Growing Rhododendrons and Azaleas

Growing Rhododendrons Successfully

Nicholas Yarmoshuk | June 11, 2004


Despite providing seemingly ideal conditions for their newly purchased Rhododendron and Azalea plants, many gardeners have found that their “well cared for” plantings start to languish in the second and third years of their growth and finally die in the third or fourth year. This experience has given Rhododendrons and Azaleas an unwarranted modern reputation as plants that are very difficult to grow.

This article describes a major contributor to this reputation, the commercially grown, pot bound plant. It suggests how to select commercially grown plants and how to grow them. These methods may lead to higher survival rates and less frustration.

Basic Conditions Required by Rhododendrons and Azaleas

Rhododendron and Azaleas growing in natural, open field conditions have a root mass of very fine, hair-like roots that grow in a thick mass very close to the surface of the surrounding soil. They tend to spread out in a circle to a radius of 2 to 3 feet from the center stalk of the plant. To survive and flourish, Rhododendrons and Azaleas continuously require air, moisture and nutrients at the roots. Healthy development of rhododendrons and azaleas requires that the following conditions be met.

  1. Soil should be loose and crumbly with a very high percentage of humus. A minimum of 50 percent extra- coarse, chunky, peat moss is recommended, plus other organic material. When amending existing soil, especially hard clay or very fine sandy soil, use equal parts of original soil, compost and coarse chunky peat moss.
  2. Oxygen in the root system is a vital factor in maintaining healthy rhododendrons.
  3. Soil must be consistently moist but never soggy. Adequate humus should hold enough moisture between rains, but watering may be necessary in drought conditions.
  4. Sharp drainage is critical. Rhododendrons are mountain plants, growing where there is always down grade for excess water to drain away.
  5. If existing garden soil is very heavy and/or poorly drained, keep the entire root system 6 to 8 inches above grade by creating a mound or berm, or a raised planting bed using a retaining curb such as logs, timbers or rocks.
  6. Some sun, or at least bright light is necessary to encourage bud formation and compact growth.
  7. Plants should be sheltered from the prevailing winter winds which desiccate the foliage.
  8. Ideally, shade should be provided during the hottest part of the day in summer.
  9. Shade in late February and through March, when the ground is frozen, may be important for some varieties as the strengthening sun may scorch the leaves.

Pot Bound Rhododendrons

Rhododendrons growing in open fields, Figure 1, have roots that are:

Figure 1. Rhododendron growing in open field

This is the entire root ball of a 20 year old Rhododendron. This root ball is 8 inches thick and approximately 4 feet wide. It was grown in a raised bed over a clay base.

Figure 2. A Rhododendron grown in a pot

Most rhododendrons & azaleas today are sold through garden centers and are grown in plastic pots.

When you take the rhododendron out of its container you may find white tip ends of roots or you may find a mass of white roots plastered against the inner side of the container.

This means that the plant has been in the container for a long time. Planted without corrective action the result will likely be that shown in Figure 3.

Figure 3. Three years after planting a pot bound Rhododendron

This is the likely result for a pot bound Rhododendron or Azalea 3 to 4 years after planting. There is: (1) no evidence of fine roots growing out of the root ball; (2) likely smell of rotting roots; (3) solid, non fibrous soil mass.

Symptoms of a languishing previously pot bound Rhododendron or Azalea are: (1) drooping leaves even while the soil is moist; (2) little or no flower production; (3) pale green leaves.

Enhancing the Chances of Success

Planting for Success

Wash the roots with the help of a nozzle and a strong jet of water.

Figure 4. Unpacking the roots of a pot bound plant

This nozzle has to be soft enough not to break the smallest roots and strong enough to brush away everything surrounding them to a depth of about 4 to 5 cm.

Tease out the roots from their bound condition and try to fluff them out.

The roots are at this time similar to your hairs after a shower; they are all together.

Keep the root ball in the shade for a few hours so that most of the most can drip of.

Place the root ball into a container that has moist, but not soggy course peat moss. Or surround the root ball with damp course peat moss on the ground in a shaded area.

Figure 5. Roots of a previously pot bound Rhododendron now unbound and ready to plant

Attaining this stage in opening up a root bound Rhododendron will enhance the probability of success of the plant 3 to 4 years after planting.

This root ball must be planted as described in detail in Figure 6 on the next page.

Do not let the fine roots become dry.

Do not plant Rhododendrons or Azaleas in clay soil. If your garden soil contains a great deal of clay, it is advisable to plant in a raised bed on the clay. In preparing the planting bed, 45-50 cm (18-20") is an adequate depth. Remember, the peatmoss and compost will decompose over time to ½ the original depth. Loosen the compacted garden soil, removing at least 10-20 cm (4-8"), especially if it is a clay soil. Dig in coarse chunky peat moss to improve drainage. The finished mound should be high enough to keep most of the root system above the grade level as shown below.

Figure 6. An ideal technique

Flood the plant in with a strong jet of water from a hose, in order to blend the soil from the root with the new planting medium. Allow the water to settle, then adjust the level of the root as necessary. Apply 8 cm (3") of mulch over the roots. Use pine needles, oak leaves, bark chips, or well moistened damp chunky peat moss.

Some growers recommend that a raised bed of some 12 to 18 inches (18-27 cms) be created using a mixture of 50% coarse chunky peat moss and 50% well rotted compost. One grower has quite successfully planted Rhododendrons and Azaleas on top of 12 to 18 inches of the chunky coarse peat moss/compost mixture and then back filled around the plant with the same material. He always waters the plant very well after planting and he feeds the rhododendrons with Peters Rhododendron Fertilizer. Feeding is always done in the early spring.

Growing Rhododendrons on clay soil is an insurmountable challenge.

Raised beds with lots of coarse chunky peat moss, well mixed with compost, with careful watering during hot dry periods will reward the grower with beautiful Spring colour.

Figure 7. One sure way to kill a Rhododendron or Azalea

The diagram in Figure 7 shows a sure way to KILL a rhododendron or azalea.

Rhododendrons and Azaleas are Surface Rooters

The fact that rhododendrons and azaleas are “surface rooters” (i.e. their roots grow just below the soil surface) makes certain cultural techniques essential. At planting time, for example, the upper surface of the root ball must be placed at the same level as the surrounding soil surface (or even several inches higher) so that the roots can continue to function normally; nor should too much pressure be applied in packing down the soil lest the roots be damaged. Later, weeds should be pulled by hand (or prevented by mulching), as even light hoeing will damage the roots; moisture must be supplied whenever the upper levels of the soil begin to dry out. There is one advantage to this surface rooting feature, however: even relatively large specimens can be moved with a minimum of damage since relocation does not require digging a deep root ball (see Figure 1).