The old standard practice was to apply herbicides in the spring. The old herbicides such as simazine, terbacil and diuron, were soil residual products that were mobile. These were to be applied in spring rather than in the fall when rains could cause the active ingredient to be moved down slopes.
However, with the increase of perennial weed problems and the need to prevent weeds from gaining a foothold in orchards, fall application of pre-emergent materials makes a lot of sense. Many newer products actually perform better when applied in the fall since soil moisture is increased due to rains. Below are some additional advantages as well as some points to consider.
Advantages of fall herbicide applications
- Eliminates vegetative cover for voles allowing predators easier access to them.
- Can control late germinating weeds such as yellow rocket and chickweed and newly germinated biennials such as wild carrot.
- Reduces spring "trash weeds" allows for better coverage of pre-emergent and post emergent materials.
- Can extend the window of herbicide application into late spring early summer when hectic spring chores such as of early season fungicide and chemical thinning sprays are completed.
- Provides extended suppression of vegetative weed growth may allow fewer applications of post emergent materials during the following summer growth season.
- Perennial weeds are more susceptible to systemic materials like glyphosate and 2,4-D in the fall.
- Newer pre-emergent materials work better when applied to moist soils and are moved downward into the weed seed germination zone with autumn rains.
- Fall rains help move herbicides into the soil and provide moisture activation.
- Cooler soil temperatures reduce herbicide degradation and dissipation.
Things to consider when applying herbicides in the fall
Weed types and species
Different pre-emergent herbicides control different types of weeds. Some work best on grasses with some effects on broadleaf weeds. Some work best on broadleaf weeds with some effects on grasses. Therefore, know what weeds were a problem in the block and choose the appropriate material.
Crop or leaf residue
Application of pre-emergent herbicides after tree leaves have dropped may provide less than satisfactory results due to the inability of the herbicide to reach the soil surface through the leaf litter. Pre-emergent materials that work best with no crop or leaf residue include indaziflam, rimsulfuron, and flumioxazin.
Leaching and runoff
Because residual herbicides are applied to the soil and persist for several months, there can be serious concerns about movement into ground or surface waters. Some residual herbicides are weakly bound to soil while others are more strongly bound and less likely to leach or move off site. Some mobile pre-emergent materials include simazine, diuron, terbacil, norflurzon and dichlobenil. Non-mobile pre-emergent materials include indaziflam, rimsulfuron, oryzalin, pendimethalin and oxyflurofen.
Some residual herbicides have a range of suggested rates depending upon the soil type. Heavier soils with higher organic matter content or more clay usually require higher rates; while sandy or gravelly soils may require a lower rate.
Only a few herbicides are available to use in newly planted orchards and this can also vary by fruit type. Norflurazon (Solicam) can be applied to newly planted apples once the soil has settled. However, for peaches there is a 6 month establishment period and for apricots, pears and plums a 12 month establishment period.