Plant Spacing Secrets

Plant Spacing

If you are growing vegetables to feed your family, you want to get as much from as small a space with as little effort as possible.  Most gardening books tell you how to space the plants – but the thing is they disagree – and they’re nearly all over generous with their spacing.

Once upon a time we had a really excellent research structure for horticulture in the UK – and they did all the research that we need. They managed to establish the principles of vegetable-crop spacing and worked out how to control the yield per unit area and the size of the individual bulbs, roots and hearts.

All gardening is an attempt to control the conditions under which our chosen plants grow. But unlike weeds or pests or weather or soil – we CAN actually control crop spacing.

This means that we can control yield. If we take a space and put more and more plants in, the yield will go up until they start to compete with each other. Eventually the yield will stabilise and if the density goes up the yield will start to go down as we get more and more tiny plants.  The research has been done to establish the maximum yields for all common vegetables.

However not all plants behave the same way. Carrots for instance hold their maximum yields and all that happens is you get smaller roots as the density goes up. Onions have a minor loss but nothing that we can’t live with.

However other plants don’t work so well – while the amount of vegetation produced per acre stays at a maximum, the bit we want goes down – red-beet for instance.  In other case the maximum yield gives a vegetable that’s of a size that’s not useful.

Most gardening books are written by professional gardeners who, aiming at shows, believe that big is beautiful.  Commercial growers  follow the needs of the cook who wants smaller things. However, Onions give maximum yield at 32mm bulbs – too big for pickling but needing too much peeling to be practical for cooking. So commercial growers aim at 38mm taking a hit on the yield. If you want big onions you take a bigger hit on the yield.

Now let’s think about planting layout. In the soil the water and nutrients are more or less evenly distributed. As we said earlier, plants are capitalists and compete for what’s there.  So plants grow better if evenly distributed.  Tradtionally, however, we grow them in rows which flies in the face of the common sense view that growing on a triangular, equispaced grid would give the best results.

history A bit of History.

Before the agrarian revolution, vegetables were broadcast on 4 foot strips ( rather like we use in our glasshouses) and then weeded by cheap labour to give a uniform distribution. When the cheap labour disappeared into factories, then they went over to a field based system which depended on horse hoeing between the rows to keep the weeds down.

If you relied on weedkillers  to control weeds you could in fact manage a staggered planting.  However growing organically as we do,  weed control is a real problem. If we grow through mipex as we do with the larger plants such as peppers, aubergines, big brassicas and tomatoes, then a staggered pattern is possible.

The limiting factor when growing commercially however is the time taken to harvest. This means that even if you are growing a crop like chard or other cut and come again salad leaves, it’s more convenient to grow in rows, even if that impacts the theoretical yield.  However if you are growing in an intensive space like a raised bed in a small back garden, then you will want to pack the plants in for maximum yield.

Weeds within the row are still a real problem – even if we get rid of the ones between the rows .  With onions, weeds in the row lead to a crop write off as the onion leaves have no smothering power. With red beet for instance it’s different.  If weed free represents 100% yield, then weeds between the rows gives a 78% yield and weeds between the rows where the weeds are not so smothered gives only a  64% yield.

We know for instance, that if an evenly distributed crop of carrots is kept clean until it has two true leaves then the crop itself will smother all subsequent growth of annual weeds.  Crops with broad leaves can be used for weed control if they can rapidly and completely shadow the soil surface if diligent weeding occurs up to that time.  If you are growing organically you will have to rely on hoeing between the rows. Hoe shallowly and organise the spacing of the rows to facilitate the process.

Principles to remember.

Population is how many per square yard or metre. Ideally we want to use the spacing that gives maximum yield but we may need to reduce it a bit if it’s an inconvenient size. Some plants have a wide range of populations at maximum yield so we can control size by spacing quite effectively.

Pattern should be as close to an even distribution as possible allowing for access for hoeing or for convenience of harvest.  We are assuming here that we are in an environment like the UK where nutrients and water are not a limiting factor. In desert conditions wide spacing to give good root growth is essential.

Planting spacing and timing is critical for plants grown from seed to over go the winter.

To get rapid emergence and consistent size, onions must be sown be first week in August in North of England and Middle of the month in the South. If the plants are too small when the winter starts many will die. If they are too big, many will go to seed. You will need to have  65-86 plants per sq m in the spring so sow 108 per sq m. For Spring sown crops plant at 86 / sq m in rows 23 cm apart.   For salad onions like white Lisbon 323 plants per square meter in 7.5 cm bands with 30cm between centres gives best results. This makes it easier to get the spacing right within the rows at a cost of extra weeding time. Carrots can also benefit from this approach.

French beans are found to give the highest yield commercially at a spacing of 32-43 plants per sq meter.  However this is from a single destructive mechanical harvest. If you want to pick them over, then 45 cm rows with 5 cm apart in the row is optimum. If you go over 10cm apart in the row yield suffers.

For Runners,  20 plants per sq meter gives the best yield – 91 cm paths with a 61 cm twin row gives a 12% higher yield than 122 cm path   and a 30cm twin row. Single plants at each cane gave the best yield. High populations delay the first pick by 3 days and the half life of the crop by 6 days.

22 plants per sq m for Broad Beans gives maximum yield with a single destructive harvest.  For peas, 64-86 plants per sq m give max yield for a single destructive harvest while 40 plants per sq m is best for picking over. For ease of picking run the rows across a 122 cm bed.

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