QUESTION: Now that the days are getting longer and the temperatures warmer, the weeds in our garden are growing like gangbusters! Some of them are already beginning to flower and we are worried they will go to seed and produce another crop just in time to compete with our vegetable plants.
ANSWER: Although we typically think of seeds germinating in the spring, some weed seeds actually germinate in the fall. These weeds are called winter annuals. The young plants are very cold hardy and often stay green late in the season. The same plants then flower in early spring and form many seeds. Common Chickweed is probably the best example of a winter annual that is actively growing and flowering in most of our coastal gardens this time of year. It is easily recognized by its light green, small and ovate shaped leaves with pointed tips. It often grows in the shade of trees and shrubs or the north sides of buildings. Chickweed is a low spreading plant that can grow four to 12 inches tall. The stems are creeping and often root at the leaf nodes. The dense mat of chickweed and other winter annuals can slow soil drying and warming and interfere with planting and soil cultivation in the spring.
Another fairly common winter annual, which is the bane of many home gardeners in our coastal area, is Little Bittercress. The plant is easily identified by the fact that when its mature seed pods are touched, they split apart explosively, giving rise to the common name "shotweed." This weed produces a flat rosette of leaves, followed by a flower stem ranging from two inches to a foot tall. Little Bittercress blooms in early spring only, from mid-March to mid-April. Its flowers are tiny and white, borne at the top of the plant.
One of the winter annuals that is easily recognized for both its foliage and flower color this time of year is Red Deadnettle. The leaves of this low growing weed are green at the bottom and shade to purplish at the top. The flowers are bright red-purple. Flowers this time of year allow bees to gather nectar for food when few other nectar sources are available. Though superficially similar to a nettle in appearance, it is not related and does not sting, hence the name "deadnettle." Despite the aesthetic appeal of its flowers and foliage, Red Deadnettle can be an invasive weed in cultivated garden soils.
Control of winter annuals includes removing plants now by hoeing or hand pulling. Non-selective herbicides containing glyphosate (Roundup) can be effective when used according to labeled directions. Spraying weeds before they flower will prevent the plants from producing another crop of seeds.
In established ornamental landscape plantings, the pre-emergent herbicide Casoron is effective at preventing the germination of both summer and winter annual weeds when used according to labeled directions. Casoron should not be used around herbaceous plant materials like peonies and other perennials. It is most effective when applied during the winter months when temperatures are cool and there is sufficient rainfall to dissolve herbicide granules.
Trifluralin is yet another pre-emergent herbicide registered for use in home gardens. Commonly sold under trade name of Preen Garden Weed Preventer this pre-emergent herbicide is registered for use around flowering herbaceous plants and in vegetable gardens once seeds have germinated. When used according to labeled directions, Preen can prevent weed seed from germinating for up to three months.
Most garden soils unfortunately contain an abundant amount of weed seeds. Frequently referred to as a "Weed Seed Bank," these seeds may last for several years or longer until exposed to sufficient light, temperature and moisture for germination. One of the easiest ways to prevent the on-going challenge with weed germination and growth is through the use of mulches. A three-inch layer of mulch will prevent the majority of both winter and summer annuals from germinating and competing with garden plants.
EDITOR'S NOTE: For answers to local gardening questions, contact Master Gardener Earl Miller at 642-0541 or e-mail him at firstname.lastname@example.org.