MOFGA Pest Report 2005
3 June
(View List Of All 2005 Pest Reports)
The wet weather has past and it is warm and everything is growing quickly. I doubt that it will catch up to a normal year, but in a few weeks this delay will be only memory. At this time farmers are trying to do two weeks of work (or more) in the next few days. This issue of the Pest Report is a warning that you should not forget weed management while you try to catch up on field preparation and planting.
Weed management on organic farms is primarily based on two practices, crop rotation and cultivation. Both of these have the same 3 goals, using up seeds in the weed seed bank, preventing new seeds from going into that bank, and starving out perennial weeds by using up the food in their storage organs while not letting the plants put new food back.
Cultivation is basically the same practice whether you do it with tractor mounted equipment or a hoe. Basically, the goal of cultivation is to scratch the top layer of soil just deep enough to kick up germinated weeds and have them dry out in the sun. If you do not cultivated deep enough then the roots of the weeds remain in contact with the soil and the plant lives. If you cultivate too deeply then you stimulate new weed seeds to germinate by bringing them up to the surface and exposing them to light. Obviously, cultivation works best in sunny dry weather. The goal of cultivation during the cropping season is to use up the seeds in the top layer of soil as soon as possible. Deeper weed seeds will be dealt with later with crop rotation and field tillage.
In order to make these practices work the best they can for you they should be specifically adapted to the specific life history of the particular species of weeds you are dealing with. Using crop rotation to meet these goals is a longer topic for discussion and something that you need to plan, but cultivation is something that you need to be doing nowŠ..even though you are behind on everything else. As an example of how important it is to fit your practices to specific weed biology in order to get best control here is a discussion of 3 weed species that are out there right now.
Shepherd's Purse
For pictures of this weed in different stages see http://www.ppws.vt.edu/ scott/weed_id/capbp.htm
Shepherd's purse is in the mustard family. It is usually a winter annual plant. Seeds germinate when the soil is below 60 degrees F, in the fall or spring. Typically, seeds that germinate in the fall form a rosette that overwinters. In the very early spring the rosette resumes growth and sends up flowers that quickly produce seeds. Each plant can produce about 50,000 seeds. Sherpherd's purse becomes a problem on organic farms for two primary reasons. First, organic farmers tend to cover crop their fields on the fall and the shepherd's Purse may germinate and survive under the cover crop unnoticed. Second, the seeds can continue to mature and become viable even on plants that have been pulled or mowed down before the seeds were mature.
This year there are many fields out there that because they are too wet have not been worked at all. I have seen lots of Shepherd's Purse in these fields happily going to seed and all this seed will be added to the seed bank when the field is finally worked. If this is the case in your fields, I suggest getting out there and hand pulling all of these plants and taking them out of the field with their seeds still attached. If Shepherd's Purse is a problem already for you, then you will have to time your fall covering to kill the rosettes, or kill them in the spring before seeds begin to form. Shepherd's Purse seeds may live for a while in the soil seed bank before germinating (half the seeds will be gone in less than three years) but not really that long compared to lambsquarters (it takes nearly 8 years for half the seeds to be gone).
Galinsoga
For pictures of this weed in different stages see http://www.ppws.vt.edu/ scott/weed_id/galci.htm
Galinsoga is in the composite family. It is a summer annual that is becoming more of a problem on organic farms. It is just beginning to germinate in the fields now. This is the time to kill it by cultivation.
Galinsoga is becoming a problem because it so quickly can produce new seeds. It is not day length sensitive and goes from a germinating seed to producing new seeds in a bit over a month. By the time you see the flowers open wide they probably have viable seed in them. Pulling up plants with flowers and leaving them in the garden is just adding to the weed seed bank. Get out and cultivate today.
Galinsoga has no seed dormancy. That means that the seeds produced or laying in the seed bank will germinate as soon as the conditions are right (exposed to light when it is warm and moist enough). This makes Galinsoga easier to get rid of than something like lambsquarters that has a physiological dormancy. Lambsquarters produces two types of seeds. The brown ones are not dormant, but the black ones are dormant and will sometimes sit in the seed bank for many years before being triggered to germinate. All your crop rotations and cultivation will not get these seeds until they are ready to germinate.
Quack Grass
For a picture of quack grass with its rhizome see http://www.rce.rutgers.edu/ weeds/weed.asp?quackgrass
Quack Grass is a perennial, native grass in Maine. It is very common in old fields and is a primary reason that farmers and gardeners are advised to get their fields prepared a year in advance before trying to grow crops. Those who do not follow the advice spend years battling quack grass.
Quack grass is often referred to as witchgrass here in Maine. I do not know why because witchgrass is a different weed
(see http://www.noble.org/ imagegallery/Grasshtml/CommonWitchgrass.html for a picture of witch grass) that does not look like quack grass at all and has a different life history.
Quack grass is easily recognized by its rhizomes, which are underground stems that send up shoots as they spread under the soil surface. If you chop up the rhizomes into small pieces by tilling you have made the problem worse because each piece of the rhizome is likely to have a node and send up a new shoot from that node. And you are likely to have dragged these pieces of rhizomes all over the field.
This year is going to be very bad for quack grass. Quack grass begins growth early in the spring and has become well established in fields that were too wet to work early. A quick tilling and planting in these fields is going to produce a mess later in the year as all the pieces of rhizomes start growing with the crops and a single cultivation does not kill the quack grass. Cultivation tends just to cut off the shoot but leaves the rhizome to send up a new one. Actually, that is the best method left if you do not have the quack grass gone before planting. BUT, you have to stay with it and cultivate off the shoots every time they come up, and if you miss a cultivation you are essentially back to ground zero. The goal of this practice is to starve out the rhizome. The rhizome is a food storage organ and each time it sends up a shoot it uses some food. If you never let the shoot get large enough to replenish this food eventually the rhizome will be starved to death. Good luck.
(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)