Student Research: Broccoli - Living Mulch

Agroecologists are interested in food production systems that resemble natural systems in important ways. For example, natural systems are often species diverse – forests, meadows, lakes, and oceans can have many different species living within one community. Generally, our agricultural practices don’t look like these communities, favoring monocultures instead. Growing our food in diversified fields can be challenging, especially given that conventional monocropping has led to high yields, low weed pressure, and relatively easy harvesting.

Broccoli transplants in plastic-covered raised beds at Woodman Farm, University of New Hampshire. 

Despite the challenges, there are many benefits to diversification that are important for long term health and stability of our agricultural land. Rotating crops diversifies species over time and has been recognized as an important method of keeping soils productive and disease pressure down. Diversity can also occur in space, such as with intercropping (planting more than one species together at the same time). However, this method in particular has the potential to lower cash crop yields if the species used compete with each other for resources.

Our lab “diversified” a broccoli field by intercropping the broccoli with a cover crop. The cover crop in this case is called a living mulch, because it is grown between the broccoli rows and one of its functions is to keep weeds down, similar to other types of mulches. Past research has shown there is a tendency for living mulches to have a variable effect on yields, and understanding why, or even how to prevent yield loss, is an important step in making this a viable option for growers.

To see if we could optimize this diversified system, that is, to create a scenario in which competition does not reduce yields, we grew broccoli with or without a living mulch, and with different amounts of fertilizer. We expected that some amount of fertility would cancel out any competitive effects from the living mulch, and then we could decide if that much fertilizer was an acceptable investment.

Establishing the experiment: a living mulch (perennial ryegrass and white clover) was planted in between half of the raised beds, while  plots in the other half of the field were kept weed free. For each type of field management (living mulch and bare soil), sections of the bed row were given different fertilizer rates to see if this resource changed broccoli yields in the presence or absence of the living mulch. 

The results, in a nutshell, were mixed. In the first year we did the experiment, the broccoli yields were not different between living mulch plots and bare soil plots at the recommended fertilizer rates. In the second year, the yields in the living mulch plots were lower for all fertilizer rates. Why the difference?

When the broccoli was mature, the heads were weighed from living mulch or bare soil sections (seen above). Living mulch can provide many ecological services, such as nitrogen fixation, soil stabilization and erosion prevention, or increased soil organic matter. However, they can also compete with the cash crop for available nutrients during the growing season. 

Well, we don’t know for sure, but we have some ideas. We don’t think that competition for water is the problem because the broccoli was irrigated. We also don’t think that the living mulch competed with broccoli for light because it was occasionally mowed. It’s possible that despite high levels of fertilization, the living mulch could still compete with broccoli for nutrients. There are environmental circumstances where the roots of the living mulch can grow aggressively to find nutrients, and this was certainly a possibility, especially in the second year when yields were lower.

Interestingly, some plants react to the presence of neighbors by detecting and responding to the ratio of red to far-red light. A nearby plant (or living mulch), reflects a different light signature than say, the bare ground does. We may be observing the broccoli’s response to this light signal, which is ultimately expressed through lower yields.

The take-away is that efforts to diversify agricultural systems remain important, and we should continue to work on methods that balance short and long-term productivity. Living mulches have a lot of potential, and under some circumstances they have been successfully used without negative yield consequences. Understanding why and when they compete with crops will be important information in order to make them a more useful tool to growers.

The results of this study will be appearing soon in HortScience (in press).