Excerpt from Publication 310, Integrated Pest Management
Table of Contents
What are weeds?
Weeds are plants growing in the wrong place. For apple orchards, this usually means plants growing directly under the trees. Most apple orchards benefit from a grass sod between the rows, but some plants that establish in the grass, such as dandelions, may be undesirable.
Why are weeds a problem?
Weeds compete with trees for moisture and nutrients. Some weeds are also alternate hosts of diseases, nematodes or insects, e.g. black rot fungi, phytoplasms, root-lesion nematodes or tarnished plant bug (e.g. on chickweed). Interference from weeds may also cause labour inefficiencies e.g. tall weeds at harvest, discomfort from allergies (e.g. ragweed) or skin irritations from poison ivy, stinging nettles or thistles.
Why scout for weeds?
Although weeds are present in every orchard, there are wide variations in the species growing and density of each population. Just as scouting for insects and diseases is well established in integrated pest management, scouting for weeds is the basis for Integrated Weed Management (IWM). Information gathered from weed scouting allows growers to:
How to scout for weeds
Scouting for weeds can be done while looking for insects and diseases, although a separate walk through the orchard allows for more detailed observations and collections. Basic first steps are:
How to record weed scouting
Recording information on scouting reports helps make decisions this season, and provides a long-term record of weed emergence patterns and problems in the orchard. Record the following:
Tools needed for weed scouting
To do a good job of field scouting, bring along:
It is easy to distinguish between broadleaf and grassy weeds. Beyond this, learn the growth habit of the weed, and target management strategies at susceptible stages.
Weeds have one (or more) of the following life cycles.
Common orchard weeds are listed in Table 4-13.
The goal in managing weeds in tree fruit orchards is to maximize yields by suppressing weed competition during critical periods of crop development. This concept is the critical weed-free period - when it is most important to control weeds to prevent competition with trees. It is important to understand these facts about the critical weed-free period:
There are other valid reasons to control weeds outside of these critical periods such as attractiveness for pick-your-own customers, harvest efficiency and reduction of weed seed banks. However, controlling weeds outside of the critical weed-free period does not increase yields.
Weed competition in the planting year drastically reduces tree growth. After tree planting, it is challenging to maintain a weed-free strip under trees, but even a small amount of weed growth reduces tree growth. Figure 4-176 shows the reduction in tree growth caused by weed competition during the first three months (from planting in May until July). The herbicide treatment maintained a complete weed-free strip from May until July, and resulted in the most tree growth. Trees in the hand weeded check had less growth, because of some weed growth between hoeings, emphasizing the need to maintain a weed-free area. Other tree research shows this effect remains with the tree for many years.
Figure 4-176. Weeds competing with newly planted apple trees cause severe growth reduction in only three months (Unpublished, Harrow 1990)
The critical weed-free period for new orchards is the first three
months after planting. Research on weed effects on newly planted
apple trees shows the need to control weeds during May, June and
July. Figure 4-177 shows that controlling weeds in May allows for
the most tree growth compared to controlling weeds during June only
or July only. If the length of the weed-free period is extended
to May and June, tree growth is greater than during only one month,
and is also greater compared to June and July, or July and August.
Maximum growth is achieved when the under tree area is kept weed-free
for the three months of May, June and July.
Figure 4-177. Tree growth in newly planted
Gala/M26 trees under one month, two month or three month weed-free
periods (Dr. Ian Merwin, Cornell University)
Figure 4-178. The yield on young Gala/M26 trees is directly related to the length of weed-free period in the planting year - the greatest yield was produced when trees were kept weed-free during May, June and July of the planting year (Dr. Ian Merwin, Cornell University)
Critical weed-free period for bearing trees
For bearing apple trees, the effects of weed competition are the greatest from bud break until terminal bud set, i.e. spring until early July. This corresponds with four stages of crop development:
Conditions during flowering, fruit set and fruit enlargement effect
this year's crop, and conditions during flower bud initiation affect
next year's crop. Weed competition must be suppressed during this
critical weed-free period - bud break to July (Figure 4-179). Inadequate
grass seeding between the rows may result in early weed problems
in young orchards (Figure 4-180).
