Cool-season cover crop mixtures are well researched and are cropped around the world, but less emphasis has been placed on warm-season cover crop mixtures. Former ECHO Farm Manager and now Organic Grower, Danny Blank, informed me that he observed the benefits of planting lablab (Lablab purpureus) with velvet bean (Mucuna pruriens), two leguminous vines, at the same time in Florida’s hot and humid summer. Benefits from this mixture include:
- Large amounts of biomass production that could be chopped down in-place for the next season’s mulch (growing mulch in place)
- Increased biological nitrogen fixation
- Weed control over the summer from the very dense vining mat made by the mixture
To test Danny’s suggestions of biomass benefits of the mixture, we planted a small area with both monocultures of lablab and velvet bean as well as mixtures at two different seeding ratios: 75/25 of lablab/velvet bean and 50/50.
All seeds were jab-planted into planting stations with ~50 cm between-row and ~20 cm in-row spacing. Soil in the area is sandy. Before flowering, square meter biomass samples were taken of each treatment with a quadrat made from polyvinyl pipe. Fresh aboveground biomass was recorded for each sample. Subsamples of each species were dried down for dry-weight conversion. The figure below displays the fresh biomass (left) and dry biomass (right) for each cover crop treatment in tons/hectare.
Land equivalence ratios were calculated for the two mixtures based on yields of aboveground biomass (Table 1). Land equivalence ratio relates the yield of a mixture to the yield of the related monocultures grown under the same conditions. Mathematically, it is the sum of two fractions: each fraction of the intercrop yield divided by the monocrop yield. In the below example Y is yield, LL is lablab and VB is velvet bean:
Given these results, there seems to be a biomass benefit of adding velvet bean to a lablab planting for the context in which these crops were grown. Further research and replication would be needed to further support this observation. Testing these mixtures in different climates and soil types is also important.
The LER fraction of lablab for both mixtures was much greater than 0.5, meaning that lablab gave the yield benefit in both mixtures. When grown by itself, velvet bean outperformed all other treatments. These results relate to the growth habit and the competition between these two species.
Lablab has a very slow initial growth rate, often taking up to a month before vines start to take off. Velvet bean, on the other hand has a very fast growth rate, taking off rapidly after germination (which can be relatively slow). Lablab in monoculture, may experience greater competition with weeds which may take up canopy space and resources. On the other hand, when mixed with velvet bean, the velvet bean takes over early on in the season smothering weeds and lablab later emerges overtop of the mucuna canopy (which is not very high), and covers the mucuna. This is what was observed in this small trial. Early in the season, the mucuna predominates, but the lablab becomes more competitive in the later part of the season.
Results suggest that mixtures could be a tool to extend the time over which GMCCs cover the ground, thereby providing weed suppression. There are many different warm-season annual cover crops that could be grown in different combinations or seeding ratios.
Do you have ideas for other warm-season cover crop mixtures? Have you tried any mixtures in your area?