How to Grow Rhododendrons

Rhododendrons can be deciduous or evergreen, although many of the most popular garden varieties are evergreen. Rhododendrons are undemanding and easy to grow shrubs. Their only essential requirement is to be planted in an acid soil, a pH is 7.0 or below. The images show that Rhododendrons have showy blooms, which are attractive to bees as an excellent source of nectar. Rhododendrons and Azaleas will not grow in alkaline soil. This is not just a case of growing better in acid soil, Rhododendrons simply will not grow in any soil other than acid soil. Most Rhododendrons flower from March-May.

It is easy to be put off by Rhododendrons, which in recent years have had a bad press. It is correct that the common Rhododendron, known as pontic rhododendron, is a menace, and you can identify it from the image below. In the wild this variety of Rhododendron is invasive, out competes native plants and is to be avoided. More information about pontic rhododendron below.

However, Rhododendrons are a vast genus of over 900 in all shapes, sizes, and heights and a wide colour range: pink, purple, yellow, white, red and orange, just about everything. The colours are bright and bold; a Rhododendron in full bloom is eye-catching. Rhododendrons are a woodland plant and dappled shade is their native conditions. That said, Rhododendrons are tough and will grow anywhere in the right (acid) soil. If the growing position is very sunny, the smaller compact and deciduous varieties will do best. 

Because Rhododendrons are such a wide genus with many varieties, it is important to check the plant label for information about the eventual size. There are many on the market which are compact, especially the Azaleas.  

If you are thinking of growing Rhododendrons, but unsure of your soil type, you can do one of two things. You can buy a soil kit to test the Phd of your soil, which will tell you if it is acidic: a result of pH 7 below is acid soil, pH7 neutral and pH7+ alkaline soil. Alternately, you can check out your neighbour's garden and see what's growing there. If there are Rhododendrons and Azaleas in the neighbourhood, the chances are your soil will be similar.

Rhododendrons are toxic to dogs, even if ingested in small quantities. It may be prudent either to not plant if you have dogs, or fence off. There are other shrubs and plants toxic to dogs, a useful list from the Blue Cross.

Many rhododendrons are large and suitable for big gardens with large woodland areas. However, there are good compact varieties which offer lots of springtime colour, illustrated below.

Rhododendrons can also be grown as an excellent flowering evergreen hedge. Because Rhododendrons tolerate hard pruning, if it outgrows its space or gets too tall, cut it back as we did this hedge a couple of years later, more pruning information below.

Rhododendrons are spectacular as single shrub, or massed together and mix well with other shrubs. The image below is a bold spring combination of strong yellow Rhododendron Luteum, combined with the variegated Euonymus fortuneii 'Emerald Gold' and flowering Aquilegia 'Swan Lavender' which makes a really bright spring combination.

There are a few scented Rhododendrons which are often the deciduous varieties. R. Fragrantissimum which is hardy down to -10 and bears white flowers. R.Luteum which has garden merit award, it is a fully hardy deciduous azalea growing up to around 2.5m/8ft. R. Loderi has coral pink flowers, Dufthecke white, Inkarho is evergreen and fragrant with white flowers and finally Antelope which is a sun loving hardy deciduous azalea.  

If Rhododendron is not the shrub for you, check out shrubs and bushes; spring flowering shrubs; summer flowering shrubs; shrubs with autumn and winter interest; and evergreen shrubs.

Some Rhododendrons can get large over time and outgrow their space. Although a woody shrub, it is possible to hard prune Rhododendrons, even severely, and they will grow back. Initially, they may look a bit of an eyesore with bare wood, but fairly soon the Rhododendron will produce fresh growth. 

Rhododendrons respond well to pruning, and the best time to do this is mid summer after flowering. There is always a risk to the next season's flowers, but after that the shrub will pick up. The same applies if you hard prune, initially it will look decimated, as in the images below, but fresh growth appears and the shrub soon gets re-established. This is helpful if the shrub is too big, or overhanging a path or creating too much shade. Cut back how you want and it will regrow. The image left shows a Rhododendron after a hard prune, centre growing back and image right, there is no bare wood visible after a prune.

There is a view that Rhododendrons can look untidy after flowering and that dead heading encourages better buds. If you are a keen gardener, you can dead head after flowering, which is said to encourage better blooms the next year and certainly looks better, but be careful not to remove the embryo bud forming behind. The important point is to be clear about what is old bud and new bud. The images below illustrated, first left has the spent flower and the new bud; the second centre image shows removing the old bud and right the plant with just the new bud.