Plant roots eventually fill containers and this often reduces growth. This is not necessarily a bad thing as slightly stressed plants are often attractive and the slower growth reduces the maintenance needed. However, eventually the plant will need to be moved to a bigger container or the compost refreshed in the same pot, as composts lose their structure over time. Shrubs and trees that stay in a pot for years are especially vulnerable unless re-potted.
These steps will ensure success when re-potting into a larger container:
- When moving plants to a larger container, (one size larger at each stage), re-pot in early spring as soon as they show signs of growth
- Remove a little of the old compost, slide the plant out and tease out roots, cutting them if necessary
- When it is no longer convenient to repot them every year into a bigger pot, they should be repotted in the same pot at least every other year. Replace one-third of existing compost and roots with fresh compost
In years when re-potting is not carried out, topdress by removing 5cm (2in) old compost from the top of the pot and replacing with fresh compost.
Plants in containers need attention all year, but summer is the most critical period as plants can soon run short of water and nutrients.
- Check for moisture daily from April to September (twice daily in hot weather)
- Water thoroughly, filling the container to the rim and allowing it to drain, then filling it a second time to ensure that the whole of the compost is adequately moistened
- If water is not draining out freely, check the drainage holes for blockage and assess compost structure
- Lining or sealing terracotta pots with waterproof materials is unlikely to significantly reduce the need to water as most water is lost though plant leaves
- Grouping pots for mutual shade will reduce heat stress on pots
- Mulching pots will help reduce heating and suppress weeds, but as most water is lost through plant leaves careful watering will still be needed
- From April to the end of August use a general-purpose proprietary liquid feed or, preferably, a high-nitrogen feed
- Alternatively, add a controlled-release fertilizer at planting time
- With soil-less composts, make sure fertilizer includes essential trace elements
- After late summer feeding is usually suspended until mid-sping; however bedding plants and other short-lived annuals will still benefit from feeding until early autumn
- Feed when the compost is moist
In winter, the main danger is compost freezing, which may kill plants.
- Protect pots with bubble plastic or bring them under temporary cover
- In very wet periods move plants under temporary cover if the compost becomes sodden, until it has dried out a little. The ‘rain shadow’ of walls can be sufficient
- In wet weather, raise pots up off the ground on ‘feet’ or similar to keep the bottom of the pot out of the water
- Remove saucers in winter
- Watering may still be necessary for conifers and other evergreens, especially if you have moved them under cover, so that they recieve no rain. Check evergreens and conifers at least weekly and water if needed
- Watering is seldom necessary for deciduous or other dormant plants
- Feeding is not necessary during the winter months
Over-watering is the most common cause of loss of container plants; watering should aim to keep the compost moist, never soggy and avoid alternating dryness and saturation.Plants grown in containers suffer from many of the same pests and diseases as when grown in beds and borders, such as aphids, algae, liverworts and moss and scale insects. Vine weevil and fungus gnats are particularly common pests of container-grown plants.