Elimination of winter annual weeds, which are hosts for insects, from the orchard floor reduces cat facing insect populations. Rodents find orchards infested with winter annual weeds attractive. Ground cover provided by weeds creates a desirable rodent habitat.
The radiant heat benefit associated with bare soil under trees is often overlooked. Eliminating winter annual weeds allows the soil surface to maximize heat absorption from the sun. During frost events in the spring, heat from the soil is released overnight. The released heat elevates orchard temperatures. The increase in temperature may only be a couple of degrees, however fruit loss associated with freeze damage can be minimized with small increases in temperature.
Fall herbicides aid spring and summer weed control by delaying spring application time. A fall herbicide spray can delay the spring application for 6 to 8 weeks. The result is a 6 to 8 week extension of pre-emergence weed control into the summer.
There are three common targets with fall applications--annuals, biennials such as wild carrot, and perennial weeds. The winter annual weeds include chickweed, henbit and the mustards. In order to have an effective weed control program you need to distinguish the different types of weeds and when they germinate.
- Annuals are weeds that live less than one year. Annuals are further divided into two classes, summer annuals and winter annuals. Summer annuals germinate in the late spring and early summer, flower and set seed in late summer or early fall, and die when it gets cool. Winter annuals germinate in the fall or early spring, flower and set seed in late spring and die when it gets hot.
- Biennials are weeds that live longer than one year but less than two full years. Biennials often grow vegetatively during the first year, then flower and die during the second year. Yellow rocket and bull thistle are two common ones in Pennsylvania.
- Perennials are weeds that live longer than two years, often reproducing vegetatively by horizontal shoots, roots, nutlets or rhizomes, as well as by seed.
Most of our herbicides labeled for tree fruit work best when applied before the seedlings emerge or while the weeds are small and are less effective when weed seedlings have become established. Established perennial weeds are much more difficult to eradicate so it is best to prevent their seed germination. The table below lists some common annual or biennial weeds in Pennsylvania orchards. The column labeled "Type" indicates whether they are winter annuals, summer annuals or biennials. If they fit into more than one type, the first type listed is the dominant time of germination. It also lists what weed species can look similar to them; some of the similar looking ones could actually be perennials.
Fall application of herbicides is a good means of controlling the winter annuals (WA) and if you include a post-emergent material, burn off any summer annuals that emerged late in the summer. This is also a good time to attack the perennial weeds such as dandelion, thistle and yellow toadflax (also known as butter and eggs).
There are a few steps in making effective fall applications. First, scout your orchards to determine what weeds you have. When scouting, note if the weeds are predominantly annuals (easier to control), perennials (tougher to control) or a mix of both. Most herbicides have days to harvest limitations if there is still fruit on the tree, so make sure you comply with those requirements.
If you are primarily after perennial weeds there are two classes of weeds that affect the timing of your treatment. Warm season perennials, such as johnsongrass, pokeweed, hemp dogbane and horsenettle should be treated before frost. The first frost will cause these types of weeds to shut down, if they have not matured and senesced. Herbicides are not effective once the plant has been exposed to frost. On the other hand, winter annuals, biennials and cool season perennials (dandelion, Canada thistle, quackgrass) are most effectively controlled when herbicides are applied mid-October to mid-November. Winter annuals which include chickweed, mustards and groundsel emerged in late summer into fall are still responsive to herbicides albeit at a slower pace. It is not necessary to wait until frost to apply herbicides. But be aware that treatments that include a residual pre-emergent applied early when soil temperatures are still warm can result in a shorter period of control next spring. The early timing can reduce the length of control the next spring because of greater herbicide degradation in the fall. On the plus side fall rains help move the herbicides into the soil and activate the materials.