Using Green Manures

Using Green Manures: What I’m Learning in My Own Garden

When I was filling my second raised bed, I realised I had made a bit of a mistake with the first one. I had forgotten to add my home compost from the hot bin as the third layer, on top of the composted bark. That would have given the bed a lovely rich layer of organic matter.

Rather than worrying about it, I decided to work with what I had.

I have sown the second bed with some of my quick-growing August autumn and winter crops. In short rows I have sown wild rocket, pak choi, kailaan, mizuna, mustard and sorrel. I've also tucked some baby turnips into one corner and dwarf green curled kale into the other. The uprights at the corners of the bed are useful too. I can suspend netting over them to protect the young plants, or use them to provide some shade if we get another spell of hot weather.

 

What about the other raised bed?

Because I hadn't added the same organic matter to the other raised bed, I decided to try something different rather than leave the soil bare. I've sown phacelia as a green manure. I'm hoping the phacelia will establish quickly, cover the bare soil and give me plenty of biomass. My plan is to cut it down after around 6–8 weeks and leave the cut material on the surface as a mulch. Later in the autumn, I’m hoping to sow some dwarf broad beans through the bed to overwinter, with the aim of getting an early crop next spring.

Phacelia is a good all-rounder for this sort of job. It grows quickly, provides good ground cover and, if allowed to flower, is particularly attractive to pollinators. It isn't a legume, though, so unlike clover, vetch or field beans it doesn't fix atmospheric nitrogen.

 

I've left my strawberries at the front edge along the base of the beds. I don't want to leave any remaining soil around the beds completely bare until I sow my herb seeds in spring, so a green manure would give me a productive way of making use of that space in the meantime. Different green manures suit different soils, seasons and purposes, so choosing the right plant is important. The obvious choice for my heavy clay soil would be grazing rye, its roots delve down into the soil and overwinter. However I'm trying to avoid digging my soil, so I'm going to sow mustard instead. Another quick grower that is killed by frost. No digging in required.

 

 

What exactly is a green manure?

A green manure is simply a plant that you grow primarily to benefit the soil rather than to harvest as a crop.

Instead of leaving a vegetable bed empty after harvesting, you grow something that covers the soil and can later be cut down and either incorporated into the soil or, as I'm planning to do with my phacelia, left on the surface as a mulch. There are several reasons I think this is particularly useful in a garden like mine.

Green manures can:

• protect bare soil from heavy rain and wind;
• help suppress weeds;
• add organic matter when the plants are cut down;
• help retain nutrients that might otherwise be lost from bare soil;
• improve soil structure over time;
• provide valuable cover and habitat for insects and other wildlife;
• and, in the case of legumes, add nitrogen to the soil system through nitrogen fixation.

They aren't a magic solution, and they don't replace compost or other soil improvements. But they are another useful tool for looking after the soil.

 

Choosing the right green manure

This is where it gets interesting because not all green manures do the same job.

I've got heavy clay soil, so I'm particularly interested in plants that can help with soil structure and add organic matter. But someone gardening on light, sandy soil may have very different priorities.

 

Legumes such as clover, vetch and field beans

Legumes are particularly interesting because they work with bacteria associated with their roots to fix nitrogen from the atmosphere. Examples include:

• crimson clover;
• white clover;
• vetch;
• field beans;
• lupins.

I've used broad beans and field beans as crops, but they can also be useful as green manures. Field beans are particularly useful for autumn sowing because they are hardy and produce plenty of growth and roots. Vetch is another vigorous option, particularly for overwintering. If your main aim is improving fertility and contributing nitrogen for following crops, legumes are worth considering.

 

Rye, oats and other cereals

Cereals are useful when your priority is protecting the soil and adding plenty of organic material.

Grazing Rye is particularly useful for overwintering because it is hardy and produces a good covering of growth and roots.

Oats are another option when you want something relatively quick-growing, although they aren't as reliably hardy through winter as rye.

 

I've become much more conscious of protecting soil over winter. With heavy clay, leaving the surface bare means it can take a real battering from rain, and wet clay isn't something you want to be constantly digging and walking on. A living cover gives the soil some protection.

 

Mustard and other brassica green manures

Mustard is another fast-growing option and can be useful when you want to cover an empty space fairly quickly.But there is a downside for anyone growing lots of vegetables from the brassica family.

Mustard is itself a brassica, so I wouldn't automatically choose it for a bed where I intend to grow cabbages, broccoli, sprouts or other brassicas next.