Figure 4-179. Weed competition early in the growing season robs trees of ground-applied nutrients - note the chickweed and shepherd's purse growing under the trees during the critical weed-free period
Figure 4-180. Improper or inadequate grass seeding between tree rows results in early weed problems in a young orchard
Benefits of weed competition
Trees prepare for winter's cold by hardening off, starting in the
late summer and through the fall. At this time, some small weed
growth under the trees is acceptable, and may encourage trees to
harden off (Figure 4-181). Weedy cover later in the season also
reduces the risk of soil erosion and creates overwintering habitat
for beneficial insects.
Figure 4-181. A well-established orchard sod with minimal weed growth - allowing weed growth in herbicide strips late in the season may help trees "shut down" for winter
Remember that weed suppression does not mean weed-free. Suppression of weeds to prevent competition for moisture and nutrients is usually enough to maximize crop yields. Where irrigation is used and nutrient levels are high, trees tolerate a higher level of weed competition than low nutrient or drier soils (Figure 4-182).
Figure 4-182. Early weed escapes in the planting year compete with young trees for moisture and nutrients - the addition of water through irrigation (note irrigation tubing) allows trees to tolerate more weed competition
Herbicides are one of the tools available to control weeds in orchards.
Mulching, mowing and cultivation are also effective in managing
weeds, especially in the year of planting. Each strategy has certain
strengths and weaknesses as indicated in Table 4-14.
Weed management strategies may combine several of these tools and it is important to be aware of the pros and cons of each. Mulching, cultivation and/or mowing may affect the performance of herbicides applied under trees, as well as using irrigation (Figure 4-183).
Figure 4-183. Weed "escapes" can occur around trickle irrigation emitters due to leaching of herbicides. Use spot treatments if escapes occur during the critical weed-free periods.
How herbicides work
Herbicides registered for orchards kill weeds in several different ways, but there are three broad categories of control methods.
There are advantages and disadvantages to using these types of herbicides as indicated in Table 4-15. Advantage and disadvantages of herbicide control categories.
Herbicides are grouped by modes of action by the Weed Science Society
of America (WSSA), and are listed in the OMAFRA
Publication 75, Guide to Weed Control. For each of the herbicide
categories listed, there may be herbicides from several mode of
action groups. For example, glyphosate (e.g. Roundup, Touchdown,
Vantage) and paraquat (Gramoxone) are common herbicides used for
burndown, but glyphosate is a Group 9 herbicide and paraquat is
a Group 22 herbicide.
Issues with using herbicides
Managing weed resistance
Figure 4-184. In many orchards, pigweed is a challenging weed, which often escapes after simazine is applied
Contamination of surface and ground water
Many herbicides are water soluble, and may leach through the soil
profile, eventually reaching the ground water. Heavy rainfall or
irrigation immediately after application contributes to this problem.
Herbicide spills near wells are another potential source of ground
water contamination. Avoid these situations, especially on low organic
Managing spray drift
As spray applicators, growers are required by the Pesticides Act to ensure no pesticide lands off the intended target. There are two types of spray drift to avoid:
The Ontario Grower Pesticide Safety Course suggests 10 ways to reduce spray drift:
Figure 4-185. Mount shields to protect nozzles from wind and prevent drift - an added benefit is protecting low branches from herbicide
Figure 4-186. Mount wick wipers to swipe weeds growing under trees without tree contact or herbicide drift
Figure 4-187. Use mowers with wing arms to cut weeds around tree trunks - a sensor kicks out the mower arm around the tree to prevent mechanical injury
Effect of herbicides on non-target pests
Herbicides can affect organisms in orchards, not just weeds:
Herbicides can cause injury to apple trees if used improperly. Read the label to determine restrictions on soil type or age of tree.
Herbicide residues in soils
For orchards scheduled for removal in the next three years, avoid soil-applied herbicides with a long carryover period. Do not use simazine (Princep, Simadex), Devrinol and Sinbar for several years before orchards are removed.
