Approaches to growing floricane red raspberry (Rubus idaeus) in Washington, the state that dominates U.S. red raspberry production, are changing. This change is fueled by growers’ desire to create more efficient and productive systems for growing this delicate crop. One of those changes is the use of tissue culture (TC) transplants at planting, which are produced using micropropagation techniques.
Raspberry growers in Washington have traditionally used bare root canes and root cuttings as their source of planting material. However, use of TC transplants is increasing because they are disease and pest free and can be produced more rapidly in nurseries. In fact, some cultivars are now exclusively available as TC transplants.
In 2018 alone, approximately 34% of raspberry plantings in Washington were done using TC transplants. Use of TC transplants is projected to increase, and growers will need to adapt their growing approaches to ensure success.
While there are benefits to using TC transplants, there are also drawbacks. First, the cost of TC transplants is typically higher than the cost of traditional sources of plant material. Second, TC transplants are weak competitors with weeds and often establish poorly. Raspberry in Washington has traditionally been grown on bare ground raised beds with growers depending on hand weeding and/or pre- and post-plant herbicide applications to manage weeds. Because TC transplants have foliage, use of herbicides is limited.
Furthermore, labor shortages and increasing costs present another limitation — labor may not be available or too costly to provide adequate weed management. Poor weed control during establishment also reduces the long-term productivity of a planting and growers’ profitability.
New Mulching Methods
This is where a second change enters our story — the use of plastic mulches to improve establishment of raspberry fields planted with TC transplants. This more recent change has been influenced by our collaborative research team, including Washington State University and USDA scientists.
Mulching is a strategy to manage weeds and promote plant establishment. Polyethylene (PE) mulch has been used in annual cropping systems for decades because it can provide weed control, increase soil temperature and moisture, and improve plant growth and yield. However, use of PE mulch generates plastic waste and disposal can be costly. Recycling PE mulch also is limited due to a lack of recycling facilities. Used PE mulch is contaminated with soil and plant debris, which adds weight and needs to be cleaned, adding to the cost of disposal and recycling.
Biodegradable plastic mulches (BDMs) are manufactured with different feedstocks and additives compared to PE mulch and are designed to biodegrade in the soil by microbial activities. BDMs could be an alternative to PE mulch if they provide similar benefits as PE mulch and biodegrade in soil or composting conditions.
Research teams across the globe have shown that BDMs can be an alternative to PE mulch in many production systems, such as pumpkin, pepper, corn, and strawberry. However, the study and use of BDMs is predominantly concentrated in vegetable and annual strawberry production systems.
Application of both PE mulch and BDMs has not been studied in floricane raspberry. However, both PE mulch and BDMs may be a solution to the problem of poor TC transplant establishment. The goal of this research was to investigate the application of both PE mulch and BDMs in floricane raspberry established using TC transplants.
To determine if PE mulch and/or BDMs are a potential solution to improve TC plant establishment, we conducted a field study comparing PE mulch and four BDMs to growers’ standard practice of bare ground cultivation. The specific treatments we compared are shown in Table 1. Plots were established on a commercial farm using ‘Wakefield’ raspberry. We evaluated weed management, soil temperature and moisture, plant growth, and yield in 2017 and 2018. We also monitored populations of root-lesion nematodes because growers were concerned that mulches could increase population densities of this parasitic nematode.
We found that PE and BDMs successfully controlled weeds during the 2017 growing season. Although the grower/cooperator hand-weeded the bare ground plots three times during that period, cumulative weed number and biomass were still greater in the bare ground control than all mulched treatments. Furthermore, no hand-weeding was required in the mulched plots, demonstrating that mulching can significantly reduce labor needs.
Plant growth was greater among plants grown with PE and BDMs relative to the bare ground control. By October 2017, average primocane height and number were 14 inches and 5 primocanes/plant greater, respectively, across all mulched treatments than the bare ground control.
Our yield results mirrored our plant growth data. Plants grown with PE and BDMs had greater total yields compared to the bare ground plots. Total average yield across mulched treatments was 8,976 lbs./acre, which was 34% (2,743 lbs./acre) more than the bare ground control.
Differences in plant growth were likely due to soil temperatures. Soil covered with PE and BDMs were on average 2.3 degrees higher than the bare ground control. Soil temperature under BDMs was slightly lower than under PE mulch. Reduced soil temperature under BDMs compared to PE is attributed to differences in mulch thickness and permeability.
When we looked at root lesion nematode populations in September 2018, we found they were higher in plant roots treated with PE than those treated with BASF 0.6 — one of our BDM treatments — and the bare ground control. Elevated soil temperatures likely increased nematode reproduction, which led to higher population densities. However, there were no reductions in plant growth and yield. We infer that the increased growth observed with PE and BDMs outweighed any potential negative impacts that plastic mulches had on nematode population dynamics.
Overall, PE mulch and BDMs provided excellent weed suppression, increased soil temperature, and promoted plant growth and fruit yield relative to traditional bare ground cultivation in commercial floricane raspberry established using TC transplants. These results demonstrate that both PE mulch and BDMs are promising tools. Washington growers have been encouraged by our research findings and are starting to adopt mulching on their farms.