Weed management for apple orchards

Introduction

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.

Scouting for weeds

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:

• identify weeds present early in the season when they can cause yield losses
• match herbicides to weeds
• choose cultivator for weed stage
• alter cultural practices for different weed life cycles
• discover weed patches before they spread through field
• identify areas to avoid cultivation
• identify areas for spot treatments
• choose the optimum timing for maximum control

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:

• identify type of weed — e.g. grass or broadleaf
• identify life cycle — e.g. annual, biennial, perennial
• identify name of weed, if possible
• identify source of weed — e.g. mulch, fence row, weed escapes, irrigation water
• take samples for verification with roots, flowers and shoots — in paper bag inside a plastic bag, with a label
• work a zig-zag pattern through the field — remember weeds grow in patches
• check field edges for weed invaders

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:

• identify the weed, if possible
• the growth stage of the weed and crop
• the number and shape of the leaves
• the stage of flowering
• areas and percentage of the field infested - mark on a field map

Tools needed for weed scouting

To do a good job of field scouting, bring along:

• a keen eye
• a hand lens
• field maps
• weed identification books
• bags for samples and marker
• field history

Basic biology of weeds for field scouting

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.

• Annual weeds — grow and flower in one year. Some weeds in orchards are winter annuals, i.e. they begin their growth in the fall, forming a rosette, and flower the following spring or summer.
• Biennial weeds — have a two-year life cycle, producing leaves in the first year and flowering in the second year.
  Annual and biennial weeds compete for nutrients and water as they grow under trees. After they flower, they die. However, their seeds may cause recurring problems for several years by forming a soil seed bank.
• Perennial weeds — live for many years, and generally establish from various types of root systems, and many also spread by seeds. They usually flower every year, expanding their root system and spreading by both methods through orchards. Perennial weeds can be very competitive, especially if they grow in thick patches.

Common orchard weeds are listed in Table 4-13.

Goals of weed management

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:

• if the crop is kept weed-free for the critical weed-free period, no yield reduction occurs
• weeds emerging after the critical weed-free period do not affect yield
• when weeds compete with trees during the critical weed-free period (especially during the planting year), effects may last several years, and possibly for the life of the orchard

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 effects

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.

Image

Critical weed-free period for new orchards

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.

Image

Figure 4-178 shows results from the same trees when they produced their first harvest two years later. Note that trees kept weed-free for May only, May and June, or May, June and July produced the most yield per tree in their first harvest. This indicates that the critical weed-free period for newly planted trees is the first three months after planting.

Image

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:

• flowering
• fruit set
• fruit enlargement
• flower bud initiation

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).

Image

Image

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.

Image

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).

Image

Weed management tools

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).

Image

How herbicides work

Herbicides registered for orchards kill weeds in several different ways, but there are three broad categories of control methods.

• Burndown (contact) — herbicide is applied to existing vegetation and kills the top growth. Herbicides are directed under the trees to create a weed-free strip and/or applied to the row alleyways. Some burndown herbicides are systemic - they are absorbed into the plant and move to the growing point and/or the roots, and give longer-term control for perennial weeds. Some burndown herbicides are not systemic and target only above-ground green tissue. These contact herbicides control top growth, but do not give long-term control of perennials.
• Soil residual (pre-emergent) — herbicide is applied to the soil surface before weed seedlings germinate. The chemical remains in the soil for several weeks to months, killing germinating seedlings. Generally, a rainfall of at least 12 mm is required to activate the herbicide, resulting in a longer period of control.
• Post-emergent (selective) — these herbicides kill only certain weeds out of existing vegetation e.g. graminicides kill grasses only, or auxinic (hormone) herbicides kill only broadleaves. These are usually targeted for weed escapes or for problem weeds like thistles or annual grasses.

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.

Be sure to know both the control category and herbicide group for herbicides used in the orchard. For example, a weed management strategy may require a burndown herbicide in the spring may use a Group 22 herbicide because a Group 9 herbicide was used the previous fall.

Rotating herbicides with different modes of action is an important strategy to manage weed resistance.

Many herbicides only control grasses or broadleaf weeds. For broad-spectrum control of weeds usually growing in orchards, apply a tank mix of two herbicides. Choose products with strengths on different weeds, e.g. Dual II Magnum (controls grasses well) is often tank mixed with Princep (controls broadleaf weeds well). If weed growth is present, include a burndown herbicide in the tank mix.

Selective herbicides are useful for treatments on weed escapes, for specific problem weeds like grasses or vetch, or for reducing pressure from broadleaf weeds with fall treatments of 2,4-D. Waiting until weeds appear before using herbicides is a preferred approach in a truly integrated weed management, as long as practical considerations such as timeliness of application, resistance management and critical weed-free period have been considered.

Issues with using herbicides

Managing weed resistance

There is concern that continuous use of the same herbicide selects for resistant weeds, and herbicide resistance has been identified in a growing number of weeds and locations around the world. In Ontario orchards, resistance to paraquat has been identified where repeated applications each year over several years was practiced.

