Every last drop counts in Catalonia...
Polyculture: higher yields; less water
Patrick Whitefield explores how the necessity of irrigation has defined a growing system
On a recent visit to Catalonia I came across an interesting example of mixed cropping, otherwise known as polyculture.
My friends Charlie and Mika have a market garden, set in a little valley cradled by rocky hills, scattered with pines and wild flowers. It's an idyllic spot but it does have its limitations, and the first of these is water. Rainfall is scarce and the only source of water for irrigation and domestic use is a tiny spring, which runs faithfully but only with the flow of a bathroom tap. The limited water supply obviously has a great influence on their system of growing.
First and foremost it means that all irrigation must be by the most efficient system - drip irrigation. They have a network of pipes laid out over all their productive land, with drippers every 30cm along the pipes.
This works fine for smaller crops but for larger ones, such as brassicas, it's too close together. These plants need to be at 60cm but if they only planted at every other dripper a lot of precious water would be wasted during the first half of the crop's life when the plants are small and their roots couldn't reach the intermediate drippers.
So Charlie and Mika alternate the brassicas with a smaller crop, such as onions, or a faster growing one, such as turnips and beetroot. The onions are either harvested green or, in summer when there's less problem with fungal diseases, left to mature as the brassicas grow to full size. The turnips and beetroot are harvested before the cabbages expand to fill their full 60cm.
Polyculture: advantages and disadvantages
The result is a polyculture, or at least a series of bicultures, and with this increase in diversity comes a number of advantages. Firstly, full use is made of the soil throughout the growing season; whereas in a monoculture of brassicas the space between the plants would stay unused for half the season. Secondly, the different shapes and sizes of the root systems of the component plants makes fuller use of the soil than a monoculture would. Competition with vigorous plants like turnips and beetroot may reduce the yield of brassicas a little, but the overall yield is much greater than it would be if only brassicas were grown.
There are also disadvantages. One that Charlie mentioned is that the double harvesting means that there's more treading on the soil, which can cause compaction on this heavy clay. I asked him why they had chosen the mixed crop rather than having different pipes for larger crops with the drippers at 60cm intervals. His answer was that it would be too complicated. The land is on a series of terraces, all of different sizes and all irregularly shaped. It's much more practical to leave the pipes where they are than to be constantly moving them around, searching for ones of the right length.
They do practise other polycultures, such as growing spinach between rows of tall climbing peas. But what interests me about the brassica combinations is that they have come about as a result of the irrigation method. Permaculture is all about systems, and 'the system' means the farm or garden as a whole. The nature of one part of the system affects other parts of the system which at first may seem unrelated to it. In this case the shortage of water has led to polycultures and the polycultures bring extra benefits in terms of increased yield and efficiency in the use of land. You could call it a win-win solution.