All potting composts contain a supply of essential plant foods, so newly purchased houseplants will continue to grow quite happily for some time provided that they are regularly watered. The amount of nitrogen, phosphate and potash in the growing compost, however, is limited and after a couple of months it will begin to run out. Indeed the plant food content of soil-less composts may well be exhausted in as lithe as five or six weeks. Regular feeding is therefore needed to keep the plant growing actively. Cacti are the one exception since they tolerate being short of food although their ability to flower may be reduced.
BASIC PLANT FOODS
The major plant foods are nitrogen, phosphate and potash. Each of these has its own special role in plant nutrition. Nitrogen encourages the growth of shoots and leaves whilst phosphate plays a dominant role in root development. Potash is important in the production of flowers but also helps to give sturdy shoot growth.
Trace elements In addition to these major nutrients, minute quantities of other elements are necessary to support healthy plant growth. Adequate supplies of these so-called trace elements are normally present in fertile soil whilst soil-less composts are artificially enriched with these key elements. Impoverished soil may need special feeding.
Various brands of specially prepared houseplant feeds are readily available. These are convenient to use and supply the basic nutritional needs of the plants if they are used as directed. Most are applied as liquid feeds but some slow release solid fertilizers are also sold for this purpose.
This is much the most popular way of supplying houseplants with essential nutrients. All you have to do is to prepare a dilute solution and then water it on to the compost. An alternative method of application, recommended for some products, is to add a few drops of the concentrate to the compost immediately before watering in the usual way. This approach is particularly economical when only a few plants have to be treated and it would be wasteful to make up a large volume of dilute solution.
Slow release fertilizers
These are available as fertilizer sticks or pills. They are simply pushed into the growing compost where they slowly release plant foods over a period of about a couple of months. They are certainly convenient to use but have the disadvantage that the nutrients are not evenly distributed throughout the compost. Also, unlike liquid feeds, these fertilizers are not immediately available for uptake by the plant roots. Another difficulty is that these slow release fertilizers may still be acting when the plant has gone into its resting phase.
WHEN TO FEED
Plants only benefit from feeding when they are growing actively. This means from spring to autumn in the case of most houseplants. Plants that make a lot of growth will need more than those of more restrained habit. Bromeliads and cacti do not need much food. Applying fertilizer to dormant plants is not only wasteful, but can lead to a build-up of unused chemicals in the compost which could affect root growth with consequent ill effects on the plant as a whole.