring. Blossoms are removed and the field is allowed to establish runners the first year, and strawberry picking occurs in June and early July the second year. There are day-neutral varieties that produce strawberries in the fall, and many other production systems such as annual beds, raised bed plasticulture or ribbon row, but the matted row is still the most common system in the Northeast for early summer berries.
Strawberries will grow on most soil types, but very sandy soil may lead to problems with drought, and heavy, clay soils with poor drainage may lead to disease problems, such as red stele. Strawberries do best in soil that is high in organic matter and fertility; well-drained yet able to hold a continuous supply of moisture; and has a pH of 5.7 to 6.
Air Drainage and Frost Protection
Ideally, a strawberry bed should be slightly higher than the surrounding land so that cold air will drain away to lower ground. A few feet of elevation often will be enough to protect delicate blossoms from a late spring frost, which will kill them. Strawberry blossoms can tolerate temperatures from 22 degrees to 29 degrees F., depending on the stage of development, but this is the temperature of the blossom, not the air, and radiational cooling can cause the blossom temperature to be significantly colder than the air. If conditions favor radiational cooling, air temperatures even slightly above 32 degrees F. could induce frost damage. A frosted blossom will develop a dark center and will not produce fruit.
A slight slope to the land often aids air drainage. Berries on a southern slope may be more susceptible to late spring frosts, because blossoms may open earlier than those on a northern slope. On the other hand, berries on a southern slope will ripen earlier.
Commercial growers should not attempt strawberry production without some means of frost protection. Some growers cover beds with floating row cover on nights when frost is predicted, uncovering beds during the day for pollination. Most commercial growers use overhead irrigation to protect blossoms from low temperatures. Irrigation is started as the air temperature approaches 32 degrees F., and as the temperature drops, ice forms over the blossoms. As long as liquid water is applied to the field, the temperature of the blossom will not drop below 32 degrees F., because water releases heat as ice forms. Once irrigation begins, it should continue until the sun comes out in the morning and the ice melts. This is important because just as liquid water turning to ice gives off heat, ice turning to liquid water removes heat from the blossom and could cause blossoms to get colder than the air.
Preparing the Soil
Soil must be prepared well before strawberries are planted in order to build fertility, organic matter and soil structure, and to get rid of pests and weeds. Once strawberry plants send out runners and form the matted row, organic growers can control weeds only by pulling them by hand. Since weeds can quickly outcompete and take over a strawberry field, a field must be free of weeds and have a reduced weed seed bank BEFORE crops are planted. Grubs (larvae of June beetles, Japanese beetles and others) that live happily under sod are another reason to prepare a field well before strawberry production. Organic growers starve grubs by getting rid of sod at least a year before planting strawberries. Building soil fertility and soil health before planting is most important for organic strawberry production; this creates a farm system where soil structure, cation exchange capacity (CEC) and reservoirs of plant nutrients are built, and then the soil supports the crop. Growers should begin improving the soil at least a year before planting.
Preparing New Land (Pastures, Fields, etc.) or Fields in Weedy Condition
One could prepare sod ground for berries by:
1) plowing in late summer (a year and a half before planting berry plants);
2) testing soil and applying the recommended amount of lime;
3) planting a winter cover crop in early September. Winter rye works well for growers with a tractor implement; annual ryegrass or oats, both of which are winterkilled, are recommended for growers working with a tiller or by hand.
4) The following spring, till in the fall cover crop and add recommended amounts of rock powders, such as Sul-Po-Mag for magnesium and potassium, and rock phosphate for phosphorus. (See MOFGA Fact Sheet #1 for details on reading soil test results and meeting nutrient needs with natural amendments; and MOFGA Fact Sheet # 11 for natural sources of crop nutrients. Fact sheets are available at www.mofga.org.) Also, this is an ideal time to add manure. Common rates of manure applications are 5 to 8 tons per acre (35 lbs./100 square feet) of poultry or rabbit manure, or 10 to 15 tons per acre (65 lbs./100 square feet) of cow manure. If you use poultry manure, use less lime and rock phosphate; poultry manure will supply approximately 15 pounds of phosphorus per ton.
5) Plant another green manure crop or series of crops the year before planting berries. The choice of green manure depends on many factors, such as your major goal. Usually, when trying to condition sod ground for strawberry production, the primary concern is weed control. The following is a good plan in most cases:
a) In the spring, plant oats;
b) Plow oats under in early summer. Plant sorghum-sudangrass if you use tractor implements, buckwheat if you use a tiller or turn by hand;
c) Plow the summer crop under in late August or very early September and plant a cover crop of oats for the fall and winter.
