Green Manures – the Good, the Bad and the Ugly

, written by Jeremy Dore gb flag

Phacelia

How to improve the fertility of your soil is a question that all good gardeners take seriously.  One of the most under-used methods of soil improvement is the use of green manures (often called ‘cover crops’ in America), 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.

As regular readers will know, I live in an area with very sandy soil: great for digging and growing carrots but not at all good at retaining nutrients which are easily washed away.  I’d love to say I can produce enough compost for my garden but despite my best attempts at bins of rotting organic material and leaf-mould there just never seems to be enough.  The charity Garden Organic recently found that growing green manure can reduce the loss of the key nutrient nitrogen in the soil by up to 97 percent compared to soil left bare.  So green manures seem to be the perfect solution.

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.

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.
  • Field beans
  • 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.
Poached egg plant

[The above list includes most of the available green manures in the UK.  For a US-list, see the article on the National Garden Association website]

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.

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 and you can see how well it did in the picture above.  It required little weeding, quickly producing lush growth up to about 40cm (16 inches) 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.

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Comments

 
"this is very nice, i like clover very much...."
Ceth on Monday 1 September 2008
"which of your top three would be good for my heavy clay soil and when do I plant them? Am a total novice gardener, please help...."
Holly on Friday 5 September 2008
"Holly, winter field beans are definitely the best for heavy soils as they prefer them and should do very well. You can plant them any time from now right through to the end of October, just follow the instructions on the packet."
Jeremy Dore on Friday 5 September 2008
"thanks jeremy, I will let you know on here how it goes.x"
Holly on Friday 5 September 2008
"Thanks for posting such an interesting article, wish i had read it before i planted rye in a bare patch! I am also a relatively new at allottmenting so do you have any tips on how to dig it in successfully?"
Samantha on Saturday 6 September 2008
"Samantha, For digging in green manures I generally use two methods: either dig it in with a garden fork very early when the plants are small and easily buried or for larger plants cut them off at the base and put them through a shredder or run a lawnmower over them to break them into small bits before forking in. Unfortunatly, shredders and lawnmowers are less accessible at an allotment, so my advice would be to fork it in early."
Jeremy Dore on Sunday 7 September 2008
"Hi Samantha, don't fret, I have used Hungarian rye as a winter cover crop for several years. In the spring I cut the grass down using a pair of shears, let it wilt for a few days and then dig it in. After a few days I either plant potatoes or put out young transplants. If you want to sow seeds you must leave the ground for a few weeks to the inhibiting chemicals that some plants release. "
Daibread on Monday 8 September 2008
"thanks guys for your comments but i have another question(sorry did say i was new to this so apologies for my niavity!). To make the most out of green manures and their resulting nutrients do you need to be ready to plant once it have been dug in?"
Samantha on Tuesday 9 September 2008
"Daibread, that's really useful feedback about using Hungarian rye - thanks for adding it. Samantha, you usually need to dig the green manure in a month or two before planting to give it time to start breaking down and releasing nutrients. In my experience this gap also gives you a chance to remove any weeds from seeds you have brought to the surface when digging in the green manure."
Jeremy Dore on Tuesday 9 September 2008
"Hi Jeremy, As a novice vege grower i was wondering on the value of planting watercress in my glass covered patch as a green manure.I have a large surface area to cover, about half an acre, so could a green manure be planted and then around December, January could i cover the same area with my own compost that i have been making using old cow muck, hay from the farm and composting from my kitchen, how would this work in preparation for my next seasons growing. Thanks for your fab sight it's a wonderful resource - Adele"
Adele McDermott on Friday 26 September 2008
"Adele, I've not heard of watercress being used as a green manure, although most quick-growing lush plants are generally suitable as green manures. The only thing to be careful of is that watercress is a member of the brassica family, so you may wish to avoid using it in areas where you are going to plant cabbage, broccoli, cauliflower, sprouts etc next year."
Jeremy Dore on Friday 26 September 2008
"Hi Jeremy, My husband and I have planned our first vege garden. The space was previously pasture. We plowed it about 2 months ago. We're wondering what other preperations we should make to the soil to maximize our harvest."
Wanda on Sunday 15 February 2009
"Wanda, Take a look at our new GrowGuide on how to prepare new land - http://www.growveg.com/growguides/starting-a-new-plot.aspx. Basically you need to get it covered with permeable plastic sheeting or cardboard to supress weeds and you may wish to get some good compost or organic soil improver mixed in, depending on the condition of the soil. It is too late to sow a green manure in Spring, unless you are going to do that on part of the land which won't be used until later in the year. Good luck!"
Jeremy Dore on Sunday 15 February 2009
