CHAMBERSBURG, Pa. — Forage producers should take advantage of plant life cycles this Fall to most effectively control many types of weeds. A final, timely herbicide application can be very effective as temperatures cool, and days run shorter. In the Fall, foliar, systemic herbicides are especially effective on perennial and biennial weeds, as herbicides move more readily to plant root systems. This occurs as sugar and carbohydrates are transported down into root systems as plants prepare for winter.
Winter annual weeds such as chickweed, henbit, marestail and mustard species are germinating now that soil temperature, day length, angle of the sun and air temperatures are falling. Winter annuals will “bolt” in the spring and produce a seedhead before dying. At this time of year these species are easiest to control as plants are physically smaller. If plants are not controlled now, these species will slow photosynthesis and respiration until they ultimately become dormant. Unfortunately, once plants go dormant, herbicide activity, translocation within plants and ultimately control will be reduced or ineffective. When targeting winter annual weeds in the Fall, ensure that foliar herbicide applications are targeting plants that are green and actively growing. Applications should be delayed until air temperature is above 40 degrees and soil surface temperatures are above freezing.
Biennial species such as poison hemlock, wild carrot, common teasel and common burdock have a life cycle that occurs over two years, where the seedlings form a rosette in Summer or Fall, overwinter, then flower and produce seed the following year. Biennials are most easily controlled newly emerged or at the rosette stage in the Fall, when root systems are smaller, and plants have less accumulated reserves. Similar to winter annual weeds, target herbicide application to biennial species when air and soil temperatures are above freezing.
Cool-season herbaceous perennial weeds like Canada thistle, buttercup species, quackgrass, wild garlic and dandelion flourish during cool Fall temperatures, remaining in active growth throughout several light frost events. While these species will eventually die back to root systems over winter, they resprout from root systems the following Spring. The adaptation of these species to tolerate several frost events and continue growth adds several more weeks into the Fall to consider an herbicide application.
On the other hand, some perennial species grow more as warm-season herbaceous perennials– jimsonweed, hemp dogbane, common milkweed, horsenettle and common pokeweed. Generally, warm-season herbaceous perennials may be treated with a systemic herbicide up until a frost occurs provided there is some green leaf tissue remaining. Once nighttime temperatures move towards freezing, these species start to senesce or die back to root systems for the year. You can visually observe this change as leaves die back, turn yellow or drop to the ground. Certainly, once a frost kills the top growth of these plants back to a root system, foliar herbicide application to warm-season herbaceous perennials will be ineffective.
Woody perennials, such as bush honeysuckle, privet, autumn-olive, multiflora rose and Eastern redcedar may be effectively treated in the fall, provided that plants are actively growing. One advantage to control at this time is that woody perennial shrubs such as bush honeysuckle often retain green leaves longer than other native species, which makes identification easier. Effective application techniques for woody perennials include foliar systemic herbicide application to green foliage when air temperatures are above freezing. Other application techniques include cut stump and basal-bark treatment, which may be utilized throughout the dormant season. However, keep in mind that herbicides begin to break down immediately once applied, so control may be reduced during the dormant season as plant growth, herbicide absorption and translocation is limited during this time.
Pasture owners and forage producers can consult their local Extension office for help in identifying weeds. Refer to the Penn State Agronomy Guide for more information on forage weed control. When considering pesticide usage, always consult a current herbicide label for safety, use recommendations and restrictions. General pasture and hay management recommendations may be found on the Penn State Extension Website.
–Tosh Rung Mazzone,Penn State Extension
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