Tag Archive: weed control


Winter Tares- good at protecting the soil, smothering weeds and maintaining nitrogen in the soil, once dug in. Picture from Garden Organic
Winter Tares- good at protecting the soil, smothering weeds and maintaining nitrogen in the soil, once dug in. Picture from Garden Organic

At one of my recent ‘Grow Your Own’ classes, one of the participants raised an interesting question about Green Manures (GM’s).

He wondered if it was actually worth growing green manures as additional sources of nutrients. He reasoned that as they use up nutrients from the ground there isn’t any real gain in the nutrients avialable to follow on crops. Like me, he had also heard that legumes, (peas and beans) do fix nitrogen from the air and therefore their roots are a source of additional supplies of this if dug into the soil. And he also mentioned that deeper rooting plants like Comfrey tap into nutrients that wouldn’t otherwise be available to plants with shallower roots, so making these available via their leaves once composted, and also from a  ‘tea’ made from these and applied as a liquid feed. So is this all correct?

I decided to contact my colleagues at Garden Organic and ask for their advice on all  this and got a very interesting reply from Francis, their Horticultural Research Manager:

Green manures and nitrogen

Legumes, when the temperature is warm enough and they have the right bacteria, will fix the nitrogen they need from the atmosphere. It is very true, and seldom appreciated, that if a legume crop (eg beans) is harvested then most of this nitrogen is taken away and not left in the soil. However, if the legume is grown as a green manure and dug in whole (usually in an immature state) rather than being harvested then there will certainly be a net benefit; nitrogen fixation (directly or indirectly via animal manures) is the main source of nitrogen for agriculture and horticulture in the absence of artificial fertilisers.

Non leguminous plants can only take up nitrogen from the soil as inorganic ions (ammonium but mainly nitrate). The latter is very soluble in water and so easily washed out by the rain and so lost from the soil, contaminating drinking water and rivers etc. A lot of work was done (some by Garden Organic) to demonstrate that one of the best ways of preventing this was by growing winter green manures such as rye. When this is dug in the nitrogen they have taken up is mineralised to be made use of by following vegetable crops.

Green manures and other nutrients

Other nutrients (especially the metals such as K, Mg etc) are more tightly held on the surface of the soil particles and so are not easily leached so it is true that green manures are less important for keeping them in the soil. However, some do have specific effects (eg buckwheat can help mobilise phosphorus and chicory is deep rooting and so is a source of trace elements from the subsoil that may have been depleted nearer the surface). All green manures will add organic matter to the soil which helps with structure and also stimulates microbial activity, important for general nutrient cycling.”

So, the net result is that there are several good reasons for using GM’s over the winter, including maintaining nitrogen where this would leach away from unprotected soil, weed reduction, protecting soil structure, and the addition of organic matter to help moisture retention and soil structure. However, the legume contribution to soil fertility (assuming you grow these to produce food), is of questionable value if left in the soil and dug in. Better to add animal manure or your own compost to boost nitrogen levels.

Comfrey- reaches the nutrients other plants cannot reach...and you can out them into your soil via a (smelly) tea made from their leaves
Comfrey- reaches the nutrients other plants cannot reach…and you can out them into your soil via a (smelly) tea made from their leaves

Further information:

Garden Organic and Cotswold Seeds have produced a useful advice booklet on soil improvement. It’s available as a pdf to download for free at  ‘Sort out your Soil’

Linked articles:

Green Gold – 7 reasons for using green manures

Green Gold: Where and when to use Green Manures

Green Gold: Making the most of green manures

Green Gold: 12 plants for soil improvement

Old School Gardener

Phacelia tanacetifolia, a  'green manure' that's good to look at and attractive to beneficial insects.

Phacelia tanacetifolia, a ‘green manure’ that’s good to look at and attractive to beneficial insects.

I’ve just been reading about green manures, from a small prize I won at my induction training as a ‘Master Composter’. The prize is a slim booklet produced by Garden Organic and it focuses on the use of green or ‘living’ manure in the garden. So, what is a green manure?

‘a plant which is grown to benefit the soil, not, as some might suppose, under-ripe animal dung!’

I’ve had a couple of tries with green manure (mainly because I like the flowers of Phacelia), but have not been totally convinced of its value – it’s hard to check what benefits it brings unless you conduct some sort of rigourous trial, of course. Anyway, this booklet is giving me the confidence to do more and so I thought I’d share its contents with you in a short series of ‘bite sized’ articles over the next few weeks.

Apparently green manures have been used by farmers for centuries to improve their land and gardeners have begun to realise their value too. Seed companies have begun to stock green manure seeds in packets sized for the average garden. They are most often used in the vegetable plot, but can also be used in other areas. In later articles I’ll cover where and when to use them; some of the plants and their benefits; how to choose and grow the right plants and what to do when you’re ready to use them. Today I’m focusing on seven reasons why to use green manure.

1.To feed the soil – green manure crops ‘mop up’ and hold onto soil nutrients and some deep-rooted types can actually gather nutrients from depths that other plants cannot access. By absorbing nutrients the roots prevent it being washed down into sub soil. Once green manures are turned into the soil the nutrients are ready to be taken up by the next crop.

2. To protect and improve soil structure – green manures help to protect the soil surface from the effects of heavy rain (mainly soil compaction and surface ‘panning’). This is a benefit for both clay and sand – dominated soils where organic matter reduces compaction in the former and helps water and nutrient retention in the latter.

3. To stimulate soil micro organisms – when dug in green manures feed and stimulate microscopic creatures that in the process of decomposing this organic matter boost soil health, which in turn helps to develop strong plants.

4. To prevent weed invasion – nature abhors a vacuum/ bare soil – as soon as plants are removed new ones will try to move in and these can often be weeds. Green manures tend to germinate quickly so can be a quick way of covering bare soil and smothering young weed seedlings, also eliminating the need for constant hoeing to remove the weeds.

5. To control pests – some beneficial crittters (like frogs and beetles) love the shady, damp ground under a green manure. Some green manures can be planted to distract flying insects away from crops you want to protect; e.g. underplanting Brassicas with Trefoil disguises the outline of the crop and seems to deter cabbage root fly. Likewise a small patch of Phacelia tanacetifolia or Clover, if allowed to flower, will attract insects that prey on many garden pests.

6. To improve the look of the garden – a green manure or ‘cover crop’ will not ony help to prevent weeds but can look attractive of itself. Some also help to fix nitrogen in the soil which will help plant growth.

7. To ‘rest’ your soil – after a period of intensive cultivation, soil can benefit from lying fallow for a season. Most usual in the vegetable garden, it’s a technique that can be useful in the ornamental garden especially where a new border is to be planted up.

Crimson Clover - another green manure that looks good and helps to 'fix' ntrogen in the soil

Crimson Clover – another green manure that looks good and helps to ‘fix’ ntrogen in the soil

So, on paper the case for using green manures is a strong one. My kitchen garden is currently straining under the weight of the many different crops I have growing in every available patch of soil (and some containers too). But in a month or two, once some crops have been harvested, and where I haven’t planned for any new crops, I’m going to put in a green manure. In next week’s article I’ll cover just where and when to use these valuable plants.

Source: ‘Green Manures’- Garden Organic Guide. September 2010

Old School Gardener

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