Rhododendrons: planting and feeding

The best time to plant rhododendrons is either in spring, from early April to mid-May, as soon as soil warms up, or in autumn, from early September to early October, so that plants would still have time to root the ground freezes over. Rhododendrons can be planted any other time, if need be, except when in flower or when new shoots are growing fast, which is usually after, and sometimes during, the flowering period. Potted rhododendrons can be planted any time during growing season. Even mature rhododendron bushes tolerate replanting well. Their roots suffer hardly any damage, being compact and shallow-rooted, which is rather an exceptional quality, comparing to other plants. Before moving rhododendrons, water them well, which will make replanting easier, and the plants will not be subjected to the moisture shortage.

Having replanted the bushes, water them thoroughly. If the rhododendron has set many flower buds, it is best to remove some of them, so that the plant would not waste valuable nutrients on flowering before it gets established.

Large flowers of rhododendrons come in abundance, so a generous supply of nutrients is important to keep up their vigour. Both organic and mineral fertilisers can be used on young and adult plants alike. Ideally, a well-balanced fertilizer should contain main elements N:P:K in ratio of 3:1:2. Nitrogen (N) is essential in spring and early summer, when plants are growing fast. Phosphorus (P) is required for flowering and for the flower bud setting. Potash (K) is necessary throughout the growing season, since it aids absorption of other nutrients. Phosphorus and potassium also play a major role in reproduction of plants and seed setting. Due to these elements, shoots grow faster and harden off in due time to withstand the winter freeze.

The best organic fertilizer is well-rotted manure. In addition to feeding plants, it improves soil structure, making it crumbly and well-drained. Mulching plants will also help to replenish the soil with organic matter.

Mineral fertilisers are mostly synthetic. They provide rhododendrons with macro- and microelements. Plants require considerable quantities of macroelements (nitrogen, phosphorus, potassium, magnesium) and only traces of microlemements (boron, copper, iron, manganese, molybdenum and zinc), but the latter ones are very important. The well-balanced complete fertiliser containing all required microelements is ideal for the purpose. Rhododendrons, [banner] grown on acid soil, absorb minerals much better. Larger quantities of microlements could poison plants grown on alkaline soil. Acid fertilisers – nitrogen sulfate, superphosphate, potassium sulphate, magnesium sulphate – are preferable, and, of course, complete fertilisers balanced especially for rhododendrons, both in liquid and granular forms. Granular fertilisers generally are slow-release type, and are used once during the growing season. Liquid feeding can be applied regularly, every 10-14 days. Mineral fertilisers, containing chlorine, are best avoided, since they prevent absorption of the other nutrients. The vigour of the plants, the abundance of blooms, the colour and the size of the leaves are the tell-tale signs whether the plants are getting sufficient nutrients. But bear in mind, that too much of fertilizers is just as bad as too little of them.

The best time to apply slow-release fertiliser is in early spring (March-April), while liquid feeding should stop in mid- to late July. If you continue to fertilise plants later than this, especially in damp and warm weather, rhododendrons will respond with a new flush of growth, which will not have sufficient time to harden off before onset of winter, and will suffer winter damage.

© Mygarden.lt, 2009

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