The Importance of Weed Control in Commercial Fruit Production

By Forrest Scharf, PAg, Provincial Specialist Fruit Crops, Crops & Irrigation Branch, Regina

Most fruit species are perennial plants and are expensive to purchase and grow. So, when investment is made to develop a large area where the plants are expected to thrive and produce bountiful crops, it makes sense to do everything possible to ensure they have the best chance to overcome natural challenges. A consistently significant challenge is controlling weeds.

Weeds have numerous negative impacts on production. Most researchers support the idea weeds limit access to resources. They need water, nutrients, space to grow, sunlight to thrive and they compete with other plants to ensure they get those resources. If the crop plants do not get the optimum amounts of nutrients and water, their growth is hindered. Some people have challenged that concept by suggesting if fertilizers and extra water are applied, the crops would not be impacted by weed growth. However, weeds have additional mechanisms to cause negative impacts including allelopathic exudates. Allelopathy occurs when plants exude chemical substances that inhibit the growth of other plants. Many weeds are fast growing whereas most fruit species are comparatively slow growing. This gives weeds the upper hand, especially when the fruit plants are young and struggling to establish strong root systems themselves. When fruit crops are young and struggling to compete they are also more susceptible to allelopathic chemicals that hinder growth, and due to resource competition combined with allelopathy, it takes them much longer to grow to normal sizes and become highly productive or more competitive.

The University of Saskatchewan Fruit Program experienced this several years ago when young cherries were being hand weeded by students. After weeding a few rows, the students got reassigned to do other duties and never returned to weed the remaining cherry rows. The growth in the non-weeded rows was significantly reduced compared to the weeded rows in the year when the oversight happened, and those rows continued to experience growth inhibition for several years following the oversight. Productivity was also greatly reduced in the non-weeded rows.

Another negative aspect of poor weed control is that weeds tend to form dense canopies that serve as hospitable moist protected environments for insects and diseases to thrive. Not all insects or diseases transfer directly from weeds to cause damage in fruit crops, but many do. So, it is best to reduce the weed growth that serves as a hospitable space where weeds and diseases can sustain themselves and then move over into fruit crops and other plants.

From an orchard management perspective there are a few categories of weeds to consider. Grassy weeds, broadleaf weeds and woody perennial species.

• Grassy weeds can be annuals or perennials and are characterized by long linear, narrow and parallel sided leaves where the apical meristem is located at the base of the plant on the inside from where the leaves emerge. Flowers in grassy weeds are non-showy and develop as spikelets that are grouped into numerous forms of inflorescence. Grassy weeds can be mowed down prior to seed set to reduce their spread, but some perennial grassy weeds like quack grass spread via underground rhizomes and mowing does not keep those weeds at bay.
• Broadleaf weeds have wide leaves and grow from a stem. The shoot apical meristem in a broadleaf weed is found at the end of stems or branches. Broadleaf weeds also often have showy flowers of different forms depending upon the species and can produce immense quantities of seeds. One functional difference of where the meristem is on a plant is that in broadleaf plants the shoot apical meristem can direct new growth towards sunlight using hormonal signalling, whereas grassy weeds, with their meristem at the base of the plant, cannot redirect towards sunlight. This makes a difference when weedy plants are growing under mulch, because broadleaf weeds can grow through openings to reach sunlight, whereas grassy weeds will not grow towards openings.
• Woody perennial weeds are like most orchard crops and can be difficult to manage once they have established themselves. Aside from depleting soil, water and nutrient resources from the crop species they can grow larger than the fruit plants and block out access to sunlight which causes persistent reduction in fruit quality.

There are differences between broadleaf and grassy weeds with respect to herbicide tolerance. Often grassy weed herbicides are designed to selectively kill grassy weeds but do not harm broadleaf species. On the other hand, there are also broadleaf herbicides that selectively kill broadleaf weeds but do not harm grassy weeds. For orchard crops that are broadleaf perennials, grassy herbicides are often well tolerated by the fruit plants (displaying little plant damage). On the other hand, because fruit species are broadleaf, if broadleaf herbicides are used those can be significantly damaging to the orchard plants. Herbicide labels outline crop tolerances, and application of herbicides must strictly follow the label recommendations.

Controlling woody perennials is mainly achieved through hand pruning, and in some cases using late fall applications of herbicides (when the plants are moving carbohydrates down into the root system and the herbicide will transport downwards with them and cause total plant death, as opposed to top kill where the roots will re-establish another tree in the next growing season). Weed control is best handled when the plants are immature, before they can spread vegetatively or via seed that later gets deposited into the soil seed bank (creating a longer-term, persistent problem).

Broad spectrum or non-selective herbicides can be expected to effectively eradicate all types of vegetation including grassy and broadleaf weeds. Some non-selective herbicides can be used before an orchard is established to clean up the orchard space as much as possible (although there are usually seeds present in the seed bank). An example of a broad-spectrum herbicide is glyphosate. It is a contact herbicide so anything that gets exposed to the chemical is likely to suffer some plant injury, but for anything outside the time of application or that avoids contact, no harm should occur. It can be used prior to planting to ensure weeds are under control before orchard establishment is attempted. A few other broad-spectrum herbicides have residual efficacy and can be applied after the fruit plants are established. A product that is widely used in the fruit sector that has these characteristics is dichlobenil (marketed as Casoron G-4). It is sprinkled as a granular product within the orchard rows in fall or early spring when temperatures remain consistently below 10 C. Melting snow and rains will disperse the chemical into the upper soil layer and any germinating weed seeds that are exposed will be eliminated. It is beneficial because it serves to control weeds in registered crops for several years with reapplications recommended every third year (depending on weed pressure and other factors that can be found on the product label).

Other weed control methods include periodic cultivation using discers, cultivators and establishing plants like dwarf clover that forms a mat to cover alleyways, allowing the low-lying canopy to outcompete weeds. Often, orchard plants are placed under mulch of various forms (black plastic, white plastic, wood chips, or landscape fabric). In a Haskap agronomy project landscape fabric outperformed other types of mulch.

For more information about weed control in fruit orchards, please refer to the Ontario Crop Protection Hub and contact the Provincial Fruit Crop Specialist at 306-787-4666.