For most of its life, a container plant will depend on you to fertilize it in the growing season to ensure a good supply of flowers and strong growth.
It’s worth noting that a potting mix’s nutrient content will be washed out and used up after six weeks in the case of soilless mix, and after eight to ten weeks with the soil-based kind. Thereafter, fertilize at fixed intervals, or add a slow-release fertilizer. These small round pellets (like tiny eggs) are added to the potting mix in the spring. Lasting about a season, they absorb moisture and release it with the fertilizer.
Plants need a balanced supply of nutrients to grow, flower, and develop a strong root system, but this boost can be given only when they are actively growing. It is also crucial to follow the manufacturer’s instructions. Overfertilizing won’t produce bigger, stronger plants—in fact, a surge of excess chemicals can be incredibly damaging. If you are not using slow release pellets (or similar treatments), try a fertilizer that comes in liquid or powdered form (to be dissolved) that is applied on watering. The main foods are nitrogen (N), which promotes good top growth, phosphorous (P) for healthy roots, and potassium (K) for abundant fruit and flowers. The relative amounts are usually shown on the packet—for example, "NPK 6:4:4."
A regular high-potash (potassium) fertilizer is essential when growing the likes of tomatoes and dahlias, and plants in hanging baskets, for a good crop of fruit and flower buds (it’s even good at ripening wood). It is usually applied from the moment the first buds appear, but don’t use it too early in the season, when it’s essential that the plant builds up a good, all-around structure with plenty of new shoot and root growth.
Most plants initially benefit from a balanced fertilizer to build them up, before a fertilizer aimed at specific needs is applied. A high-nitrogen fertilizer boosts leaf growth, and some plants, such as bougainvillea, benefit from this before being given a high-potash fertilizer to promote extra flowers. Plants grown for their foliage, such as hostas and coleus (Solenostemon), need a nitrogen fertilizer in summer to promote a terrific display of leaves.
Some plants, such as azaleas, camellias, kalmias, and rhododendrons, hate alkaline conditions and need to be grown in a special acidic (ericaceous) potting mix. Widely available, ericaceous potting mix has a pH of 6.5 or less (the pH denotes the degree of acidity or alkalinity in the potting mix). Such plants are usually clearly labeled. When watering, ideally use rainwater or, if that’s impossible, cold, boiled water.
Every other year at the start of the growing season (depending on the rate of growth), permanent container plants will need rejuvenating, or they quickly decline. This involves potting them on, top-dressing, or repotting. Potting on means moving the plant up to a larger container with new potting mix to provide more room for root growth. With top-dressing, the top inch of potting mix is removed and replaced with a new layer. Repotting means taking the plant out of its pot, shaking off old soil, teasing out the roots, and adding new potting mix before putting the plant back in the same pot.
Excerpted from Simple Steps: Containers for Patios
©Dorling Kindersley Limited 2007