How Plants Grow

Plants need nutrients, space to grow, water and light.

In their natural environment plants are capitalists – they  compete for all of these resources and the ecosystem that this produces is called climax vegetation.

When we create a garden, we interfere with this natural order. However the principle to follow is that of “just enough structure”. We need to work with nature to get the best results but we need to be intelligent about it – so that we get more with less.

Plants are divided into roots and shoots. Even in the seed this division is present. Once the seed germinates the radicle elongates and goes into the ground while the shoot will elongate and push itself out of the ground. The 2 sides of the seed in most vegetable plants become what are called seed leaves and will quickly go green and start photosynthesis to make the sugar that’s the basis of much plant chemistry. That gives it a head start.

The goal of the plant is to reproduce itself. Most flowering plants do this sexually with pollen being carried from one to another by insects. Some plants, however reproduce vegetatively by producing fruits directly from axillary buds without flowering – figs and mulberries for instance are like this. Our aim in growing vegetables is to produce the conditions that favour the part of the plant we want to eat. However, plants are quite varied in their approach.

Some plants grow, flower and die in a year. Lettuce, Pak Choy, Rocket are examples. We need to keep these plants vegetative – this can be done by cutting and coming again or by playing tunes with the daylength as they are more likely to bolt when the days are long.

Other annuals like squash, courgette, tomato and beans we want to encourage to flower and fruit.

Other plants grow in one year to produce storage organs which will then sprout the next year. Carrots Turnips and Parsnips are obvious examples. Less obvious ones are Onions, Fennel and from certain points of view Broccoli. Again its important not to let these bolt in year one which some of them may do. These are biennials.

Finally perennials exist from year to year – fruit trees, asparagus, globe artichokes are examples. Permculture is very pro perennials because they reduce the amount of cultivation that the soil needs.

Plants are largely self sufficient in that they use photosynthesis to provide most of their food and structure from, literally, the zephyrs and sunbeams (well CO2 and sunlight at least). However they need some main additional nutrients to be supplied. The big three are nitrogen, phosphate and potassium (NPK) which is what conventional non organic growers use to supplement the plant’s growth. They also need various trace minerals – magnesium, calcium and sulphur are the ones needed in largest quantity but they also need some trace elements – iron, manganese, copper, zinc, boron, chlorine, molybdenum and nickel.

At one extreme, plants are grown in hydroponic systems which supply all the nutrients in solution and the plants grow in an artificial, totally controlled environment. At the other they are grown under an organic regime with a lively soil flora with lots of earthworms and soil fungi and a regular mulching with properly prepared compost.

What does science have to say? – well that depends on whether you buy into the reductionist approach or the soil ecology approach. If you take the view that the soil is just there to hold the plants up and we can just chuck in the nutrients that we know are deficient (a kind of outdoor hydroponics) then various things happen. You get run off of nitrates and phosphates which cause algal blooms and some health side effects – which is why we now have EU legislation to limit how much you fertiliser can be used. Typically agriculturalists are a bit trigger happy with the nitrogen bottle which means the plants grow too fast and sappy and are therefore prey to plant pests. The nutrients in compost tend to be released slowly over time so the plants grow in a more balanced way.

Another complication is how many of the trace minerals there are in the soil. Its reasonably straightforward to measure NPK but trace minerals need to be done professionally. You also have the complication of pH – ie how acid the soil is. At a pH of 6-7 most minerals are readily available. Most get held more tightly and are less available as the soil gets more acid – except for Iron, Manganese, Zinc, Copper and Cobalt which are most available in acid soil and are tightly bound above pH 7.

Over time soils tend to become more acid so its necessary to redress the balance with lime from time to time. Traditionally this goes in with the brassica (cabbage, broccoli etc) part of the rotation.

Plants get at nutrients through their roots which enter the soil particles. The nature of the soil can make a real difference too. Sandy soils drain freely but don’t have so many nutrients, clay soils have lots of nutrients but a very fine pore structure. This makes them prone to waterlogging and losing soil structure. Loams and silts are in between. The soil character is improved with the presence of organic matter – humus – which traditionally gets applied as compost. This allows for the presence of earthworms who improve fertility by improving the soil structure and moving the pH to the alkaline side.

So for a plant to grow well it needs ready access to nutrients in a slow release kind of way so it doesn’t get to much too soon and in soil which stays at a good level of moisture. If a plant dries out (water stress) this will push it toward fruiting (good for tomatoes, terrible for lettuce) and if it dries out enough the plant will be permanently stunted.

Two more points in this little article.

1) contrary to what the hydroponics and agribusiness boys believe, it’s actually true that no plant is an Island. Most plants (though not Brassicas) are associated with Mycorrhiza. These are beneficial soil fungi. Some live between the cells of the plant root and then extend into the soil as additional root hairs. others live in the soil but from a sheath round the plant root. The fungi take sugars from the plant that they can’t photosynthesise themselves. In return they are particularly good at phosphate uptake. Phosphate isn’t very soluble so that in a deficient soil the plant on its own would have to provide a much bigger root system (at the expense of other things the plant could be doing with materials that build the root) than is needed with the Mycorrhiza. Currently there is a lot of interest in commercially available Mycorrhiza – see separate article.

2) Having said all that, the ghastly truth is that plants are capitalists – they compete for nutrients. Thats why its often better to grow them on a triangular grid rather than in rows. It also means that its important to keep the weeds under control. You can do this mechanically by hoeing, by weed suppression using mipex as we do commercially or by spraying with herbicides. In traditional agribusiness you’re always trying to do away with labour which is why the first efforts into GM were to produced herbicide resistant spray so that Monsanto could

a) own the world’s food supply
b) sell lots of crop spray.

Its also worth noting that excess use of the nitrogen bottle causes plants to be prone to pest attack – and hence allow the agrichemistry industry to sell more pesticides.

When I was a young man this didn’t seem such a wonderful idea which is why I gave up Biochemistry as a way of earning my living. 😉

So I’ve told you that a plant left to its own devices will do what it has to do as long as it has access to enough water and nutrients to go through its life cycle.

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