Picking a Cover Crop for the Raised Beds

Over the years, I’ve grown all kinds of crops in my raised beds. The glass gem corn (pictured above) is my most recent experiment. To me, everything in the garden is an experiment because nothing in gardening is a sure thing (unless we’re talking about mint). Oftentimes, I’ve taken the soil for granted with the yearly expectation that the soil will continue to churn out the most delicious produce my family and me wait all year to relish. So this fall, for the very first time in my 20-plus years of gardening, the veg plot is getting a cover crop.

Perhaps the relative success in the veg garden can be attributed to the modest amount of compost that I add every year from my small compost bin, as well as the organic fertilizers the plants receive both at planting time and throughout the growing season. These plants have to live up to some pretty high expectations so regular feeds are necessary. A healthy layer of mulched leaves every autumn is another nutrient source.

Once I pull the last of the tomatoes and tomatillos this weekend, veg garden 2021 will be officially over. The cover crop is for veg garden 2022. Funny how gardeners are always looking ahead. By planting a cover crop or green manure, I’ll be feeding the soil at a time when it doesn’t have to put its energy into growing food for me.

What does a cover crop do?

A few things actually. It protects the surface of the soil, particularly in areas where erosion is a problem, and shades it so that weed seeds never have a chance to germinate. Improved soil texture, increased aeration and sustenance for the millions of microorganisms that call my soil home are the main reasons for choosing to use a cover crop this fall. All of which translate to a nutrient rich soil for next year’s garden. The biggest decision was which cover crop to choose. There are many so I turned to horticulturist April Shelhon at Botanical Interests. I had an idea what I wanted before we spoke, just needed confirmation from a pro that this was the cover crop best suited to my gardening style.

She suggested first considering my goals for the soil, as each cover crop does different things. The soil in the raised beds, despite being a garden mix, is very heavy and not much different from the soil in my garden beds. I’ve never had the soil tested but they have a similar look, feel and permeability. Friable soil, or something that looks more like coffee grounds than boulders, is my main goal.

I’m a least amount of effort for maximum benefit kind of gardener so I chose the Soil Builder Peas and Oats cover crop. Oats build biomass (which is a fancy way of saying “organic material”) and break up heavy soil with their extensive root system while peas fix nitrogen. Plants can’t absorb nitrogen from the air and rely on other plants like peas that harbor bacteria in their roots that convert atmospheric nitrogen into a form plants can utilize. Nitrogen in the soil is in constant flux and difficult to quantify. It’s readily consumed by crops throughout the growing season so, by planting peas and oats, I’m replenishing nitrogen AND creating a better growing environment for next year’s garden.

I also don’t want to risk destroying soil structure so I need a crop that will be killed by cold temperatures (I’m in zone 5b) and allowed to decompose into the soil. No tilling needed. For me, a pea and oat cover crop is the way to go. It suits my lazy inclinations.

The right cover crop depends on your growing zone, soil goals, the amount of effort you want to invest and timing. Peas and oats can be sowed in late summer/early fall, giving them enough time to flower before being struck down by freezing temperatures, unless we have a mild fall and it looks like the plants could potentially set seed. In which case, I’ll get in there and cut them back before they do. While in the flowering stage, all the sugars are concentrated in the flower and when cut back, those nutrients are available to the soil. If allowed to go to seed, the sugars turn to starch and the benefits are no longer available to the soil.

Botanical Interests has a helpful chart highlighting each cover crop, the best time to sow it and how it benefits the soil. They’ve also simplified things by including information on the amount of space each seed packet covers. One envelope of Soil Builder Peas and Oats covers approximately 80 square feet. I have two 8’x4′ raised beds, so one packet should be plenty. I’m an over planter and bought two, just in case.

April mentioned that oats are an allelopathic crop and excrete a chemical into the soil that prevents others seeds from germinating. I’m glad she mentioned this, especially if your interested in sowing a cover crop in late winter/early spring. She suggests allowing the plants to flower then turning them into the soil or cutting them back. Give them 4-6 weeks to decompose before sowing your first spring crops.

Soil is everything to a garden. In many ways, it reminds me of a friend that gives you a Christmas present, when you hadn’t planned on exchanging gifts, and expects nothing in return. Soil is THAT friend. Only this time, I have something to give and that feels pretty good.

Have you used a cover crop in your garden?