The green manure debate

Right, that’s it. I know what I said about green manures, but it was a load of old twaddle. Don’t believe a word of it.

Sometimes, as a gardener, you just have to admit when something isn’t working. For many, many years I’ve been using green manures: usually to follow the new potatoes, as I always find it a bit tricky to know what to plant straight after if I’m not to muck up my rotation system. So I take the easy way out and sow green manures instead.

The theory is sound: you sow fast-maturing plants which have some beneficial effect on the soil. Sometimes, as with phacelia or buckwheat, it’s simply a lot of lush nitrogen-rich green matter which rots down into the soil and adds extra organic matter and no doubt nutrients.

Sometimes it’s a more specialist function: they say deep-rooted plants like alfalfa and blue lupins draw up nutrients from deep in the soil. Leguminous green manures like red clover ‘fix’ nitrogen from the air via nodules on their roots; grazing rye breaks up claggy clay. And so on, and so on.

Now, I myself have regurgitated all this in various articles over the years, many times over. It’s not all bunkum, either: studies have been carried out by Garden Organic (among others) which undoubtedly show that soils on which green manures have been grown contain more nitrogen than those left to their own devices.

I have also followed the theory myself: I’ve tried phacelia (pretty, and the bees love it) buckwheat (don’t let it seed whatever you do) red clover (patchy and difficult to get rid of) and grazing rye – my own personal favourite – many times.

However – and here’s the rub – I have to admit I have never noticed any particular improvement in performance in subsequent years, and certainly not any more than other beds where I’ve just laid a thick mulch of compost and covered the lot with black plastic.

If you look closely at the above Garden Organic report you’ll find the trials were done on a field scale. And most other similar tests like this one from the HGCA (part of the Agriculture & Horticulture Development Board) are also primarily for the benefit of farmers.

I’m sure green manures do work splendidly if you’ve got a whole farm to spread them over. But they ain’t much good in a garden.

Objection no. 1: why go to all the trouble and expense of identifying, purchasing and growing special crops to get the green manure effect? You could grow any nitrogen-rich leafy crop. Even productive ones (now there’s a thought) like lettuces Then at least you’d get some bang for your buck before you dug them in.

Objection no. 2: that digging in thing. When I cover my veg beds with compost and then black plastic, all I have to do in spring is peel ‘em back and go. With green manures you have to shear off top growth, dig in the roots and then wait for them to rot down. Yada yada yada, it takes ages and is back-breaking and then you have to wait a month before you can plant anything. Why bother? The benefits just aren’t spectacular enough to justify the extra effort.

Objection no. 3: the weeds. This sudden lightbulb moment all came about while I was standing staring disconsolately at my grazing rye the other day. It was meant to be a grazing rye and vetch mix, which sounds all very nice until you realise grazing rye grows twice as fast as vetch so you actually end up with just the grazing rye. As I say, though, I like dealing with grazing rye: it doesn’t self-seed, just gets on with the job and covers the ground quite well.

Except that it didn’t quite get to the edges. And now it’s riddled with deadnettle and chickweed and a few proper nettles here and there self-seeded out of the hedgerows.

Of course, I can dig in the weeds along with the grazing rye and they’ll still have much the same effect (see objection 1 above). But I didn’t want any weeds over winter: they’ll self-seed into neighbouring beds and generally make themselves a nuisance. It is painfully obvious from the plastic-covered and weed-free bed next door that there are better, cheaper and less labour-intensive ways of doing things.

The recently-crowned Practical Gardening Journalist of the Year and no-dig gardening pioneer Charles Dowding got there long before I did. He dislikes green manures quite intensely for harbouring slugs and weeds, and adds that in clay soils they may turn putrid before they decay. He has long preferred well-rotted brown manure instead: and what’s good enough for him is good enough for me.

So I hereby retract any previous advice given about sowing green manures. Don’t bother. Grow lettuces instead. Or just cover your beds with a nice thick layer of manure and pop some black plastic over the top to keep out the rain and the weeds. You’ll be glad you did.