Cooler temperatures, shorter days and rain showers all serve to validate fall’s arrival. For gardeners, it’s time to tidy up those unsightly plants that are still lingering in the garden. A general cleanup now will not only improve the garden’s appearance, but also help in the prevention of insect and disease problems next year. Most disease-causing organisms such as fungi, bacteria and viruses as well as insect pests spend the winter on plant debris and weeds. Getting rid of spent plant material can make a significant difference in the occurrence and severity of pest problems next year.
Once vegetables are harvested, remove the entire plant. This will help with foliar diseases such as late blight on tomatoes and potatoes and will decrease nematodes and nematode eggs if roots are infested. If there is still plant material in the garden, follow up with tilling or turning the soil over. Most microorganisms that cause foliar diseases are destroyed quickly once plant foliage is in contact with the soil and starts to rot. After the first killing frost, remove annual flowers from the garden and till the soil to a depth of eight inches to bury any remaining infested plant material. And remove weeds from the garden because in addition to producing seed, they can harbor diseases and insects.
Herbaceous perennials such as hosta and iris are beginning to succumb to the short days and cool nights. After the tops have died back, cut them back close to the ground to help reduce slug populations. Fall and winter are good times to inspect trees and shrubs as well. Many of the organisms that cause leaf spots and twig and branch die back over winter on fallen leaves and stem tissue. Rake and remove fallen leaves. If the plant showed symptoms of a foliar disease, it may be advisable to also remove some of the mulch and replace it with new. Prune out branches that have died and check other branches for areas of dead tissue known as cankers. These should be pruned six to 12 inches below the visibly affected area, and pruners should be dipped in a 10 percent bleach solution (one part bleach to nine parts water) between cuts.
One of the best investments home gardeners can make this time of year is to plant winter cover crops. Often referred to as “green manures,” cover crops will produce an abundance of lush growth during the winter months that will translate into lots of organic matter for your garden next spring. They also boost soil fertility, cycle nutrients, improve soil structure, prevent weeds and protect the soil from erosion and compaction.
While cover crops produce green foliage above ground, their roots are also at work improving soil structure. Legumes such as vetches and crimson clover have relatively simple root systems that support nitrogen fixing Rhizobium bacteria and penetrate deep into the soil to loosen hard packed beds. When the cover crop root systems decompose, they release nutrients back to the soil. As a rule of thumb, legume cover crops provide more nitrogen to the soil, while grasses boost soil organic matter more effectively.
You can plant cover crops by section in your garden, planting the earliest cover crops as soon as harvest is complete. The sooner you plant cover crops, the better. Planting now will help recover more nutrients, cover the soil more quickly and produce more organic matter. By capturing more nutrients and covering the soil, early planting also improves water quality protection by reducing nutrient leaching, erosion, soil compaction and runoff. For example, WSU researchers in Mount Vernon found cereal rye planted Sept. 1 will accumulate almost four times as much nitrogen as rye planted Sept. 30.
Cover crops need a good seed bed just like garden crops. After harvesting your garden crops, turn the soil and rake it smooth. Small-seeded crops need the smoothest seedbed. Most of the common cover crops (cereals, vetches, Austrian winter pea, fava bean and buckwheat) have medium to large seeds. Cereal rye is the most commonly grown cover crop in the Northwest. It is vigorous, very cold hardy and can germinate and become established in cool weather. One commonly grown mixture is cereal rye and hairy vetch, typically planted in the garden at a seeding rate of 1.75 pounds-to-1,000 square feet of rye plus .75 lb/1,000 square feet of hairy vetch. The vetch will germinate in the fall, but grows very slowly until spring. Then it will use the more upright rye as a structure on which to grow. Other common cover crop mixtures include oats or barley plus crimson or Austrian field peas.
Simply broadcast cover crop seeds across the area to be planted. Cover the seeds by raking at least a quarter-inch deep or rototill no more than two inches deep. This provides good soil seed contact and protection from drying, which increases germination. You won’t be able to work all the seeds below the surface, so do not be concerned that some seeds remain on the surface. You do not need to fertilize cover crops in established garden. Enough nutrients will remain available in the garden to meet their needs. Natural rainfall provides enough moisture for seed germination and plant establishment.
Ideally, next spring, turn the cover crop under about three weeks before planting. This gives time for some decomposition to occur and for the soil to warm. You can turn the cover crop under in sections based on when you plant different parts of the garden. Avoid turning the cover crop when the soil is too wet to prevent harming the soil’s structure. If the top growth is too heavy to turn under easily, cut and remove most of it first and put the clippings in the compost pile.
This column is printed as a service of the Washington State University Extension Office at Elma. Donald D. Tapio is the regional gardening and farming specialist for Grays Harbor and Pacific counties. He can be reached at (360) 482-2934 or firstname.lastname@example.org.