If crop rotation is important, a cereal, legume or phacelia will be a better choice.

 

Choosing a green manure for different soils

The best green manure depends partly on what your soil needs.

 

Why I've chosen phacelia

For this particular newly filled raised  bed, phacelia seemed like a good fit. I wasn't trying to solve every soil problem at once. I mainly wanted to cover the bare soil, grow plenty of green material to add organic matter, suppress any weeds and give the bed something useful to do until I was ready to plant.

There would be an added bonus if I let some of it flower, the pollinators would benefit too. That's something I particularly like about green manures. They don't necessarily have to be purely about soil fertility. Some can benefit the wider garden as well.

 

Green manures for heavy clay

I’ve found heavy clay is a bit of a challenge. It can be wonderfully fertile, but it can also become sticky and waterlogged in wet weather and hard and difficult to work when it dries out. I'm increasingly trying to improve my clay soil by adding organic matter rather than constantly trying to dig my way out of the problem.

Green manures can be part of that approach.

Field beans, vetch, clovers and cereals such as grazing rye can be useful choices for larger areas to dig in. Their roots grow through the soil and the plant material eventually contributes organic matter. But one thing I've learned is that you can't fix clay by simply doing more to it. If the soil is saturated, leave it alone. Hence trying mustard as I've no issues sowing herbs in the space afterwards.

Digging or cultivating wet clay can damage its structure and leave you with large compacted clods when it dries.

 

What about sandy soil?

The priorities are rather different on light, sandy soil.

Sandy soil tends to drain quickly and can struggle to hold onto moisture and nutrients. Green manures can help by adding organic material back into the soil. Grazing rye, oats, clover, vetch and phacelia can all be useful choices.

Over time, adding organic matter can help sandy soil retain moisture and nutrients more effectively.

 

Poor or low-fertility soils

Legumes can be especially useful on soils that need more fertility. Field beans, vetch, clover and lupins can produce useful quantities of biomass while contributing nitrogen through their relationship with nitrogen-fixing bacteria. Not an instant remedy, but a natural, budget-friendly way to feed your soil for a healthier, happier garden next year.

 

When can you sow a green manure?

One of the things I like about green manures is that they can fit into gaps in the growing year.

Spring and summer

Quick-growing green manures such as phacelia, mustard, buckwheat and some clovers can be useful when a bed is going to be empty for several weeks or months.

Late summer and autumn

A good time for overwintering green manures. Field beans, vetch, clovers and grazing rye can establish before winter and keep the soil covered when vegetable beds would otherwise be bare.

Over winter

Hardy green manures can continue protecting the soil through the colder months, giving you a living cover rather than leaving the bed exposed to the weather. As always, check the sowing instructions for the particular seed you are using. Getting the timing right makes a big difference to how well a green manure establishes.

 

A quick guide to choosing a green manure

What you want to achieve	Green manures to consider
Increase fertility / nitrogen	Clover, vetch, field beans, lupins
Improve soil structure	Field beans, vetch, grazing rye and other cereals
Add organic matter	Grazing Rye, oats, barley, phacelia
Protect soil over winter	Grazing Rye, field beans, vetch, clovers
Get quick ground cover	Phacelia, mustard, oats, crimson clover, buckwheat
Suppress weeds	Phacelia, mustard and other vigorous growers
Improve light/sandy soil	Grazing Rye, oats, clover, vetch, phacelia
Support pollinators	Phacelia, crimson clover and other flowering green manures
Avoid before brassicas	Mustard and other brassica green manures

 

 

What I’m hoping to learn from my own beds

For me, using green manure isn't about following a perfect soil-improvement formula. I'm experimenting. I've got one raised bed where I've added plenty of organic composted material to grow my autumn and winter vegetables. In the other, I've chosen to grow phacelia and see if it makes a difference.

I'll be watching how quickly it establishes, how much growth it produces, how easy it is to cut down and what the soil looks and feels like afterwards.

And that's probably my favourite way of gardening – trying something, seeing what happens and then using that experience to decide what I'll do next time.

 

What I have learnt is that there isn't one perfect green manure. Choose one according to what you want it to do, when the bed is going to be empty, the type of soil you have and what you intend to grow afterwards. And sometimes, as I've found this year, you don't even have to have planned it from the beginning. A forgotten layer of compost can be enough to make you try a different approach!