Grow Hack: Benefits of Using Cover Crops for Green Manure

Photo by Nico Escondido

Outdoor cannabis gardens lie at the mercy of the elements, and risk contaminating their surrounding environment with fertilizers and herbicides. Before you use chemicals on the farm, make sure to take advantage of nature itself for cheap and naturally farmed outdoor cannabis.

Planting a cover crop might be one of the best, and at the same time the most environmentally friendly way to conserve and renew your outdoor soil during the off-season. Cover crops of grasses, grains, legumes, clover, etc. can be mixed and matched to provide any number of benefits to soil fertility, beneficial microbes and fungi, soil texture, water retention, erosion, you name it! In addition to helping the soil, sustained areas of cover crops attract beneficial insects that return season after season to ward away bad bugs, mites and spiders. Instead of buying manure or mulch, cover crops may simply be plowed over to create what’s called “green manure.”

The hardy and versatile cannabis plant grows in a diverse array of climates worldwide, but even North American-grown buds can blossom anywhere from arid badlands, to cold Canadian Rockies, to the bayous down South. The climate and necessities of grower will dictate which cover crop you should plant, so make sure to do your own research before investing in any seeds.

Your harvest time will also determine what cover crop you should plant in the fall. For example, ryegrass needs to be planted early enough in the fall so it doesn’t die in the winter. Indica strains ripen early enough to allow you to plant some ryegrass, winter rye, hairy vetch or crimson clover. On the other hand, pure sativas flower well into late October or even November, which may force you to plant a clover or grass early the following spring before bringing your pot plants out from under the lights indoors.

To aid with soil compaction and to provide soil organic matter, plant annual ryegrass in the early fall. Ryegrass roots stretch deep into the soil and create a thick matrix that, after decomposing, turns your soil bed into a thick sponge that retains water and nutrients. For wet climates, heavy and sustained rainfall over the winter (such as in the Pacific Northwest) puts soils at risk of becoming depleted of nitrogen, which is highly water-soluble. Ryegrass absorbs immense amount of nitrogen into its tissue as it grows, acting as a sort of nitrogen layaway for your cannabis crop next season. Plowing it all over in the spring returns all that nitrogen to the ground after nitrifying bacteria get their hands on it.

Brassicas, such as rapeseed (Brassica napus), mustards, turnips or forage radishes also root deep with long taproots that reduce compaction and increase water retention, but they also provide another benefit. Brassicas produce pungent and mildly toxic compounds called glucosinolates that suppress weed growth, soil borne pathogens and nematodes. Plant brassicas in the fall, 4 to 10 weeks before the first frost.

Clovers, which grow fast and low to the ground, fixate large amounts of nitrogen. Research from Ohio State University found that crimson clover produced 78 pounds of nitrogen per acre, while sweet clover produced 115 pounds.

You could plow through the whole field of clover before bringing the cannabis plants out in the spring, or let it keep growing around the plants for a permanent ground cover. Permanent ground covers of native grasses in the space between cannabis plants aid greatly in water retention, maintain microbial populations and attract beneficial insects. You can obtain increased benefits from cover crops using “no till” farming techniques, but these often require you to kill the cover crop with an herbicide, a big no-no for organic cannabis cultivators.

Don’t miss our previous Grow Hack: Control and Monitor Soil Water Content

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