Containers: What You Need To Know
The only limit to growing in containers is your creativity. This style of growing is, obviously, particularly useful if you don’t have a garden. Your small balcony or window ledge can become a home for an array of containers, your own portable mini garden. Recycling makes sense: try old milk cartons, tins, wooden crates, colanders or pots in all shapes and sizes.
Here are a few basic principals to bear in mind:
What to grow
Not all plants thrive in containers. Some are more fussy than others, and the clue is often found in thinking about the natural habit of each plant. Carrots, for example, send down long roots. They will not thrive in a shallow window box. Potatoes can be grown in containers, as long as they are really big – up to dustbin size.
The container material
Think about the qualities of the material. Plastic, for example, conducts heat better than terracotta so can dry out roots or freeze them up in frosts. Terracotta soaks up water, so while the pots may look lovely they will dry out quicker than plastic. So think about lining them, to retain moisture. With wooden containers, you may need to treat them if you want them to last. Rub with a wood preserving oil, a good few coats. With a square container, you might want to support the corners to prevent warping with a brace of some sort. If you have containers on a roof terrace or balcony, then weight is obviously a consideration - hauling enormous terracotta pots up several flights of stairs is not for the faint-hearted. For a cheap but effective alternative, use large galvanised-metal dustbins with holes drilled in the base.
In general, the bigger the better. Small containers need more watering and feeding. You will get stunted growth if you put a large plant in a small container. Also, check its depth – this will determine what crops will happily grow. Salads and radishes have shallow roots, for example, whereas peas need around 20cm. Root crops, such as carrots, demand even more.
This is absolutely vital. You don’t want your compost (and therefore the roots) to get waterlogged. So your container needs to have holes in the bottom. Or just above the bottom on the sides of the container - this will allow for a small water reservoir which will help with watering. If you need to make holes, drill or use a nail and hammer. Then fill bottom of container with stuff to help with drainage: shards, stones, pebbles, sharp sand, gravel or small chunks of broken-up polystyrene.
Growing in containers is different to growing in soil. Worms and other micro and macro organisms do not get on in containers. So there is less biological life, and the plant is therefore more dependent on you to add nutrients and keep a general eye. So, do not fill containers with garden soil. Use multi-purpose compost, and choose one that contains loam - proper soil, that is, which helps to keep in moisture. John Innes, a well-known formula, is now available with no added peat. You could also mix a commercial multi-purpose with a third or so of top soil.
Plants in containers need more feeding. The nutrients in the compost are depleted after a few months. If using organic principals, this means liquid seaweed, nettle or comfrey feeds. You can also mix dried fertilisers such as dried comfrey or seaweed meal into pots. Every year, you may need to rake off and replace the top layer of compost.
A good layer of mulch will help to keep moisture in (and thus to extend the length of time between waterings), keep nutrients in and weeds down and look good. You can use almost any organic material - bark, wood-shavings, grass clippings, shells, stones - as a mulch, and the perk of having containers is that you generally need much less of it than you would on a conventional bed. And if you're using fresh wood shavings, then don't forget to stick on an extra dose of Blood, Fish & Bone - they deplete the nitrogen in the soil and will cause yellowing.
Containers dry out quickly. Keep a close eye on them. Prod the compost with your fingers to gauge how wet it is. When well watered, pick up the container. This way you can develop an instinct for its weight when watered.
Slugs need hiding places, so if you have pots it’s a good idea to move them away from sanctuaries such as walls with cracks. Grow bags offer excellent hiding places for slugs, so can be a bother in that respect.
Spare a thought for how your containers look. For example, if you snip a row of salad leaves in a window box you walk past every day, it may look rather sad and barren. Ugly pots can be wrapped. Plastic pots can be hidden in terracotta pots.
You can sink containers into the soil in your garden if you want to stop a plant from spreading. Mint, for example, is rather rampant.