Almost all of this web site is designed for experienced growers and it occurred to me that a person new to rhododendron growing might stumble on this web site via Google and not find any information that would be meaningful to them. So what follows is an introduction to rhododendron culture.
Location: Most rhododendrons do best in light shade, especially in the afternoon. The north or east side of a building is best. A windy spot should be avoided as the leaves can be damaged by the wind, especially in the winter. Acid soil is required, a ph of from 5 to 5.8. This acid soil requirement restricts the growing of them to a limited area, mainly on the East and West coasts. A sign that the ph of the soil is wrong is if the leaves are a yellow-green or if the plant shows chlorosis where the veins of the leaf are green but the tissue between the veins is yellow. Certainly some people grow them in the mid west where the soil ph is 7 or higher but they are usually grown in raised peat beds or the soil is acidified for the plants.
Rhododendrons vary in their ability to survive very cold winters or very hot summers. Before purchasing a plant find out how “hardy” it is. In other words what is the lowest winter temperature the pant can take without being damaged or outright killed. Some rhododendrons can not do well in very hot summer climates. Just be sure that the plant you are getting will survive your weather. If you are getting a plant from a local nursery, the chances are that the nursery is only selling plants that will do well in your area. Buying by mail is very risky as a plant from a nursery far from your garden might be selling plants hardy for their location, not yours.
Water drainage is also very important. The soil must drain water. The plants can not grow in sodden soil. If after a rain store water puddles in an area you can not grow rhododendrons in that area.
Soil: In addition to acid soil, the soil must have a large amount of organic matter in it. This can be composted leaves or pine needles or purchased peat moss. High organic matter soil provides many features that the plant’s roots need. The open, lightness of a high organic soil allows the soil to hold air which the rhododendron’s roots need. The organic matter act as a buffer in the soil, absorbing fertilizer then slowly releasing it back to the plant. It helps to prevent over fertilizing of the plants. Because the organic matter can hold water, hut not exclude air, it has a large ‘mass’ and thus prevents rapid temperature changes in the soil.
Mulch: Probably the most important cultural practice in growing rhododendrons is providing a natural mulch on top of the soil around the plant. Leaves, pine needles and wood chips are perfect mulches. The mulch should NEVER be removed. No “spring clean up” where all the fallen leaves are raked out from under the plant and NEVER cultivate or scratch the soil at the base of the plant. The roots are right under of rotting mulch. When you remove the mulch or scratch ted soil you inadvertently damage the roots. Peat moss is a soil additive, not a mulch.
Fertilizer: Yes, you should fertilize rhododendrons. I apply a 50% organic, high nitrogen fertilizer in late February or very early March. It should be sprinkled on top of the mulch. Rain and snow will bring it to the roots. Applying it this late in the winter gives it time to reach the roots and have the plant absorb it for the spring growth. The “50% organic” means that half of the nitrogen is not available to the plant until micro organisms in the soil convert it to a usable form for the plant. The plants should not be fertilized after May 1st as it will encourage the plant to grow too late in the summer and be hardened off for winter. The amount of fertilizer depends on the size of the plant. A cup full for a 6 foot high plant all the way down to a 1/4 cup for a 2 foot high plant.
Planting: Look at this very important video:
Yes I know it is shocking but you must tear apart the roots as shown in the video if you want to be successful. Two things were left out at the end of the video: you must place the mulch around the base of the plant immediately after planting and you MUST water the plant immediately after planting. During the late spring and summer the plant will get almost no water from the surrounding mass of garden soil so you MUST water the plant with a hose at the base of the plant every week or 10 days depending how hot the weather is even if you have rain during the week. This watering must be done until September. The canopy of leaves shunts rain and sprinkled water out beyond the root zone and none of that water will reach the roots. It is very common for these newly planted rhododendrons to wilt a week or two after planting. If the plant wilts it will generally recover if you water it but even if it recovers there has been damage done to the roots. Several wiltings over the summer will probably cause to plant to abort many leaves and greatly weaken it going into the winter.
General Maintenance: Over the year there isn’t to much to do to maintain the plant. After the petals drop in the spring the foundation of the flower is left on the plant. This, if left on the plant, will grow into seed pods and take much of the energy from the plant and thus can cause a lack of flowers next year as the flower buds are formed in the summer for next year. Removing this flower foundation as soon as the petals drop will allow the plant to put all its energy into next years flower buds. This is called “dead heading”. On an old, large plant this can be a daunting task. You can help to overcome this problem by increasing the amount of fertilizer you feed the plant so the plant has enough energy to produce both seed and flower buds over the summer. It is much preferred to dead head the plant.
Insect Problems: Insects can attack the leaves, stems and roots. Leaves can attacked by caterpillars, lace wing flies and weevils all of which can be controlled with insecticides. Only apply insecticides when you see damage on the plant.