If your land is really weedy, be much more aggressive in getting rid of perennials such as quackgrass and depleting the seed bank of annual weeds. (See MOFGA Fact Sheet #10 on using a series of green manures alternating with fallow periods to get weeds under control. You must do this BEFORE you plant the berries.)
6) Plow under oats as early as possible in the spring, wait a few weeks, cultivate and plant
The choice of strawberry varieties is based on factors including taste, use of berry, winter hardiness, disease resistance, and ripening date. The University of Maine Cooperative Extension Bulletin #2184, “Strawberry Varieties for Maine,” describes most of the varieties suitable for New England and is available from your local Extension office and at www.umext.maine.edu/onlinepubs/htmpubs/2184.htm.
Strawberries should be planted early in the spring. Late spring plantings may have problems when hot, dry weather arrives, because they did not have time to develop adequate roots. Planting depth is extremely important: Crowns should be just level with the ground.
In the matted row planting system, plants are set 18 to 24 inches apart in rows 4 feet apart. All runners are allowed to root, but beds are kept 18 to 24 inches wide by cutting off runners that extend into the alley; use a hand hoe, rolling coulter wheel, cultivator or rototiller.
To ensure early runners and vigorous plants, remove all blossoms during the planting year. Failure to do this may result in weaker plants with lower yields.
As mentioned, for the most part fertility is established before planting and is based on soil testing. However, since June-bearing strawberries set buds for the following year’s fruit in the fall, adequate available fertility, especially of nitrogen (N), is crucial then. To get good bud set, N fertilizers are applied in mid to late summer before the bearing year, so that organic fertilizers have enough time to break down and become available by early fall. A common practice is to apply enough high-N materials, such as fish meals, seed meals or alfalfa meal, to supply 30 pounds of actual N per acre. Compost can be used as a supplement and serves well to condition the soil and balance nutrients but does not provide enough available N at the time needed. An inch of compost and the appropriate amount of a high-N organic material spread directly over the rows is recommended.
Strawberries should be mulched over winter to protect the plants from extreme cold and, more importantly, from damage by rapid freezing and thawing of soil. Although threshed straw is the most commonly used mulch, any material that will provide protection without matting can be used. This includes marsh hay, sudangrass, pine needles or coarse wood shavings (not sawdust).
Apply mulch after 15 to 20 frosts, but before the temperature drops below 200 F. The exact time will vary from year to year and place to place. In central Maine, growers lay the mulch around the week before Thanksgiving.
Apply mulch 4 to 6 inches deep over the rows. This will take 100 to 150 bales of straw per acre.
Remove mulch as soon as possible in the spring by raking it into the aisles where it can retard weeds and conserve soil moisture. Many growers like to leave it on the plants somewhat longer to delay blossoming (beyond the danger of a late frost, they hope). This does work but has been shown to reduce total yield.
Renovate or Rotate?
Conventional growers renovate their fields after harvest. Within a week after harvest they mow the old foliage to 1 inch above the crown and narrow the rows to 10 to 12 inches with a rotovator or tiller. This practice is questionable for organic growers, because without herbicides, weeds often compete better and take over when released from the shade of the old strawberry foliage. Organic growers may be better off not mowing and just narrowing the rows and fertilizing at this time.
Better yet, growers with enough land may profit most when strawberries are picked for only one year and then the field is rotated. Growers have to decide what works best for them, but organic growers will probably not be able to harvest for more than two years from a field before weed and pest management is lost.
Crop rotation is the best tool organic growers have against insects, diseases and weeds. As soon as the crop is harvested, plow the land. Deciding what to do next depends on which problem is the worst. Learn the biology of the weeds and pests that have become major problems during strawberry production and practice a crop rotation that interrupts their success best. If no problem has gotten out of hand, then sow green manures that will best improve the soil and reduce the need for off-farm sources of fertility.
A rotation that works well on organic strawberry farms in New England is to plow under strawberries after harvest; then bare fallow land until late August; and then plant a winter rye/hairy vetch mix. Allow that to overwinter and grow into mid-May the following year and then plow it. Bare fallow the field for two to four weeks and then plant a summer cover crop (cow peas or soybeans if a legume is desired, sorghum-sudangrass or buckwheat if not). In late August take down the summer cover crop and plant an oat cover for the fall, then plant strawberries the following spring. If you have the land, the soil will benefit and pests will suffer more with a longer rotation out of strawberries.
About the author: Eric is MOFGA’s organic crops specialist. You can contact him at 568-4142 or [email protected].