Undersowing crops and green manure are not the easiest topics to understand in market farming. In fact, it takes a lot of experience to be good and efficient at either of these tasks.

That is why, for the sake of this article, I am going to be relying heavily upon the trusted advice of Mr. Eliot Coleman, of The New Organic Grower, who has more experience with oversowing and green manures than anyone else in the field. 

Undersowing is the practice of a growing green manure along with the market crop. It is also called overseeding or companion seeding.

The practice of undersowing is like planting desirable weeds between the crop rows. The effect of the undersown plant (deliberate weed) on the crop plant depends on the age of the crop.

The advantage of undersowing is that the green manure crop is already established at harvest time.

How Do You Incorporate Green Manure?

Weeds can overwhelm your crops if they both start at the same time. Most crops do fine if they are kept weed-free for the first 4-5 weeks after establishment.

Later competition from low growing weeds will have little effect on them.

The best crops for undersowing are low growing, and the best date for green manures are four to five weeks after the establishment of the crop plants, whether direct sown or transplanted.

Timing of Undersowing Crops

It is important not to wait too long when undersowing your crops as then the balance is tipped too far in the other direction. 

Since the undersown ‘weed’ is deliberate, you need to make sure it grows. If you wait too long before undersowing, the crop plant will be large enough to overwhelm the green manure.

The trick is to undersow when the crop plants are well enough along not to be adversely affected by the undersowing, but not so well established as to hinder the growth of the undersown manure.

For example, Eliot Coleman lives in a cool climate where corn, beans, squash, and late brassicas are not planted or set out until June 1.

He finds that the Fourth of July is just about a perfect time year in and year out, to undersow those crops.

Once you acquire the experience you will also be able to judge the timing by the size of the crop plants.

Before Undersowing

Successful undersowing requires a clean, weed-free seedbed. Sowing the green manure is no different from sowing the crop. When seeds are planted into a weedy mess they become the seeds for failure.

A clean seedbed prepared for undersowing is a by-product of early weed control. At least three cultivations should be made prior to undersowing, the last one just a day or two beforehand. 

The goal of the market farmer is to provide every opportunity for the undersowing to get well established without weed competition. The canopy of the undersowing will join with the crop canopy to keep later weeds from germinating. 

How to Seed the Undersown Crop

Eliot Coleman recommends drilling as the preferred method for undersowing crops.

If the undersown crop is drilled between the crop row with a single or multi-row drill, the seeds are planted at the proper depth, in contact with moist soil, where they are much more likely to germinate. 

Here are some examples (I hope you like my accurate drawings!): 

Undersowing with 12-inch spacing

For crops such as carrots, onions, and lettuce planted in 12-inch rows, there are four rows side-by-side with a 12in spacing (see picture above). 

The one-row seeder can be used to drill three rows in the path. A single undersown row can be drilled between each crop row. In this case, Eliot sows dwarf white clover in the paths.

White clover can be used between rows of carrots or onions. The row space at 18 in are similarly undersown with three rows in the path and one row between the crops. 

Undersowing with 30-inch spacing

For 30 inch spacing at which corn, beans, brassicas, and others are planted, the 5-row drill is used.

One pass is made down the centre between each crop row. Depending on the vegetable crop, dwarf white clover, sweet clover, red clover, hairy vetch, and soybeans all work well as green manure. 

Undersowing with 60-inch spacing

In the crops spaced at 60 inches (tomatoes and melons, for example), two passes are made with the five-row drill.

Dwarf white, red, and sweet clover are all good choices here.

Undersowing with 120-inch spacing

At the widest spacing, 120 inches for pumpkins and winter squash, everything except a strip about two feet wide on either side of the row is drilled.

Eliot Coleman’s favourite undersown crop for squash and pumpkin is biennial sweet clover.

What Is The Best Green Manure Crop?

Here are Eliot Coleman’s choices of green manure crops for different uses:

• Tall crops: sweet. Clover, vetch, red clover, or alsike clover
• Sodlike clover: dwarf white clover
• Resistance to foot traffic in picking: dwarf white clover or vetch
• Before potatoes: soybeans or sweet clover
• Under corn: soybeans, sweet clover, or red clover
• Between rows of root crops: sweet clover or dwarf white clover
• Soil protection that will winter-kill: spring oats, spring barley, or in warmer climates a winter legume that will complete its growth in spring and can then be mowed off
• For the latest fall planting in cold climates: rye or winter wheat

Eliot Coleman’s Green Manures In Rotation

The following is the sequence Eliot Coleman uses for his undersowing rotations.

Potatoes cannot be undersown easily if the cultivation method is hilling.

The Cabbage Family is undersown to sweet clover, which is one of the best leguminous green manure to turn under for next year’s corn crop.

It grows well under the Cabbage family because it is a tap-rooted crop that does not seem to interfere with the more shallowly rooted brassicas.

Sweet corn is undersown to soybeans because research shows a soybean crop almost totally inhibits potato scab organisms in the soil.

The soybeans also grow well in the understory of the corn and provide excellent weed suppression. 

Peas are not undersown but are followed by a mix of clovers as soon as space can be cleared.

This combination of legumes grows until it is turned under the following spring, by which time, enough nitrogen has been fixed to ensure a terrific crop of brassicas.

Tomatoes are undersown to oats or some other non-winter-hardy grass crop. Certain grasses have been found to be excellent preceding crops for legumes such as peas since they produce an allopathic effect that suppresses grasses and other weeds, but not legumes.

It is important to choose a non-winter-hardy cultivar so there will not be a massive fresh green growth in the spring to impede early soil preparation and planting of the pea crop. 

Beans are undersown for winter veg. It is a dependable preceding green manure crop for tomatoes.

Root crops are undersown to dwarf white clover because it will grow in the crop understory and because it provides good erosion protection for the soil over winter.

Squash is undersown to sweet clover in the empty strips between the squash rows. Beets, carrots, and other root crops grow very well following sweet clover.

The onion crop, on the other hand, grows best with no preceding green manure, so plant onions in the strips that were occupied by the squash plant themselves.

Bottom line

Sowing dates and equipment for undersown green manures should be as well-thought-out as those for the cash crops.

Sowing dates should be marked on the calendar. The seeds should be ordered ahead. The equipment should be quick, simple to use, and in good working order. 

Green manure is most effective when it is considered as an important component part of the crop rotation planning. 

Variety in green manures is as important as variety in the market crops. Because green manure plants also have different benefits and faults that affect the soil and following crops in different ways, green manure should be rotated to include as many different varieties as possible. 

Remember, the ground is never bare. The soil is always growing either a market crop or next year’s fertility (green manure). For much of the summer, it is growing both!

Over time and with practice, you will develop an undersowing plan that is right for your market garden and climate.

This is simply a guideline based on the best practices of Eliot Coleman. There are no hard and fast rules, the right crops are truly dependent on the veg you grow and the climate you live in.

Experimenting with many different undersowing crops will allow you to see which ones are appropriate for your particular situation.

You can bet we will be documenting our experience with undersowing as soon as we have any! (nervous laugh…)

Do you undersow your market garden and why? Let us know in the comments below!

Stay Local,

Kathy & Jon

your friendly neighbourhood growers