The soil or compost in which roots grow is a living media that provides the plant with moisture and nutrients.
Where plants are kept in containers it is vital to ensure the soil or compost is able to provide these ingredients – it needs to be managed.
In almost all situations container plants have a limited shelf life and planned replacement is the only realistic long term option. Two to three years in a pot is a good rule of thumb, and after that period the alternatives are either to transfer the plant to a larger pot with new
compost, or replace the plant and start again.
Due consideration of all of the following points will help to keep a potted plant healthy for as long as possible. A plant in a pot is not there for ever but while it is there it can be perfectly healthy!
Compost is made up of many components: mineral particles, microscopic in size, but important sources of potassium and phosphorus, vital to plant growth; organic matter which helps retain nitrogen and moisture; a population of micro-organisms which help growth and the release of nutrients: and finally air spaces that are equally important.
Plants in containers need regular watering. Weather conditions can create waterlogging so good drainage is very important. Pots in exposed situations – wind and sun – can dry out incredibly quickly. Use the squeeze test, see watering guidelines
Nutrients are vital to healthy plant growth and although often present at the time of planting, can run low in the subsequent seasons.The plant itself will absorb some but others can be leached out by over-watering. Organic matter retains nitrogen, but encourages the soil flora and fauna, which in turn decomposes the organic matter. In normal conditions this would be replenished by leaf fall.
Composts in pots therefore develop a higher and higher proportion of mineral matter, increasing density and reducing the air spaces. The pot becomes full of root and more irrigation is needed. In time the plant becomes sickly and will eventually die.This will depend on the vigour of the plant and the size of the pot.
Regular feeding will maintain the plant for several seasons – this may need doing every 2- 3 weeks with a good, well-balanced fertilizer throughout the growing season (eg Growmore and/or seaweed extract). The species of plant, type and size of pot, and the quality of the compost are all variables – take care with ericaceous plants.
Resist the temptation to top-up a pot with more compost on the surface where the rest has sunk, as this can lead to surface rooting and will make the plants more prone to drying out.
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