What do you do if you have a garden full of soil with a pH factor in excess of 7, and somebody gives you a shrub like a rhododendron or azalea which loves acid soil? You could plant it and watch it fade away just to show your friend how silly he or she was. You could dig a large hole and line it with plastic, putting a few drainage holes in the bottom before filling it with ericaceous compost and planting your new shrub. Even this is not foolproof, because lime can leach into the compost and eventually cause damage, though it may take quite a long time. You could, of course go to the trouble and expense of building a raised bed to plant your new shrub in. Or you could take what to some people is the coward's way out and put it into a container.
A container is not a natural habitat for any plant. Plants in containers can be a lot of time-consuming work. Many of the troubles that afflict plants in the garden border (such as poor drainage, drought and shortage of nutrients,) are more likely to strike down plants in pots, tubs, troughs, window boxes and hanging baskets. Extra care when planting and special attention to their later needs is therefore essential.
Drainage is the most vital point. Your container must have plenty of drainage holes, large enough so that they will not become blocked easily; roots rot in waterlogged compost. Put some broken crocks, small stones or chippings in a layer over these holes, to prevent the growing compost from getting in to them and clogging them. It is a good idea to stand tubs and troughs on bricks, stones or other low supports, so as to keep the base of the container off the ground. This will allow water to drain freely. For the same reason, a window box on a sill should have strips of wood (or other low supports) underneath, in order to keep the drainage holes free.
Having sorted out the drainage, you now have to fill it. Don't use garden soil. It may be cheap but it is a false economy. It is probably of poor quality, it is likely to be heavy (and you have to move the container when it is full at some time or other), slow-draining and short of moisture-holding organic material, and so will be prone to waterlogging in wet weather and to drying out fast during hot spells. Your garden soil will almost certainly contain weed seeds, and possibly weed roots, which will cause problems later. And it is probably full of pests and disease spores which can wreak havoc in a container. So always use a prepared potting compost, which should be comparatively sterile, free draining and enriched with a balanced dose of plant foods. If you are going to plant fast growing things such as bulbs and bedding plants, a peat based compost is quite suitable. Permanent plants, such as shrubs and conifers, will generally do better with a soil-based compost, which will remain in good condition and retain a store of plant foods longer than peat. And do remember to use a lime free 'ericaceous' potting compost when growing lime hating plants, such as rhododendrons, azaleas, and camellias, in tubs.
Unfortunately, plants need water, and plants in containers need much more frequent watering in dry weather than plants in the open ground. Whenever the weather is hot or drought conditions exist, you should check them at least once a day. Small containers and hanging baskets dry out very quickly and may need watering twice a day. If they are allowed to become bone dry they will be difficult to remoisten. Watering with a can is not much good - the water runs down the outside of the shrunken compost and drains away. It is better to dunk the container in a bowl of water until the compost is well soaked. By the way, never hang a newly-planted basket in a sunny position straight away. Give it a soaking, and keep it in a shady place for at least a couple of days to let the plants settle in. And here is a handy hint. Place a saucer on the liner in the bottom of a wire basket before filling it with compost. This will act as a reservoir, and keep the compost moist for a little longer between waterings.
Unfortunately, regular watering washes plant nutrients out of the compost in containers quite quickly, so plants in containers also require more frequent doses of fertilizer than plants in garden borders. Not only that, a peat compost will lose its goodness much faster than a soil-based one. So give bedding displays in containers regular feeds throughout the growing season with a liquid fertilizer (a high potash feed, such as liquid tomato fertilizer, will promote flowering). Permanent plants like shrubs can be given a liquid feed to give them the occasional quick boost, but they will also need a couple of top dressings a year (in spring and autumn) of a longer lasting, solid, balanced fertilizer.
Happy Gardening.
written by Bill Hutchings
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page last updated 1 MAY 1998