Garden Green Manures

By Steve Reiners, Professor and Chair, Horticulture Section, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic that ran Spring-Summer 2020. Read more articles from 2020 | Read later articles in this series.

July 3, 2020

You may have heard the saying, “nature abhors a vacuum.”  Although it applies to many situations, farmers and gardeners have a unique understanding.  They know that leaving a plot of land open encourages weeds to invade, bringing all the unpleasant things associated with them.

I’m not saying that weeds don’t have a place and at times a certain usefulness.  But how about planting a crop that can add nutrients?  Smother weeds ?  Increase organic matter?  Encourage beneficial insects? And discourage diseases?  I’m talking about adding a cover crop or green manure.

Farmers have used this technique for centuries, but it’s seldom seen in gardens.  Green manures are crops that are grown not to be eaten by you, but to be turned into the soil and consumed by earthworms, insects and microbes.  They add soil organic matter that feeds your plants and makes crops less susceptible to drought.

Do you have an empty bed in your garden this summer?  Maybe where you had early peas or greens and plan to plant spinach in the fall.  That might give you an open bed for four to five weeks.  More than enough time to plant a green manure crop instead of filling up with weeds. 

Two that would fit perfectly in that window are buckwheat and mustard.


Buckwheat grows very quickly and can be turned under or mowed 35 days after planting.  Its wide, broad leaves help smother weeds and its fibrous root system is efficient at making phosphorus and calcium more available to crops that follow.  Its attractive white flowers attract honeybees as well as beneficial insects like lady beetles and parasitic wasps that help control insect pests.

Prepare the seedbed as you would for any crop.  Uniformly sprinkle about 1/2 to 1 cup of buckwheat seed across a 100 ft2 bed and lightly rake in – just like you would if you were planting grass seed.    Keep the bed moist until the buckwheat sprouts and then leave it alone.

Knock the buckwheat down after flowering but before it sets seed.  Once incorporated, it is quick to break down.  It could be followed with a late planting of spinach, lettuce, radish or garlic, or even another cover crop for fall and winter.


Another crop that will work is mustard.  It’s probably better in the spring or fall as it doesn’t like the heat of summer.  But it will still work.  One of the great things about mustard is that if you chop it up and incorporate while still green, it can suppress some soil-borne diseases.  It’s actually a natural fumigant.

Prepare the seedbed like you would with buckwheat but only add 1 to 2 tablespoons of seed per 100 ft2 bed.  And again, rake into the soil.  Lightly pat down the soil to encourage good seed-to-soil contact.

It will likely flower in 5 to 6 weeks and once it does, that’s the time to kill and incorporate.  Feel free to take a few leaves and add to your salad or stir fry as it will give you some spicy flavors.

My colleague in Horticulture, Thomas Björkman, has developed a wonderful website ( on cover crops.  Although the site is designed for commercial vegetable growers, it can be a useful tool for serious home gardeners.  There is a recommended seeding rate for each crop, given in pounds per acre.  For gardens, take these rates and multiply by 0.037 to give you the ounces needed per 100 ft2.

Mustard and buckwheat are just two potential cover crops you can use.  In the fall, crops like oats, rye and vetch make good additions and early spring is good for adding clovers.  If you have two or more months open in the summer, think about adding Sunn Hemp or Sudangrass.  Look online for many of these seeds as they may not be available where you normally get your garden supplies.

This week in the garden

Summer squash is starting to flower, but don’t panic if you don’t see fruit right away.  The first flowers are typically male, sources of pollen for foraging bees.  The bees move the pollen over to female flowers which then results in fruit set.  We sometimes see only male flowers for a week or so before the females open.  It’s easy to tell the difference as female flowers have shorter stems and have small, immature fruit at their base.

Keep weeding!  10 minutes weekly when they are small is likely all you need to keep them under control.  Letting them grow makes the job much tougher.

Recent rains have helped but keep the garden watered.  Remember, we need at least an inch of rain or irrigation every week.