Melissa Matthewson, OSU Extension Small Farms
Spring and Summer: increasing day length, warmer weather and the spring bloom of trees, shrubs and flowers, but what we do not
welcome is the emergence of weeds on our farms. In some cases, with biennial weeds like thistles and teasel, rosettes have already appeared
on the farm, but many of our nasty summer annual and perennial weeds show their faces in the coming months. Weeds can
have various impacts on our farms including decreased grazing potential, decreased crop yields, increased erosion, toxicity to our animals
and family, and decreased property values. They can also be costly to control. So, what can we do as small farmers and landowners
to control weeds on our farms?
The first step in controlling weeds is to identify exactly what weeds you do have on your property. If you do not have any identification
books on hand, you can use the internet or you can bring in a sample to the Extension office and we can help you identify it. Once identified,
it is important to know the life cycle of the plant as well. Knowing whether it is an annual, perennial or biennial will affect your
management decisions and control options.
An annual weed completes its life cycle in a single year. Preventing annual weeds from going to seed is the best control option, although
it is important to remember that you may have a build-up of weed seeds in your soil from past plant life cycles. Summer annuals
live from spring to fall and winter annuals germinate in the fall and winter. Examples of common annual weeds are pigweed, lamb’s
quarters, and yellow star thistle (winter annual).
A biennial weed requires two seasons to complete its life cycle. In the first year, it develops roots and low-growing foliage. In the second
year, it will send up a flower stalk, set seed and mature. They are easiest to control in the first year by removing the rosettes. Common
biennial weeds include teasel, bull thistle and poison hemlock.
A perennial weed lives more than two seasons. Year after year, perennial weeds will produce foliage, seed and mature. Most invasive
weeds that we encounter are perennials. Some perennial seed can remain viable in the soil for years, sometimes germinating ten years
later. Common perennials include dandelion, curly dock and field bindweed.
Prevention is crucial in managing the spread of weeds. Plant clean, weed-free seed in your pastures, minimize bare ground and overgrazing,
plant and maintain desirable plant species and sanitize your equipment. Beyond prevention, there are mechanical, cultural, biological
and chemical controls for each type of weed. Choose the method of control based on the plant’s life cycle and your future objectives
for the site.
Mechanical control of weeds physically disrupts weed growth. This is the oldest method of control and usually, the most effective for
annual weeds as long as you apply the mechanical method before seed sets. Various methods of mechanical control include mowing,
cultivation, hand-pulling, burning, mulching or solarizing.
Cultural control of weeds includes practices that encourage desirable plant growth. Fertilization, irrigation, and planting appropriate
species at optimum densities are practices that help desirable plants out-compete weeds. Common cultural methods of control include
shading, water and nutrient management, proper seedbed preparation, planting of native species, and destruction of weed competition
before planting.
Biological control is the intentional use of living organisms to reduce the population of a pest. Biological control can reduce pesticide use
and can efficiently suppress weed populations, but it is not a total eradication option. Biological control in the state of Oregon has been
used on many weeds including yellow star thistle, leafy spurge and bull thistle, just to name a few.
Chemical control is the use of existing herbicides to control weeds. Effective use of herbicides is determined by knowing the life cycle of
your plant, understanding pesticide safety, and reading the label for proper application and protections. Think about what you would
like to do with the area when the weeds are eradicated. If you are going to use herbicides, make sure the residues will not affect the
future use of that site. It is also important to know if the herbicide is selective or non-selective, so you do not accidentally kill plants that
you had not meant to kill. The Pacific Northwest Weed Handbook is a great resource and lists various weeds and their chemical control
with proper application rates and times.
In general, inventory the weeds on your property and identify them properly. Ask yourself
what land management changes you can make on the farm in order to minimize
your weed problems. Decide on the best method of control based on the plant’s biology
and then monitor your success over time. Weed management is a long-term process and
there are usually no short-term solutions for large infestations, but successful weed control
is possible with correct information and management techniques. For more in-depth
information on weed control, please go to the OSU Extension website at http://
extension.oregonstate.edu/catalog/.