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(Covered) cover cropping

July 10, 2024  By Talia Plaskett, protected crop specialist, Perennia

Figure 1. An oat-pea cover crop mix planted on four different dates. This demonstration was funded by the On-Farm Climate Action Fund (OFCAF) program. Photos courtesy of Talia Plaskett, Perennia.

Protected spaces tend to house the hardest-worked soils on market garden farms, growing year-to-year, back-to-back cash crops. This intensive cycling degrades the chemical, ecosystem and physical health of the soil through compaction, loss of soil structure, build-up of salts and increase in pH over time. While profitability and maximizing the use of on-farm infrastructure is crucial, sustainable agriculture requires the maintenance of soils to remain productive into the future.

Cover cropping is commonly implemented in field production to protect the soil against erosion between cash crops. Where erosion is not the primary concern in protected spaces, cover crops offer a wide range of benefits to soil health. These benefits include nitrogen fixation, building organic matter, supporting microbial communities, breaking up compaction, contributing to diverse crop rotation schedules, promoting soil aggregation and contributing to disease, pest and weed management.

Over the last two years, Perennia has had the opportunity to demonstrate strategies for winter cover crop implementation in Nova Scotia in protected spaces.

Below is a summary of what we have learned:

  • Seeding date is incredibly important to maximize cover crop establishment and growth before light becomes limiting.
    • Biomass production was highly impacted by seeding date, like the trends observed in winter cash crops. Figure 1 shows four different planting dates in an unheated high tunnel, with the first planting date happening mid-September, and the final planting date taking place mid-October. Crops seeded earlier produced significantly more biomass compared to the same mix that was seeded later in the year. These differences were maintained through the winter until the end of March, when the cover crop was terminated.
  • While there are some cover crop species with increased winter hardiness compared to others, that does not guarantee they can withstand the full brunt of cold temperatures in an inflated, unheated tunnel.
    • Row covers were installed over winter cover crops to protect against cold temperatures and help boost production once sunlight is no longer the limiting factor to plant growth.

Figure 2. Comparing cover crop survival across row cover and no row-cover treatments through the winter of 2022-23.

In Figure 2, you can see two sections of a winter cover crop – one which was covered with row cover, and one which was accidentally uncovered. Nova Scotia experienced a polar vortex in February 2023, which provided an extreme test of the row cover’s capacity to protect a winter cover crop. The insulating layer was able to sustain the covered crops through the winter, whereas the exposed plants did not survive. The added protection meant increased biomass production and a more effective weed suppressant compared to a crop left to winterkill. Keeping in mind that the damage done to the unprotected cover crops would likely be less severe through a more typical winter, the argument can still be made that row cover is a cost-effective way to preserve the crop through the winter months and provide additional protection for unexpected extremes. In our case, it maximized the benefits generated during the off-season without having to provide costly supplemental heating.

Judson Reid at Cornell University has been studying winter cover cropping in tunnels for a few years now and presented some of their findings to us. In a less extreme season where cold-tolerant cover crops survived through the winter, there was consistently more biomass produced in the row-covered treatments compared to those left to grow in ambient tunnel temperatures. Figure 3 shows the impact of row cover on two different cover crop mixes, in two different tunnels, highlighting the trend associated with added insulation.

Figure 3. A comparison of the biomass produced by two cover crop mixes grown in unheated tunnels throughout the winter. Chart courtesy of Judson Reid via Perennia.

Reid recommends the installation of row cover after a few days of tunnel temperatures below 0 C. This should be enough of a cold snap to help reset insect pest populations in the tunnel without significantly compromising the cover crop.

For a summary of Reid’s work with cover crops at Cornell University, check out this presentation he did for Perennia.*

As with any practice, there are drawbacks associated with the practice of implementing protected cover crops in the off-season:

  • Potential to bridge the gap between last seasons pests and the next, with lots of places for pests and diseases to survive compared to a bare soil;
  • Reduces the amount of time that a greenhouse can be used to produce cash crops;
  • Good establishment of winter cover crops requires early establishment in September, when cash crops may still be productive;
  • Incorporating cover crops can translate to increased tillage when tarping, spraying and/or winterkill are not an option for termination; and
  • Benefits are not necessarily noticeable from year to year;
    • Cover crops should be viewed as a long-term investment, versus a quick solution to a long-term problem.

What’s next?

While the majority of cover crop talk for greenhouses is focused on the winter months, some producers find themselves with empty tunnels for short periods of time during the summer months. This summer, with funding from the On-Farm Climate Action Fund, Perennia will demonstrate implementation of summer cover crops. Water infrastructure, irrigation scheduling, mowing requirements, as well as selected species are going to look different through the heat of the summer compared to a winter cover crop, so stay tuned to our social media channels to see how it goes!

*Funding for this project has been provided by Agriculture and Agri-Food Canada through the Agricultural Climate Solutions (ACS) – On-Farm Climate Action Fund (OFCAF).

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