How To

How To: Green Manure

“You wouldn’t go out naked in the Winter, don’t expect your beds to”

Wise words from Mark @ learnhowtogarden.com on overwintering with green manure – I haven’t ventured over to his site yet, but this YouTube video sums up his thoughts.

There is also an excellent article Green Manures – the Good, the Bad and the Ugly written by Jeremy Dore on growveg.co.uk which is worth a read.

There seem to be many conflicting articles on why / how you might use a green manure & after digging deeper (no pun intended), this appears to come down to whether you are employing a ‘no dig’ technique, if you are using raised beds, whether you have certain plant pests / diseases, what you intend to grow in that bed afterwards – the list goes on and on…

There are two things however that the internet & books do agree on;

  1. Soil left bare could be considered a waste & if left to its own devices, nature will decide what grows in that bed – not you!
  2. Some green manures are members of the legume / brassica family and therefore can affect your crop rotation (I haven’t written that article yet..)

So, with this in mind, lets first look at the ‘why’ of green manure, then tackle the ‘how’ of green manure & finally, sum up with what will1 work for you.

Why use a green manure?

There are four main roles that green manures play (some fall into multiple roles or a seed mix could also be used), but make sure to read the ‘how’ section as well before choosing …

Nitrogen Fixer – Nodules on the root structure will store nitrogen as it grows & then slowly release this back into the soil as it decomposes.
Biofumigant – Releases gases into the soil to cleanse any pests / diseases.
Soil Improver – Has deep tap roots that break up the soil structure & can draw additional nutrients up from the sub soil.
Weed Suppressor – Lots of foliage means less light for weeds to grow & plenty of organic matter to dig in / add to the compost bin.

How to use a green manure?

“To dig or not to dig, that is the question”

Now you know why you would select a certain green manure, we can nearly cover how to select which one of the many options will benefit you & your allotment / vegetable patch.

First we need to answer an age old question – to dig or not to dig (that will be a link to another article, but for now, a summary will have to do!).

If you are employing the method of ‘no dig’, then you have effectively decided to never turn over your soil. All the organisms in the top few inches of the soil will remain undisturbed and all composting will only ever be added to the top of the soil – you should only select green manure that can be cut to soil level and therefore you should avoid all grasses (unless they will be killed off over the winter). Any organic matter can then be shredded and left as a mulch, or added to your compost bin. A benefit of this approach is that as the roots rot down, pathways will open up to allow more air into the soil.

The alternative to this, is digging (or in some cases, double digging) – this is how I was taught to turn the soil over when helping dad in the garden & really the only way I have ever known to get goodness into the soil – digging a trench & then adding grass cuttings / vegetable waste / manure. For this approach, it’s open season on all green manures & the only thing you need to be wary of is crop rotation. You also have the option of digging in the organic matter or composting it, however for nitrogen fixers, it is still recommended to leave the roots in the ground – these should still be cut to soil level & not pulled out or you will lose the nitrogen nodules.

There are so many different opinions each method, (we’re going to try both) but once you’ve established which camp you live in, then you can finally make a choice…

What green manure to pick?

Bottom line comes down to this – are you trying to repair a bed that’s broken, preparing a bed for a new crop, or looking to maintain the soil over winter?

If you have a bed that has contracted a disease, then get some caliente mustard in there pronto to get the bed back to healthy – but remember, mustard is a brassica, so ideally you should avoid planting cabbages & other brassica’s into that bed in the next rotation. As a bonus, the mustard will die off in the frost, so this can just be left in and allow to die down to a mulch if you’re in the no dig camp.

If you’re looking to prepare a bed for the next crop, such as delicious purple sprouting, get a nitrogen fixer in that bed (that’s not also a brassica) and you’ll be well away. I’ll cover this in more detail in the crop rotation post.

If you just want to keep that bed warm and the soil in great condition over winter, it will need to be frost proof & able to keep the weeds suppressed – something like Phacelia tanacetifolia (the image at the top of this post) is a great choice, or Hungarian grazing rye.

At this point, I need to point out that I had started to compile a list of the green manures that I had been reading about, along with their affect on crop rotation and growing periods. However, it seems the people at greenmanure.co.uk (of which I am no way affiliated) have already done the hard work and made a very useful chart to assist with making a decision – you can take the information on this page & then apply it to their chart for easy way to get it right every time.

Happy Growing

Carlos

 


12 – By ‘will’ I of course mean ‘in theory’ or ‘should’ – this may be only the second article on a blog about starting a new allotment, but if I don’t make assertions, how can I reference them at a later date and then gloat3 if I’m right, or use it as a ‘learned lesson’ if I was wrong!

2 – Anyone that has ever read a Terry Pratchett novel, will recognise this style of commenting with a hint of humour & assuming no-one has any issues, I will continue to do this as it makes me smile when I type.

3 – No-one likes a gloater

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