The allotment and companion planting

This year I have spent some time planting companion plants alongside my vegetables.
I know there is some scientific research for plants like marigold, mustard and maybe chamomile but there seems to be very little research on any of the other recommended plants. I must admit that the poached egg plants that I planted under the sweet peas may have helped them. I would much rather think that it was the inoculated charcoal that was more effective though. I am the belt and braces kind of gardener that uses every technique he can to produce the best results.

The companion planting lists seem to be repeated from web site to web site and book to book with very little passing through people’s brains.

The lists of pests they are reported to be effective against read like the old quack remedies such as snake oil.

Is there anyone that can suggest some good research that has been done on companion planting?


I am always amazed at what academics research, because they seem to go for stuff that is not very good for everyday life. In horticulture research, I think that the funding comes from industry so research will be focused on what they think are important.
My main worry is the thoughtless copying of long lists of companion planting that I have seen in many books and websites.


I find it very difficult to rely on old wives tales about gardening techniques because I have found that many of them do not work. The “plant onions or garlic with carrots to keep away carrot root fly” does not work for me and I have tried it for years. However, each time I hear about a method I have not tried I will give it a go even though my scientific training makes me very skeptical.


Now I have read on http://www.allotments-uk.com/ that planting potatoes next to raspberries encourages blight. What is that all about? I have been reading gardening books and research for years and have not heard about that one. Rubus idaeus the raspberry and Solanum tuberosum potato both totally different species from different parts of the world. Not only that, I am growing potatoes right up to my raspberries this year. I can’t put them anywhere else because it would mess up my rotation.

What fascinates me is that there are very few non cultivated plants within the lists. If there is something in companion planting, and I think there maybe, then surely the fact that a plant is cultivated cannot be the important factor.


Native uncultivated plants, weeds for short, probably have a greater affinity for native mychorrhizal fungi may give a greater help to crop plants than the list plants. Mychorrhiza will be able to transfer any beneficial chemicals to partner plants much quicker, easier and more effectively than diffusion through the soil.
It may be true what people say,
But I keep wondering anyway…

Terra Preta on a cold December day.

Although I am going to go through what is known about these black earths, I think that it cannot be decided how useful this will be for amateur gardeners until we have tried it for ourselves.  This is what several of us are doing at allotments.uk.

Could the research on Amazonian black earths be applied to British soils to increase their fertility?  Research shows large areas of fertile black earths in otherwise very poor rain forest soils.  The fertility of these soils seems to be associated with charcoal added to the soil over thousands of years by indigenous South American civilizations.  This could have been done purposefully or coincidentally when adding midden contents to fertilize fields. Regardless of their motivation, it seems that this has produced a remarkable soil with some strange characteristics. 
It seems that regardless of its use it remains fertile for generations even when these fields were reclaimed by the rain forest.

Ignoring the weeds, this end of the pea rows did not get any inoculated charcoal. 

 This is the end that got the inoculated charcoal. Not the best peas in the world because they were on relatively infertile soil.

However, when scientists add charcoal to soil there is a noticeable reduction in fertility. This is cited as a reason for not developing a slow acting soil amendment from a charcoal base.  How could such a relatively inert substance, which does not degrade substantially over hundreds of years, reduce fertility?

There is some suggestion that charcoal adsorbs or absorbs nutrients in the soil.  It certainly has the capacity to do this having an intricate structure of many small channels giving it an extremely large surface area.

So nutrients may well have a tendency to adhere to the large surface area.  As there is a depletion of nutrients in surrounding areas of soil when charcoal is added to the soil, maybe adding nutrients to charcoal before putting it on soil might mitigate any adsorption or absorption problems that adding "neat" charcoal might produce. 

This could produce a slow nutrient release system.  Affinity of the charcoal for nutrients depends on the condition and type of soil it is in, but I would suggest that this would be a buffering mechanism.  It would adsorb nutrients when they were in surplus but as concentration decreases the charcoal will lose nutrient by a diffusion process maintaining equilibrium with the surrounding soil.
What kind of nutrients could amateur gardeners inoculate the charcoal with to give the maximum long lasting effect on the fertility of their soil?
Several different methods and recipes have been suggested; however I will only give you the method and nutrients that I use personally.  My charcoal, which is just barbecue lump charcoal, is marinated in:

·         Chamomile tea

·         Nettle tea

·         Sweet Cicely tea

·         Worm tea

·         Sugar

·         Blood, fish and bone meal.

The mixture is housed in an old plastic dustbin and left for at least two months.  After this time the charcoal is removed, allowed to dry and then crushed with a bull hammer into pieces no more than 1 cm³.  This gives the charcoal a larger surface area and makes it easier to add to the soil.  I have used bigger lumps but they are cast to the top of the soil continuously by the frost rather than being incorporated within the top 15 cm of the soil. 

