Above: chickweed in flower (spring). Chickweed is a winter annual that is sprouting this fall.
At first glance, you may think, “I know what a weed is!” But, do you understand their life cycle, the type of weed, and effective control methods?
Let’s start with the question posed in the title. Besides the obvious weed in the field, garden, or flower bed, a weed can be any plant growing in the wrong place or where it is not wanted. For example, farmers grow corn, but the next year, when they grow soybeans in the same field, a “volunteer” corn plant is a weed. In home landscapes, it may be the tree seedling suddenly sprouting up in your flower bed.
Farmers reduce weed competition through cultivation, cultural methods, and herbicides to produce high-yielding crops. This multi-pronged approach can also be considered in home gardens and landscapes.
Home gardeners know all too well how difficult it is to stay ahead of weeds in their vegetable and fruit crops, and in their landscapes. Enthusiasm is high on planting day, but later on, the days spent weeding aren’t so fun.
Besides the traditional weeds we think of, invasive plant species, like garlic mustard, callery pear, and Asian bush honeysuckle, threaten natural areas and ecosystems.
So, I think most people can agree that weeds are a problem. How do we best go about controlling these plant pests?
One fairly universal principle is that it is best to control weeds when they are smallest and most vulnerable. However, many of us feel the urgency to control weeds only when they are large and out of control.
Farmers understand the importance of controlling weeds early. Although somewhat rare compared to several years ago, some farmers still use rotary hoes and similar early-season weeding equipment. These tillage passes are crucial for USDA Certified Organic producers.
Many herbicides used are selective pre-emergent herbicides, meaning that they must be in place to kill weeds before they emerge. Other selective post-emergent herbicide labels direct farmers to use products before weeds reach a maximum height – perhaps 4 inches, depending on the product. And, some herbicides are non-selective, meaning that they will affect almost anything with green leaves that they come in contact with. Herbicides containing glyphosate are an example.
Culturally, crops that canopy over (the ground is no longer visible) have a better chance of out-competing weeds.
Knowing the types of weeds and their life cycles can help us develop more effective weed control strategies.
First, there are annual weeds, which can be further classified as summer or winter annuals. Summer annuals are the ones that sprout from seed in the spring, grow and produce seeds through the growing season, and die before winter. Examples include foxtails, crabgrass, ragweed, and common lambsquarters. On the other hand, winter annuals sprout in the fall, spend the winter in a vegetative state, then flower, produce seed, and die before summer. Examples include chickweed, henbit, and purple deadnettle.
Biennials are weeds that grow foliage, called a rosette, during the first year, then produce flowers and seed, and finally die in the second year. Wild carrot (Queen Anne’s lace), poison hemlock, garlic mustard, and bull thistle are examples. Control of biennials should be done early in the first year of growth.
Perennial weeds grow year after year and are difficult to control. Many of these have extensive underground roots or stem structures that enable the plant to regrow, even if the top growth is eliminated. Examples include Canada thistle, dandelion, and buckhorn plantain. Chemical control of perennial weeds is generally most effective in the fall or at flowering time.
Additionally, there are three main types of weeds – grasses, broadleaves, and sedges. Sedges look somewhat like grasses, but have a triangular stem. Grass herbicides don’t typically work on sedges.
Finally, weed control should be approached from an integrated pest management (IPM) perspective. This means that we should consider multiple ways to combat plant pests, including cultural, mechanical, and chemical methods. For homeowners and gardeners, some non-chemical weed control methods would include hand-pulling, hoeing, and mulching. A cultural control is mowing turf at a taller height (3”) to help the grass out-compete weeds. Gardeners can lay a tarp over a garden area for a couple of weeks to kill all newly emerged weeds before planting their vegetables. Consider all the alternatives and choose the strategies that best suit your preferences.