"Hello there, I'm new here but feel I should sing the praises of this site. Have been growing organically in my allotment for twelve years now and have found that clover seems to do best in my raised beds although it's a pain to dig in it does provide a fair bit of notrogen. Didn't know about the poached egg plants though, cheers, John B"
John Backley on Sunday 15 March 2009
"Yes, since writing this article I have read good things about crimson clover and I have bought some this year to trial in my own garden."
Jeremy Dore on Sunday 15 March 2009
"Hi, am wondering on the value of planting a green manure on an area that i plan to cover with main crop potatoes, so need the green manure in the ground for only 2-3 months at the most, does anyone have any suggestions as to which green manure i can put in and where do i get this seed from - thanks Adele"
adele mc deremott on Wednesday 8 April 2009
"Adele, during the summer 2-3 months would be adequate to get a green manure grown and dug in but during spring as the ground is warming up I think it will be hard to complete before the potatoes need to go in. You could leave the green manure in while the potatoes start growing but that could risk damaging the new potoato stems when you do dig it in. If you do put a green manure in then legumes would not be a good choice as research has found little link between these and potato growth (probably because they fix Nitrogen in the soil which makes leaf growth not tubers). I would probably go for a mustard - see http://www.greenmanure.co.uk/sow_help.htm for details."
Jeremy Dore on Thursday 9 April 2009
"Wow, thanks for all the information here! I put some phaecelia on one bed this spring, and am planning to plant my courgettes out on it this weekend, but haven't dug it in yet! Can I pull it out and leave it on the soil as a mulch? Or should I just compost it and reap the benefits later? Any suggestions welcome!"
Elspeth on Friday 29 May 2009
"Elspeth, in theory you can use it as a mulch but once it grows tall or starts to flower then it doesn't break down well. However the flowers are great for beneficial insects so I tend to leave mine to this stage and then add it to the compost heap."
Jeremy Dore on Monday 1 June 2009
"hi everyone does anyone know which of the green manure is best to use for a sandy soil garden, thanks in anticipation "
Pat on Tuesday 4 August 2009
"Pat, my garden soil is very sandy so I would try the ones I recommended at the end of the article, then perhaps experiment with the others next season."
Jeremy Dore on Wednesday 5 August 2009
"thanks"
pat on Wednesday 5 August 2009
"Excellent piece and much useful discussion. I've had an allotment for about six years in Sheffield and I've experimented a bit with green manures but have a problem with green manures and crop rotation and was wondering if anyone else has a solution. I use a classic three bed rotation system (roots,brassicas,others/legumes). If I want to over-winter green manures then in theory I could use a legume,brassica and other(e.g. phacelia) in the rotation. Simple. The problem is that I have kale, purple sprouting broccoli etc that stays in the ground until April, not really leaving enough time once they come out to get in a green manure to complete the rotation. If I don't put in a green manure folowing the kale then I will have two beds that have had brassicas in them. The only solution I can see is to use a green manure in all beds that won't interfere with the crop rotation. This really leaves me with phacelia/buckwheat. Any other solutions? If I do stick to a non-legume, non-brassica green manure then are there others that I could try. I guess grazing rye is OK but I find the seed gets eaten (pigeons/mice probably). I do grow poached egg plant and could possibly harvest my own seed (though not sure I'd get enough). Is poached egg plant seed commercially available? Sorry for the detailed post but hope someone has some ideas. "
Kevin Walters on Wednesday 19 August 2009
"Kevin, You've hit on exactly the dilemma faced by many people when using green manures - many of them do interfere with crop rotation. However, my own thoughts are: 1. Yes, use plants from other crop families where possible. Phacelia and buckwheat are both good and poached egg plant seed will be commercially listed as Limnanthes Douglassii. 2. Although brassicas will introduce problems with rotation, in my experience legumes suffer less problems with rotation. So I wouldn't worry too much about sticking in a legume green manure where necessary - at least once every 3 years should be fine. 3. Any quick-sprouting plant that produces good green growth at lower temperatures will be better than leaving soil bare, so you can experiment with others. Just remember to dig them in before they start to flower and set seed. If you do find any other good green manures then please do let us know by posting a follow-up comment. Good luck!"
Jeremy Dore on Tuesday 25 August 2009
"Hi, I've just taken on an allotment and am reading this site with interest. I planted some forage pea (http://www.greenmanure.co.uk/foragepea.htm) a week or so ago - any recommendations for what are good crops to follow this with and make the most of the nutrients it provides? "
Rebecca on Wednesday 4 November 2009
"Hi Rebecca, Usually legumes such as forage pea are followed by brassicas (cabbage family plants), although some people follow them with potatoes as both these groups like the nitrogen fixed in the soil (as long as you leave the forage pea roots in!) There is some more detail on this at the end of our Crop Rotation GrowGuide at http://www.growveg.com/growguides/crop-rotation.aspx."
Jeremy Dore on Thursday 5 November 2009
"Thanks Jeremy, your crop rotation guide is very useful. Brassicas it is then! Out of interest, you say some people follow legumes with potatoes but from your response to Adele above it sounds like you think this isn't such a wise choice/the nitrogen might not be so useful?('If you do put a green manure in then legumes would not be a good choice as research has found little link between these and potato growth (probably because they fix Nitrogen in the soil which makes leaf growth not tubers)'). Thanks, Rebecca"