There are large numbers of micro pores formed from the xylem and phloem structures within the charred wood and both soil bacteria and fungi could find a habitat in these pores free of predators.

The importance of the micro fauna and flora of the soil cannot be overestimated especially when considering the number of symbiotic or mutually beneficial, relationships there are between micro organisms and crop plants.   There are some tentative indications that mychorrhizal fungi associations with crop plants are encouraged by the addition of charcoal to the soil. Mychorrhizal associations with plants enable them to obtain nutrients from a wider volume of soil than otherwise would be the case, the plant giving the fungi sugars in return. 

My contention is that cultivation disrupts this association and kills off mychorrhiza fungi through cropping, digging and forking.  Adding these fungi to the charcoal may help to replace them in the soil.  A number of scientists suggest that the soil is abundant in spores of these fungi and that adding more is superfluous.  They seem to be basing their assumptions on research into natural ecosystems rather than cultivated environments and I wonder if this still hold true.  

·         So I add mychorrhizal fungi to the charcoal as well.  But only after crushing the charcoal. 

There is some evidence that adding charcoal to compost heaps achieves the same adsorption of nutrients but I have not tried this. 

How much charcoal to add to the soil is another unanswered question and one that we are trying to assess.  There seems to be two ways of adding it.  

·         Spread it broadcast over the soil.

·         Just add it to planting areas in planting holes or along rows of seed drills.  

At the moment I am just adding charcoal to planting areas. 

A further advantage of adding charcoal seems to be its ability to help the soil to retain water while also allowing better drainage. 

 Charcoal is relatively inert in the soil so there is a great opportunity for sequestration of carbon.  The more charcoal that can be added to the soil the larger and better our crop plants will become. All these different advantages of adding charcoal to the soil seem to be both long lasting and cumulative. 

Green manures

I have been using grazing rye as a green manure for several years now. My soil is mostly a sandy clay and obviously fairly easy to work once it has been knocked into shape. Grazing rye comes up in thick soft leaves that easily shade out most weeds.  It also has a very thick mat of fibrous roots.  The advantage of this grass is that it rots down very quickly in the soil.  

Green manures, whichever you use, are useful for a number of different reasons.

• They cover the ground in winter so that nutrients are not washed away by the winter rains.
• They shade out weeds that might germinate during the autumn and winter.
• They add organic matter to the soil which eventually rots down to form humus.  Humus is a black oily sticky liquid that forms films around soil particles. 
• They help to recycle nutrients in the soil.  
• If you use one of the plants from the legume family like tares, clover, field bean or lupin then they will add nitrogen to the soil when you dig them in.  Remember that root nodules formed by the rhizobium bacteria help to fix nitrogen and pass it to green plants.  The nitrogen is therefore in the leaves,  stems and roots of the plants.  If you do not dig in all the plant then you will loose some of the fixed nitrogen unless you put the tops onto the compost heap.  If you burn the tops then you are loosing the nitrogen to the air. 

I dig it into the soil in the spring before planting or sowing seeds.  

You don't need to worry about introducing a weed onto your allotment because the only way that rye grass spreads is by seed and whether you dig in or strim you will not get seeds.  It does not have stolons like couch grass.   I sometimes mix it with tares or clover when I am sowing, although grazing rye does tend to shade these plants out as well. 

Some photographs of the allotment.

Green manures

Chilly December

I have just returned from holiday in Malta and Greece to a very cold England. The ground has been frozen for about two or three weeks now and little can be done in the allotment.

I went to the allotment yesterday with the intention of moving some of the black current bushes. Althought the top couple of centimetres were not frozen, below this was a thick layer of frost. Not the kind of ground to start planting into. The black currents will probably have to be moved in the spring if this weather continues.

Remarkably, I was able to get out some of the parsnips. They were excellent and shows you why thinning out is very important if you want big roots. I also got some carrots. I think that the ground was not so frozen here because of the horse muck I had put close to them.

Beetroot are very small but I harvested them in any case.

I took off some of the Brussel sprouts and I will prepare them today by peeling off the outer leaves. There are some plants that are producing large buds and some that are only producing small ones. I cannot remember the varieties that I planted so I will not be able to ensure I have large buds next year.

They did not burn down my shed on bonfire night which I am very greatful for. The committee put the bonfire closer to Tony's allotment and quite far away from my shed.

Someone however put a load of fireworks on my allotment right where I had planted broad beans. I am not happy. These are the fireworks you stick into the ground. Needless to say I will be asking the committee why they pick on my allotment to do these things without any consultation with me.