Cover crops are used in an agricultural production system to improve soil quality and fertility, reduce soil erosion and to reduce insect, weed and disease pests’ presence. It can also be used to aggregate soil particles, reduce soil crusting, breaking a hard pan and to capture and store available soil nutrients for a subsequent crop. It directly contributes to an ecologically sound and sustainable management plan for growing crops economically and efficiently. It does this by being a protective cover over the soil, increases soil organic matter and carbon (green manure), releases organic chemical compounds into the soil, and breaks pest and disease cycles.
Cover cropping is often associated with organic and sustainable farming systems, but it can be customized to meets individual goals of all types of agricultural production systems. In general, selected plants are planted into a field at the prescribed density and allowed to grow for a specific time period after which the plants are incorporated into the soil. Selection of cover crop and timing of incorporation are critical to its successful utilization.
To get started in developing a garden plan to incorporate cover crops into your garden, start by determining the primary reasons for using a cover crop. Once the objectives are determined, selection of cover crops can begin. Cover crop can be used individually or in combinations board on specific needs. Using combinations of cover crops can lead to improved benefits over using a single crop plant.
Although we have little seasonal changes in Hawaii, time of year will play a role in cover crop selection as some grow better in warmer weather and some in cooler. How long you maintain the growing of a cover crop is determine by objectives and type of plants used. Cover crops that present problems with reseeding and becoming a weed problem in your garden should be cut and incorporated shortly after first flower bloom.
Many guides on cover cropping indicate that planting should be done during the period after harvesting the primary cash crop and before winter sets in. In Hawaii, we enjoy year-round cropping and cover cropping can be scheduled based on type of cover crops that needs to be grown to achieve the desired results or scheduled around when primary crops need to be grown.
While some may view planting a cover crop as a waste of resources in a limited growing space, the benefits of cover crops will accrue whether used in a container garden or a large field plot. When cover cropping is done with good planning, it will pay dividends to crops down the road no matter what size it is practiced on. The use of cover cropping has shown to reduce the amount of supplemental nutrient application required, especially nitrogen following legume types of cover crops. By reducing the presence of weeds, insects and diseases, you reduce the amount of inputs needed to control these pests.
When selecting a cover crop, you need it to match your objectives.
If you would like to suppress weed growth as you fallow you garden plot, you would plant crops that grow quickly to out-compete the weeds. Plant crops like buckwheat, oats and cereal rye. These plants germinate and grow quickly to out-compete weed seedlings, shading them out. Several plants are reported to suppress nematode populations, especially root knot nematodes which are common in many Hawaiian garden soils.
Sorghum x Sudangrass hybrid, sunnhemp, marigolds and brassicas (mustard, arugula and radish) are cover crops widely used to suppress nematodes through various modes of action. Some of these are known as trap crops, which attract the nematodes to feed on their roots, but does not allow them to reproduce, thereby reducing nematode numbers. Mustard, arugula and other brassicas are known to have biocidal activities when tilled into the soil. As the plant material decomposes, it releases glucosinolates compounds that mimic the actions of synthetic soil fumigants. Collectively, this mode of action mechanism is known as biofumigation, and works well in suppressing nematode populations.
Deep-rooted cover crops can help in breaking or puncturing a hard pan and can also transport mineral nutrients from deep within the soil profile for use by shallower rooted crops. As the plant decomposes, it leaves openings through the hard pan from which water can flow through. Cover crops like sweet clover, daikon and radishes are great for this purpose.
While the use of cover crops will improve crop growth when properly done, growing the wrong cover crop can increase problems associated with insect and disease buildup problems. Rules for crop rotation about not planting members of the same family as consecutive crops do apply. Many of our cover crops are nitrogen-fixing legumes and therefore we should not plant beans or peas preceding or following these crops.
Also, there are certain cover crops that will allow the buildup of certain pests, such as buckwheat that will increase the root knot nematode population if it is present.
For more information on this and other gardening topics, visit the University of Hawaii College of Tropical Agriculture and Human Resources website at http://www.ctahr.hawaii.edu/Site/Info.aspx or a Cooperative Extension Service office. Nagata can be reached at russelln@hawaii.edu.