Good drainage is one of the most important factors in successful container gardening. To facilitate this pots must always have drainage holes. Should be raised off the ground to allow excess water to drain out of them. You can use bricks for larger pots or terracotta ‘feet’ for smaller ones. Very large containers that you wish to move around the garden can be put on a slatted wooden base with wheels. This allows drainage and enables the pot to be moved without difficulty.
When you buy a container, always check it has enough drainage holes. They are marked on many plastic troughs, but not pushed out. This can easily be done with any sharp instrument. Holes of 0.5-1 cm every 10-12 cm/4-5 in are probably adequate for most containers. If you are using a recycled object, like a tin bucket, remember you must make drainage holes in it before planting it up.
You must also ensure that the chosen site is strong enough to support the container. Remember that it will be much heavier when it is full of moist soil and may be unstable if it is in an exposed area. Also be careful when positioning and ensure that the container is not dripped on by gutters or under overhanging trees or other plants.
Large containers are best planted in situ, whereas smaller ones can be filled on a table or workbench. If you are planting up a plastic container that will sit inside something else, always plant it up inside the other pot as the sides of the plastic pot will sag outwards when it is full of soil. This is particularly important for window boxes and long troughs.
If you wish to insulate terracotta pots, you need to line them with polythene at this stage, making sufficient drainage holes in the bottom. Otherwise you can use strips of polystyrene or spray-on insulating foam, which will do the same job.
First, put a layer of stones or broken crocks into the container. This layer must be deep enough to allow the water to soak through without allowing the soil to clog up the drainage holes.
The depth will vary according to the size of the container, but 2.5-5 cm/1-2 in is usually sufficient.
Next fill the container with soil. Always use fresh specialist or multi-purpose compost rather than garden soil. If you are only growing annuals and bulbs you may be able to reuse some of the compost but remove all the old roots and add at least half fresh. If you wish to improve the drainage, you can mix in up to a quarter perlite or vermiculite. This will also allow the roots to move with ease through the soil and will lessen the overall weight of the container.
Before you start putting in plants and bulbs, make sure that the soil in the container and around the plants is moist. It is very important that it is not bone dry, but equally it should not be waterlogged. When planting you can also mix in moisture-retaining granules, bone meal and slow-release fertilizer if you wish. The goodness in the compost will only last five to six weeks and after that you will have to provide food for the plants on a regular basis.
Plant up to within 1 cm of the rim of the container. The soil will sink down leaving about 2.5 cm/ 1 in at the top, which allows for a layer of mulch or grit. This looks attractive, prevents evaporation, stops weeds growing and deters slugs and snails. Even if you are not going to use a surface layer, always make sure there is a gap between the surface of the soil and the rim of the container as otherwise any water will simply run off, probably taking the soil with it.
After about the first six weeks it will be necessary to provide food as well as water for your container plants. A fortnightly feed of liquid seaweed during the growing time will often be sufficient. Tomato food encourages the growth of flowers and is effective when given to annuals. Always follow the instructions as too much food can do as much harm as too little. When feeding, it is important to check that the soil is moist throughout. If in any doubt, water well and feed the next day.
Either the container or plant may be unable to withstand cold weather. Bear in mind that some plants, like olive trees, can put up with low temperatures but not a combination of cold and damp. Plants can be protected with sacking or straw wrapped around them or, if necessary, they can be moved indoors. Often moving a container to a more sheltered spot may be sufficient. If you cannot do this and are worried about the container’s frost hardiness, wrap it in bubble wrap, sacking or newspaper when temperatures below freezing are forecast.