Seed trays, modules and pots
21.08.2023 - 12:00 / theunconventionalgardener.com / Emma Doughty
Transpiration and evaporation
Water is drawn into a plant via its roots. They take up water from the soil, and the most accessible source of water for them is the water held in capillary spaces – the small spaces within and between particles of organic matter.
How much water a soil or compost will hold is known as its water holding (or field) capacity. It is scientifically measured by waterlogging a soil and then allowing it to drain naturally via the force gravity exerts on the water. The amount of water left in the soil is the field capacity. Think of a sponge that has been drenched in water and then left on the side to drain, without squeezing. Eventually water stops leaking out, but the sponge is still wet. The water capacity of a compost depends on what it’s made from – the addition of sand makes a compost more free draining; a higher organic matter content increases water retention.
Plant roots push water upwards by creating root pressure, a small force that is enough to supply water to the whole of a low-growing plant. If you notice water droplets appearing at the tip of plant leaves in the morning (which aren’t dew) those are guttation droplets, a direct result of root pressure.
Larger plants need water to circulate right to the top, and root pressure isn’t enough to power that, but the water gets a lift from transpiration, which we met in Chapter 2. Water loss from the plant’s leaves pulls water up from lower down the plant, and transpiration is strong enough to pull water up to the top of the tallest tree. A plant’s water pumping system is completely silent – a feat of engineering we simply can’t match.
Plants also have to contend with evaporation, which is essentially the vaporization of water from soil or plant
Seed trays, modules and pots
When plants are grown in the soil they can send out roots, make friends with fungi, and source their own nutrients from their surroundings. In gardens we help them do this by improving and feeding the soil, a topic I will be returning to in chapter four. But when they’re confined in containers plants have a limited volume of soil and therefore a limited amount of nutrients to tap into.
Soil isn’t one thing, it’s a collection of different things that come together to make the life-giving, plant-growing ‘dirt’ that we love. We have a tendency to poison it, cover it over and generally forget that it’s there, but good soil is the heart of a good garden and something we should pay a lot more attention to.
An ideal seed compost is able to retain water, whilst at the same time letting excess water drain away to provide an environment that is damp but not waterlogged. It allows penetration of plant roots and is able to anchor plants, but has space for air. Its texture is consistent, and it is free from pests, diseases and weeds that would compete with the seedlings. As we have seen, it doesn’t need to contain many nutrients if seedlings are going to be pricked out; seedlings growing in modules will either need enough nutrients in the compost to support them through their first weeks of life, or suitable supplementary feeding.
There is a big trend at the moment in recycling containers to use in the garden (we’ve already touched on it with recycled food containers used for raising seedlings). There is also a large range of containers you can buy – from cheap plastic pots right through to enormous designer urns. What you choose is as much down to your budget as it is to your tastes, but all containers need to hold a suitable volume of potting compost and retain water whilst allowing any excess to drain away. If you are recycling containers to use for food plants then be sure that they’re clean and that they weren’t used to store anything toxic in their previous life. And remember that not all plastics are UV stable – some degrade when they’re exposed to sunlight.
When a seed sends out its first shoot and it rises above the soil level, germination is over and seedling development has begun. This is a particularly vulnerable time for the plant – it is running out of stored resources and needs to start collecting its own food. In this period of rapid growth it is also particularly at risk from pests and diseases.
When I set about blogging The Peat-Free Diet it was an experiment, an journey into the unknown. My aim was to provide gardeners who want to garden without the use of peat with the information they need to do so, and the book evolved into a gardening primer that assumed peat was not on the menu. My love of science made more of an appearance than I had anticipated and there are plenty of big words to cope with, but it is my hope that they are presented in such a way that they are not hard to swallow.
Potting on
The Pantry contains information about some of the items that are useful for a peat-free gardener, and gardening terms you may come across on your peat-free travels.
One of the big differences between now and the time before gardeners relied so much on peat-based composts is the rise in container growing. An army of modern amateur gardeners has to put up with small gardens, and possibly with no soil at all. Growing plants in containers allows us to garden wherever we like, and even to grow plants that would not thrive in our soil. Some plants are grown in containers to keep them under control; others so that they can be moved indoors in winter to ensure their survival.
Buying plants
There are no diseases that particularly single out plants grown in containers, with the exception of damping off – the fungal disease that affects seedlings, which we met in Chapter 2. As long as plants are kept well-watered and suitably fed (i.e. not stressed) then container culture should be very healthy, particularly if your potting compost was a sterile mix.