Soil incorporation requirement
Devrinol and Treflan require incorporation into the soil to prevent sunlight from breaking them down. For Treflan, cultivation with tillage equipment is required, restricting its use to newly planted trees. For Devrinol, incorporation by rainfall or irrigation must take place within two days in the summer or seven days in the late fall.
Integrated weed management, like integrated pest management, is a multidisciplinary approach to controlling weeds in the orchard using chemical, biological and cultural techniques. The following strategies (and combinations) are used successfully in Ontario apple orchards before planting, in the planting year and in established orchards.
Before planting the orchard
Managing weeds in orchards prior to tree planting reduces weed
problems later in the orchard life. Using the pre-plant year to
build organic matter with a green manure crop (Figure 4-188) also
gives several opportunities to reduce weed pressure.
Figure 4-188. A green manure crop, like this Sudan-sorghum cross, can be incorporated in the fall prior to planting to suppress weeds and improve soil tilth
Begin with a general burndown treatment (e.g. glyphosate) after weed growth has begun. Plant the green manure crop, choosing one that grows thickly and suppresses annual weeds. If a grassy cover like cereal or Sudan-sorghum is used, use a selective herbicide like 2,4-D to control broadleaf perennial weeds like thistles. As well, use wiper applications with Roundup targeted when the weed is most susceptible on problem weed patches - Canada thistle at early flower bud, milkweed at flower bud, bindweed at full flower, quack grass actively growing with at least three to four new leaves on each shoot. Where perennial weeds are present, choose the higher rate of glyphosate as listed on the label.
Establish sod ground cover the year before the orchard is planted.
Early fall is the preferred time to plant the grass, and once established,
the future tree row can be killed with a burndown herbicide (Figure
Figure 4-189. Orchard sod can be established in the year prior to tree planting - sod strips, where trees are to be planted, are killed out with a burndown herbicide
First year (new plantings)
After planting the orchard in the spring, choose one of the residual
herbicides registered for new plantings (Figure 4-190). Extend the
weed-free band beyond the drip line of the tree, and maintain it
weed-free during the critical period. Most soil-applied residual
herbicides give 8-12 weeks of weed control.
Figure 4-190. Properly maintained herbicide strips, just wider than the tree's drip line, eliminate weed competition
Install plastic mulch (Figure 4-191) and plant trees directly through
holes punched in the plastic. Black mulch is recommended to keep
weeds from growing under the plastic, and soil moisture is conserved
with the mulch. The life of the mulch varies depending on the material,
e.g. thin plastics last one year and landscape fabrics may last
up to five years.
Figure 4-191. Installing black plastic mulch at planting will control weeds, conserve moisture and warm the soil
There is little shading in young orchards and weed escapes are
likely to begin in early summer. Use either a selective herbicide
(e.g. Venture or Poast) for grass, or cultivate or hoe to keep tree
rows clean until late July. Some small weeds growing under the trees
by late summer slow tree growth and help trees harden off for winter.
Either strategy is acceptable if done in a timely manner. Remember the goal is to prevent weed competition from bud break through terminal bud set and fruit bud initiation.
Some growers use organic mulch under the trees to suppress weeds
and conserve moisture. Avoid weed seeds in the mulch, and monitor
nutrient levels in the trees as the mulch breaks down. Figure 4-192
shows some mulching effect from clippings mown from the sod row
Figure 4-192. Blowing grass clippings under trees with a modified mower creates an efficient and cheap mulch
There is some concern with using translocated herbicides like glyphosate
and amitrole after June, because trees are moving stored reserves
to their roots and may move the herbicide there too. Use these herbicides
carefully at this time with directional shielded sprays or wick
wipers (Figure 4-193). This timing is useful for controlling perennial
weeds like thistles or bindweed that are most susceptible at early
bloom to systemic herbicides.
Figure 4-193. Translocated systemic herbicides can be safely applied near trees using a wick wiper - either the hockey stick model (left) or the rope wick (right) have been effective, be sure to time it for the sensitive stage of the weed
Fall applications of 2,4-D or glyphosate directed away from the
trees are useful for controlling perennial weeds. Wait until useful
fruit is removed from the orchard floor before applying.
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