Rebecca on Friday 6 November 2009
"Rebecca, Yes, I wouldn't follow legumes with potatoes myself but I know that many people do because potatoes are 'hungry' crops. But as the research article I quoted shows, you don't necessarily want a nitrogen fixing green manure before potatoes. Best to stick with the brassicas."
Jeremy Dore on Friday 6 November 2009
"good stuff, thanks Jeremy!"
Rebecca on Friday 6 November 2009
"Dear, sir, This article is very nice. in my target area (birbhum district of west bengal of India) the soil nature is clay, sandy, lateritic etc. so what major plants we can use as green manure crops? I have introduced Sesbania rostrata (Dhanche plant) in this year. I want to buy journals papers, magazines. atfirst if you send a free copy in my address i will be very oblized to you. My address: Mr. Pabitra Paramanya C/o. Jisu Niketan, Infant Jesus Church Makrampur, Birbhum district West Bengal 713204 THanking you."
Pabitra Paramanya on Thursday 5 August 2010
"VERY SMART AND BRIGHT LOVE THE COMMENTS"
bazyang on Tuesday 5 October 2010
"WHY DOSE MANURE HURT THE ENVIORMENT JUST WANTING TO KNOW"
ALISHA on Tuesday 5 October 2010
"Sir, thanks a lot for sending me updates about green manure and trap crops.It will help me to motivate the poor and marginal farmers in our area, West Bengal, Birbhum, Bolpur, INDIA."
Pabitra Paramanya on Thursday 7 October 2010
"where can u buy field beans ?"
heather on Tuesday 25 January 2011
"hi i was learning about liquid manure and was wondering if anyone has used pig manure. i was going to put it in a hessian sack and suspend it in a dustbin of water can anyone give any tips on this. "
peter on Saturday 27 August 2011
"Hello!Can anyone help me with some informations? I'm writing my license about green manures and i didn't find out until now how phacelia tanacetifolia enriches the soil with nutrients,given that this in not a plant from Fabaceae family? Thank you!"
julie on Saturday 19 May 2012
"I sowed field beans as a green manure last year but forgot to cut them down and dig them in at springtime so I now have beans about four feet high that have set seed! I am going to try saving the seed and cut down the foliage after harvesting then dig (or fork)in the roots."
Mike, Hemel Hempstead on Saturday 22 June 2013
"Thanks for this very helpful and thorough blog, and all the helpful comments from other people. I'm just clearing a neglected allotment plot with heavy clay soil and a high water table; this has given me lots of help about choosing the right green manure - it looks like winter tares for me. Could I also just mention that the poached egg plant, which is great for beneficial insects, self seeds like mad: once you've got it you've probably got it for ever, so be sure you like it!"
Jay on Sunday 25 August 2013
"Thanks for this very helpful and thorough blog, and all the helpful comments from other people. I'm just clearing a neglected allotment plot with heavy clay soil and a high water table; this has given me lots of help about choosing the right green manure - it looks like winter tares for me. Could I also just mention that the poached egg plant, which is great for beneficial insects, self seeds like mad: once you've got it you've probably got it for ever, so be sure you like it!"
Jay on Sunday 25 August 2013
"The soil around my LARGE hosta beds is tight with little roots. I can hardly get my shovel in the soil! I am thinking of planting a cover crop of something like large radishes or turnips that I will LEAVE in the ground over winter. Thought maybe the rotting vegetables would help loosen the soil and leaves wouldn't look too bad with hostas. Or would something like buckwheat work better? Does the red clover re-seed? I do not want anything that would re-seed as this is a one time plan. I will not be tilling it in but letting it die back as a natural mulch."
Jane Ellen on Monday 9 September 2013
"thanks for this great thread from Tasmania Australia. I have clay soil so will follow the advice here to break it up and add organic material. Looking forward to giving it a go, thx"
Stroganoff_au on Monday 28 October 2013
"Can I use excess salad greens for green manure in Brisbane Australia?"
Jenny on Sunday 3 November 2013
"I planted field beans but they were not dug in and have now flowered. Can I let the beans develop and eat them? And can I still dig the plants into the soil once they have finished cropping? "
Weeding Woman on Thursday 21 May 2015
"You can try a green named Sanappai in Tamil or aka Sun Hemp aka crotalaria Juntia which will be ploughedback as it stands around 4 or 5 feet tall. Gives NPK and micro nutrients to land."
Dr G Ranganathan on Sunday 27 September 2015
"Hi, I have a small urban garden with a fairly heavy clay soil. I want to improve the soil in order to sow a lawn - would using green manure help improve the soil in preparation for a lawn? Are there any downsides to this?"
Matt D on Sunday 27 September 2015
"Hi Jeremy. I know this article is rather old now but it remains on of the best on the subject, so thank you!!"
Gaby on Thursday 26 October 2017
"I have just cut down my Fava ground cover crop leaving the roots in place and simply chopping the stems on top of the roots. I covered with burlap. The question I have is, do I continue watering or should it be left to dry out unless of course it rains. We are having a very dry winter!"
Ann on Thursday 8 February 2018
"I have just cut down my Fava ground cover crop leaving the roots in place and simply chopping the stems on top of the roots. I covered with burlap. The question I have is, do I continue watering or should it be left to dry out unless of course it rains. We are having a very dry winter!"
Ann on Thursday 8 February 2018

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