Cover Crops as Living Mulch under Organic Vegetables

Cover Crops as Living Mulch under Organic Vegetables

Summary

Incorporating long-standing leguminous cover crops into vegetable rotations is commonly known to improve soil quality and increase nitrogen availability to subsequent crops.  Small-scale, organic vegetable growers, however, struggle to include season-long covers on limited land.  Further, it can be difficult for growers to establish winter covers after harvest of late season crops like winter squash and Brussels sprouts.  Using cover crops as living mulches under vegetable crops could address both of these issues if vegetable yield is still comparable to clean cultivated controls.

In the 2015 we established 2 living mulch treatments under acorn squash and Brussels sprouts on 4 cooperating organic farms in south central Wisconsin.  The living mulches were Dutch white clover and medium red clover.  To measure the effect of the living mulches, we collected weed density and biomass, cover crop density and biomass, soil moisture, labor time per treatment, and cash crop yield.

Year one results show no differences in clover density and biomass between the Dutch white or medium red clover treatments, suggesting that both cover crops perform equally as living mulches.  There also were no differences in weed density or biomass between the clover treatments and the clean cultivated control, suggesting that the cover crops did not  help to control weeds.  Cash crop yield, for both vegetables, was significantly higher in the control compared to the clover treatments.  This result does suggest that the living mulches were competing with the vegetable crops for resources, thus reducing yield.  However, labor in the control was higher compared to the clover treatments, so management costs are increased in a clean cultivated system.

Objectives/Performance Targets

Trials were established on 4 cooperating farms in southern WI.  All the farms are certified organic and have between 8 and 12 years of experience with vegetables and cover crops.  Replicated plots of acorn squash and Brussels sprouts were established at each farm with living mulch treatments of Dutch white clover and medium red clover, as well as clean-cultivated controls.  The vegetable crops were planted into freshly tilled soil on 5/20/15 and cultivated on 6/2/15.  The living mulches were seeded on 6/9/15 after a second cultivation of all the plots.  The seeding rate for each clover was 15 lbs/ac.  Seed was broadcast through the treatment plots and worked in with a third cultivation of all plots on the same day (6/9/15).  Fertility and pest management were managed as per grower standards.  Living aisles were mowed at the end of July to control weeds.  Brussels sprouts were topped on 8/25/15.  Salable yield was measured in weight for Brussels sprouts and in both weight and fruit number for acorn squash.  Treatment response data was analyzed using an analysis of variance (ANOVA).  Treatment means were separated using Fisher’s Protected LSD at the .05 significance level.  A field day was held at High Meadow Farm on 9/15/15.  See attached handout from the field day.

2015 Living Mulch Field Day Handout

Accomplishments/Milestones

Yield

Acorn squash and Brussel sprout yield in the control plots was significantly higher than in the plots with the living mulches.  However,  the Brussels sprouts yield results were only analyzed from two of the four farms due to cut worm damage on the other two farms.  Cut worms feed in pockets so some plots were more impacted than others, confounding the statistical analysis.  The affected plants lived, but in many cases did not produce harvestable Brussels sprouts. 

Anecdotally, we observed changes that might have improved yield in the living aisle plots, including:

• Leaving a clean strip for the cash crop where clovers were not seeded/weeded out in order to reduce competition for resources.
• Delaying clover seeding by an additional week or two, and adding a fourth cultivation, as a way to both reduce weeds in the cover crop and to reduce competition with the cash crop during the minimum weed free period. In this case, it would be important not to delay clover seeding so long that it is completely overwhelmed by the cash crop.
• Adding more fertility in cash crop planting strip so that there are more resources and less competition between the cash and cover crop.

Weeds

There was no difference in weed density or biomass between the living aisle plots and clean cultivated control plots, suggesting that the cover crops did help to control weeds.  Because there was high weed pressure at some of the farms, we did decide to mow the cover crops once in July.  By that time many of the weeds were well established and the mowing did not kill most of them.  It did, however, cut them back and reduce their ability to set seed.  In the future, an earlier mowing would be more beneficial and could kill more of the weeds.

Impacts and Contributions/Outcomes

Desired Impacts

• Increased grower awareness about the benefits and challenges of living mulches
• Increased grower knowledge about how to incorporate living mulches in vegetable systems
• Increased grower experimentation with and adaptation of living mulches on their farms

Impacts to Date

Twenty growers attended the “living clover mulch under vegetables” field day on 9/15/15, toured the trial plots, and discussed the results to date (as provided in a handout and presented by the ag professionals).  Ten growers submitted evaluations of the event.  All respondents indicated that their knowledge about cover crops as living aisles increased “somewhat” or “a great deal” after the event.  Six growers indicated that that cover crop practices will change as a result of what they learned.  These comments are examples of what growers said when asked “what are you planning to incorporate on your own farm as a result of this workshop:”

• “new thought process in using cover crops”
• “trial living mulch for permanent bed system”
• “try some living aisles next year”
• “underseeding clover into brassica’s after final cultivation, not seeding clover first and planting into it”
• “I might put more into off-season cover crops and maybe leave more fallow. I would like more examples of living aisles and their benefit before making a decision on that.”

Next Steps

The trial will continue in 2016.  The vegetable crops will switch positons in the established clover beds.  There will be a field day and presentation of results in the fall.  After all data analysis, we will produce an illustrated information sheet with recommendations for using clovers as living aisles under organic vegetables.

Collaborators: