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Fertilizing Rhododendrons

Rhododendron groupEarly to mid-springLate Spring / Early Summer*
Rhododendron - hybrids and deciduous azaleas
(small plants - up to 60 cm or 24 in)
50 g mineral compound fertilizer or controlled-release fertilizer50 g (1 :oz) mineral compound fertilizer
(large plants)100 g mineral compound fertilizer or controlled-release fertilizer70 g (2 2 oz) mineral compound fertilizer
Wild species and dwarf varieties10 g (a oz) mineral compound fertilizer or controlled-release fertilizer 
Japanese azaleas50 g (1 : oz) mineral compound fertilizer or controlled-release fertilizer*Disregard if controlled-release fertilizer was used for fertilizing in spring.

Sufficient fertilizing is very important for the health of your rhododendron plants because their abundant foliage means that they require large quantities of nutrients. Fertilizing increases the plants' ability to grow healthy-looking foliage and aids the development of plenty of buds.

Keep to the fertilizing periods stipulated (see fertilizing programme,above). Do not fertilize any more after the middle of summer

Symptoms of nutrient deficiency: Leaves that are light green all over, an increase in falling leaves, sparse development of buds and flowers, sparse growth and few new shoots.You can obtain the right kinds of fertilizer through the gardening trade. There are several different types, which vary in their composition and effect.

Mineral fertilizers

These are fast-working, have an immediate effect and are usually sold in the form of granules. Sprinkle them on the surface of the soil, in a circle round the shrub, with a diameter that corresponds to the spreading foliage of the shrub and about 20 cm (8 in) away from the stem. Then rake them in.

Mineral compound fertilizer contains all three of the essential main nutrients: nitrogen (chemical symbol N), phosphorous (P) and potassium (K). Also included are the essential trace elements in a balanced ratio. I have had good results with commercial preparations bought from garden centres. There are plenty of well-known brands to choose from.

Mineral, controlled-release, compound fertilizers have become much more popular over recent years. These long-term fertilizers are manufactured in such a way that they will release the nutrients (in particular, water-soluble nitrogen) into the soil very gradually. The advantage of this is that the fertilizing effect should last for up to six months, which means you will only need to fertilize once, in the spring. In addition, controlled-release fertilizer is environmentally friendly as it does not allow nitrogen to be washed into the deeper levels of the soil. There are various well-known brands on the market and you can find out more about these at your local garden centre.

Organic fertilizers

These may be plant or animal-based and will release nutrients quite slowly (particularly nitrogen). This means that their effect is long-term and therefore they should only be used for a main fertilizing in the spring.

Another useful organic fertilizer is well-decomposed stable manure, (but check the pH first)which can be distributed under your plants in a layer 1-3 cm (up to 1 in) thick, depending on the size of the plant.

Iron and magnesium-rich fertilizer

This is a remedy for acute iron or magnesium deficiency caused by a pH factor that is too high. Their effect will last for only a few months, then the procedure will have to be repeated. Check with your garden centre etc. for commercially available products.

As plants in large containers have to cope with a limited area for their rooting systems, regular fertilizing will be necessary. Having said that, fertilizing is a very simple business and generally means one dose of controlled release fertilizer per year or liquid feed in your watering can

A handful (about 50 g or 1 oz) sprinkled on the soil in mid to late spring will be sufficient. During the summer the nutrients will be released gradually and washed into the soil every time you water the plants. Japanese azaleas and dwarf rhododendrons should receive only a third of this recommended dose.

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