Green Manures (Cover Crops) 

 

How to improve the fertility of your soil is a question that all good gardeners take seriously.  Or at least should. One of the most under-used methods of soil improvement is the use of green manures (often called ‘cover crops’ in America), which are essentially plants grown specifically to be dug back into the soil to improve it.  In principle this sounds pretty easy – just sprinkle some seed on the ground after the main crop has been harvested and then dig the plants in after a few weeks.  But in practice there’s a lot more to it, so I thought I would do a little experimenting to find the perfect green manure.

Cover crops, or green manures, are a great way to protect ground that would otherwise lie bare over winter. Dig them in and they’ll help to build up your soil’s organic matter – which is great news for the vegetables that follow! The end of summer is the perfect time to sow a cover crop for winter. 

Why Grow Cover Crops?

Cover crops are plants grown to protect or improve the ground for future crops. Keeping soil covered over winter protects it from erosion and helps support all the beneficial life associated with it. It also gives weeds less opportunity to establish, meaning cleaner beds for sowing or planting in spring. Dig the cover crop into the ground at the end of winter and it will rot down to add valuable organic matter, helping to feed the plants that follow.

Green manures work by drawing goodness out of the soil and storing it in the plant’s cells and root nodules.  When the plants are then dug back into the soil they rot down and gradually release these nutrients to the next crop in a more readily-available form.  Regular use of green manures improves the soil structure, breaking down hard soils and adding organic matter to light soils like mine.  Green manures can have other benefits as well.  Many of them provide good soil cover, suppressing weed growth and preventing erosion.  Others attract beneficial insects to the garden such as bees and hoverflies which prey on pests like aphids.

Which Cover Crops to Grow

Heavy Soil

Cover crops with deep or fibrous roots such as cereal rye help to improve soil structure by breaking it up. Others, like mustard, grow very fast to produce lots of lush foliage that can be incorporated into the soil after just a few months to boost its organic content. Mustard is a particularly good cover crop for clay soil, where it can be dug in before winter so frosts have a chance to break the soil up. Prolific salads such as mache or corn salad may also be grown this way.

Poor Soil / Hungry Crops

Some cover crops directly add nutrients to the soil by fixing nitrogen at their roots. Examples include winter field beans and peas, clover and vetch. These are all types of legume and are a great choice for sowing before nitrogen-hungry brassicas such as cabbage.

So how do you choose a green manure to sow?  The following types are readily available:

• Legumes, such as winter field beans (like fava beans), lupins and fenugreek which fix nitrogen into the roots (as long as they are dug in before flowering when the nitrogen is lost). Other peas and beans, such as sweet peas, can also be used. I have used winter field beans very successfully when planting a late green manure since they will even grow when temperatures are starting to take a dive during mid-autumn.
• Clovers, red or crimson clover being the best as it dies down, also in the legume family.
• Winter tares, also known as vetches, are also winter-hardy but like rye they can be difficult to dig in.  Again, part of the legume family so they fix nitrogen into the soil.
• Rye, such as Hungarian grazing rye, will grow well at low temperatures but can be difficult to dig in and get rid of.
• Mustards, can be very effective but, as they are part of the brassica family, they can interfere with your crop rotation.
• Buckwheat and Phacelia are both excellent at attracting beneficial insects and are easily dug in.
• Winter-hardy salad crops, such as corn salad and miner’s salad (Claytonia) are easily dug in once used and can provide some extra salad leaves while growing.
• Others which are not normally regarded as green manures can also do a great job.  Poached-egg plant (Limnanthes douglassii) is a great example – bright flowers, grows well over winter and digs in easily.  I regularly plant this in my garden and leave a few to flower to attract hoverflies

Whilst this looks like a wide variety of options, there are some important factors to consider.  Firstly, many green manures are great for farmers with machinery to dig in the plants but are not half as easy for gardeners who have to do it by hand.  Well-known author Bob Flowerdew recommends that you avoid ryes, tares and vetches, fodder radish, and many clovers for exactly this reason.  Secondly, not all green manures grow well on all soils.  Tares don’t do well on dry or acid soils, clovers prefer light soils and beans prefer heavier ground.

How to Sow a Cover Crop

To sow a cover crop, start by roughly digging the ground over. Remove all weeds, especially perennial ones. Tamp down the soil with the back of a rake then scatter, or broadcast, your seeds evenly across the soil surface. Don’t sow them too thickly. Rake the seeds into the soil, tamp down with the back of your rake, then water the ground if it’s dry.

The chunky seeds of winter field beans may also be sown in rows. Use a spade or hoe to dig out trenches about two inches (5cm) deep. Space the trenches eight inches (20cm) apart. Now sow them so they’re around four inches (10cm) apart then fill in the trenches to cover them.

In most cases it’s best to dig your cover crop into the soil before it begins to flower, perhaps leaving a few for early beneficial insects. At this stage the stems will still be soft, making them easier to cut up and dig in, and quicker to rot down. Incorporate the foliage into the soil or simply cut it off and leave it on the surface as a mulch for the worms to dig in for you, covering it over with cardboard if you’re concerned about weeds springing up. Cover crops should be dug in at least one month before sowing or planting so they have enough time to begin decomposing.

This year I set apart an area where I could grow three of the best as a trial: fenugreek, phacelia and buckwheat. For me, both the buckwheat and the fenugreek struggled to provide much ground cover and were relatively poor at germinating.  Phacelia was the complete opposite.  It required little weeding, quickly producing lush growth up to about 16 inches (40cm) high and even attracted a range of bees and insects if left to flower.  I shall be sowing more of it before the end of the season and plan to incorporate it in several places across my garden next year, particularly because it’s from the waterleaf family of plants and doesn’t interfere with crop rotation.

So if I was asked to name my top three green manures they would be phacelia, poached-egg plant and winter field beans.  I’m still on the lookout for other good green manures though, so please do share your experiences below.

 

 

Author: RNW