Weeds can reduce caneberry growth and production by competing for water, nutrients, and space. Young plants grown from newly planted rhizomes or regrowing after pruning near soil surface are most susceptible to competition with weeds. Established canes normally experience little weed competition. However even in an established caneberry crop, weeds that are not controlled can negatively impact production by sequestering nutrients and water, retaining moisture in the lower canopy to the detriment of fruit quality, and harboring pests and pathogens. When not controlled, weeds will produce seed or vegetative propagules that may remain in the soil seed bank and appear in following crops.
Weed control in caneberries relies on preplant soil fumigation for management of most weed species. Additionally, herbicide applications, hand weeding, and cultivation between plant rows may be required, depending on weed species composition and density. Mowing weeds between rows and mulching have also been practiced in established caneberries.
Many different summer and winter annual and perennial weeds can be found in caneberries grown in California. To make the best choice of control practices, conduct weed surveys before planting the caneberry rhizomes and continue at least twice a year: winter and spring. Keeping written records of monitoring results will help with site-specific and timely weed control. To prevent weed shifts or herbicide resistance from occurring, monitor treated areas and remove escaped weeds chemically, mechanically, or by hand.
Weeds are controlled best in the seedling stage; therefore, it is important to be able to identify weed seedlings. A good pictorial reference, such as the online weed gallery photos on the UC IPM Web site will help assist in identifying weeds. Check the susceptibility charts to determine the best herbicide(s) to use for optimum control.
WEED MANAGEMENT BEFORE PLANTING
Soil fumigation with 1,3-D and chloropicrin followed 10-14 days later by an application of metam sodium or metam potassium for control of soilborne pests would also control most of the weeds in the soil seed bank. Soil fumigation only partially controls weeds with hard seed coats, such as little mallow or redstem filaree, and herbicide applications of carfentrazone or paraquat may be needed to control these species.
Cultivation can be an effective weed control measure, especially in organic production. Cultivation following irrigation to germinate weed seeds helps deplete the soil seed bank by destroying germinated weeds. This method will effectively control annuals but is not effective on perennial weeds and may even spread vegetative parts of perennial weeds that can become established if the soil is moist. To control perennial weeds such as bermudagrass, and johnsongrass, be sure to cultivate the soil when it is dry and to cut the rhizomes into small pieces. Rework the soil to pull new rhizomes to the surface and dry them out as well.
Interior furrows in caneberry tunnels are typically dry. However, the rows that run from one tunnel post to the next collect the water that runs off the plastic during rains. This presence of water facilitates weed emergence and growth. Weeds in anchor rows do not directly compete with canes, but can rapidly reproduce and spread into neighboring cane rows. Cover crops planted in anchor rows can minimize nutrient and sediment losses during rains and can be excellent competitors of weeds. Cover crops compete with resident soil seed bank weeds and prevent wind-dispersed weeds from reaching the wet soil surface, which is necessary for their germination. For example, barley and triticale planted at 600-800 lb/A into wet post rows controlled annual sowthistle 85 to 95%, Conyza species (hairy fleabane and horseweed) 85 to 98%, and little mallow and redstem filaree about 90%, compared to non-cover cropped anchor rows. Mallows and filaree weeds are troublesome, because they survive fumigation due to their hard impermeable seed coats, while Conyza species have developed resistance to both glyphosate and paraquat, chemicals commonly used in caneberries and elsewhere. Cereal cover crops can be managed with mowing or sethoxydim (Poast) when needed to prevent seed production.
Germinated annual weeds and aboveground parts of perennial weeds can be controlled with glyphosate. Evaluate glyphosate efficacy 10-14 days after application and note lack of control: glyphosate resistance have been documented in some weeds such as horseweed, which has wind-dispersed seed that can easily blow in from other areas. Tillage, hand weeding, or paraquat application will control the glyphosate-resistant weeds.
Rhizomes of field bindweed and tubers or nutlets of yellow nutsedge will not be controlled by any herbicide currently registered in caneberries. Repeated glyphosate treatments may help deplete soil reservoir of perennial propagules and provide partial control.
Carfentrazone and paraquat can provide partial to complete control of weeds with hard seed coats that escaped soil fumigation.
WEED MANAGEMENT AFTER PLANTING
After planting, weeds can be controlled both culturally and chemically. Hoeing or using weed knives in the row and cultivating between the rows are cultural methods that must be performed frequently to keep weeds from seeding and to reduce competition.
Use of plastic mulches in blackberries and raspberries is limited because of high costs, difficulties in management, and because it interferes with emergence of new canes. Yard waste and other organic mulches placed around plants can improve water infiltration and other soil physical properties and allows for new cane emergence. A yard waste mulch layer has to be at least 6 inches thick to prevent weed germination.
Herbicide use in established caneberries has a dual purpose: weed control and in season elimination of vegetative primocanes as needed. Carfentrazone (Shark) and paraquat (Gramoxone) can provide partial-to-complete weed and primocane control after planting. Neither herbicide is recommended for use in blackberries or young raspberries, because both crops have green cane that will be damaged by application of these herbicides. Carfentrazone and paraquat are most effective when applied before weed emergence or when weeds are small. When applying either of these materials to a mature crop of raspberries, it is imperative that care be taken to prevent drift into the upper canopy of the hedgerow, or significant damage can occur to the foliage.
Postemergence herbicides such as glyphosate can be used to spot treat perennial weeds during the growing season; take extra caution to avoid contact of the herbicide with crop green tissue.