In orchard practice, it is common to use herbicides with different modes of action, often during the same growing season. For example, triazine herbicides (e.g. Princep/simazine) have been widely used in orchards, creating concern of selecting for triazine-resistant pigweed (Figure 4-184) and lamb's-quarters. Corn growers had previously selected triazine-resistant weeds through continuous use of triazines alone. However, when orchard growers include other herbicide modes of action, e.g., Roundup, Ignite or Gramoxone, any resistant weeds are killed by these treatments. The escaped weeds are usually species that simazine does not control well e.g. pigweed and crabgrass. Only tests confirm if weeds are truly triazine-resistant. With more herbicides available to apple growers, choose different herbicide families and modes of action. Rotate herbicides within each year, and between years, in each orchard block.

Image

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 matter soils.

Soil residual herbicides such as simazine bind tightly to soil particles and may be washed into surface water when soil erosion occurs. Practice erosion control if the orchard site is at risk of wind or water erosion.

Remember herbicides lost through leaching or erosion are not available to control weeds in your orchard.

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:

• physical drift — when tiny droplets (less than 100 microns) are moved by wind and air currents off-target
• vapour drift — when volatile products (e.g. 2.4-D) become a vapour and move off-target

The Ontario Grower Pesticide Safety Course suggests 10 ways to reduce spray drift:

1. use anti-drift technology — nozzle hoods, boom hoods, perforated screen, air-assist/curtain and shields (Figure 4-185)
2. use alternative application technology, e.g. wick wiper (Figure 4-186)
3. where possible, don't spray - use cultivation, flamers, mulch biological control or mowers (Figure 4-187)
4. read the label and follow directions for stage of growth, weather and other precautions
5. watch the weather — and avoid spraying in winds over 10 km/h, temperatures above 25°C and low RH (<75%)
6. use buffer strips, check label for required distance - if not listed, stay back at least one spray boom width
7. use high water rates and larger nozzles — results in larger droplets and better spray coverage
8. use adjuvants with caution — check label, may distort spray pattern
9. lower the boom, may need wider angle nozzles
10. choose the right nozzles — drift reducing styles are now available

Image

Image

Image

Effect of herbicides on non-target pests

Herbicides can affect organisms in orchards, not just weeds:

• some herbicides are toxic to soil fauna like earthworms (e.g. linuron (Lorox)
• some insect pests thrive on weeds, control these weeds to reduce fruit damage, such as:
  • tarnished plant bugs thrive on creeping Charlie and chickweed — control these weeds to reduce injury
  • twospotted spider mites overwinter on the ground cover and build up populations on these weeds in the spring — remove host weeds early in the season to reduce problems
  • caution — where tarnished plant bug or twospotted spider mites are established on the orchard ground cover, avoid killing the weeds with herbicide and forcing pests into trees
• controlling dandelions in orchard sod (e.g. with 2,4-D) may affect bee activity during pollination time — honeybees appear to prefer apple blossoms to dandelions but bees may use dandelions to maintain hive strength in spring if flying conditions are poor during apple bloom

Crop safety

Herbicides can cause injury to apple trees if used improperly. Read the label to determine restrictions on soil type or age of tree.

• Soil organic matter — generally soils with at least 2% organic matter (OM) are required to use soil residual herbicides. For low organic matter soils (OM <2%), choose Devrinol because its crop safety is not dependent on OM. Mulch is also a good choice for low OM soils. A soil OM of 3% is recommended to avoid crop injury when using Sinbar, because it tends to be less bound to soil particles.
• Age of trees — some herbicides are only registered for new trees. For Treflan, this restriction is because the product must be incorporated. For other products, there has not been research to prove there is no residue in the fruit. Some herbicides are only used on older trees. For example, linuron is restricted to some tree species more than 10 years old because the original research was only done on trees 10 years old. In some cases, there is a need to wait until the trees have a larger root system or thicker bark to avoid tree damage.

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.

Strategies for integrated weed management (IWM)

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.

Image

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 4-189).

Image

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.

Image

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.

Image

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.

Tillage tools like the Weed Badger (rotational finger weeders) are also effective if done every three to four weeks. Many implements have a sensor that kicks equipment out around the trees to avoid damage. Avoid late tillage in the fall to prevent late tree growth and winter injury.

Established orchards

Early season

Some weeds, like chickweed, usually begin growth early in the season before bud break. As an orchard matures, patches of perennial weeds begin to establish and are usually growing by bud break. Use one of these strategies for weed control.

• Wait for weeds to emerge and use a burndown treatment, followed by mulching and/or a residual herbicide.
• Apply mulch and/or one of the residual herbicides before weed growth begins. Control weed escapes that begin before the critical period of fruit bud initiation occurs (generally in early June when terminal buds set). Herbicide choice depends on weeds are present and how early in the growing season they appear.

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.

Summer

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 middles.

Image

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.

Image

Fall

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.

Destroy flowers of weed escapes before weed seeds are allowed to spread. Mowers with swing arms that can reach under the trees are effective, especially if weeds are small and lush.

Plan a weed management strategy in advance. Focus on controlling weeds during the critical weed-free period, and integrating all the tools of weed management to maximize yields in apple trees. Use information from weed scouting to adjust the strategy as required. Be sure to make notes of results and adjust the strategy for the following season as needed.