http://www.ottawahort.org
Growing Rhododendrons/Azaleas in Hardiness Zone 5
Yearbook 1993 page 28 By: Edeltraut Schmitz Back to the web version(Ms. Schmitz is co-owner of Artistic Landscape Designs Ltd.)
The growing of these beautiful plants is only for people who are enthusiastic gardeners and who are willing and have time to give the plants some extra care.
The American Rhododendron Society has established a system of Hardiness Rating: e.g., the symbol E-l indicates the hardiest varieties, which are tested to -32DC. Hardiness, however, refers not only to the coldest degree an established plant can survive; hardiness can also mean survival in hot, dry periods, or a combination of both. Very important is the duration of a severe period, the windchill factor, the adaptation to the soil condition (pH level), consistency of the soil, etc.
Site Requirements:
Choose a location sheltered from prevailing northwest winds. The best place to plant is on the east side of a dense planting (i.e. in front of spruces, pines, a wooden fence, a building, hedge, etc.). Some sun is essential for good bud development and compact growth; some shade during the hottest part of the day will allow blossoms to last longer and keep the roots cool and moist. The rhododendrons/azaleas are surface-rooting plants; hence they can be planted under deep-rooting trees such as oak, fruit-trees, larch, pine or spruce. These, by nature, do not produce many fibre roots. Do not plant near maple, ash, birch, lilac or cedar because these have an abundance of surface roots which would be in direct competition for food and water. Do not plant under an overhang or too close to a wall either - the situation is usually too dry and without adequate snow-cover in winter.
Soil Requirements:
The soil should be acid, ideally pH 4.5 to 5.5. It should have high humus content - a minimum of 50% peatmoss and other organic materials, e.g. composted oak-leaves, pinebark nuggets, pine-needles, Turface or Perlite. To increase the acidity of the soil, incorporate half a pound of ground sulphur into 100 square feet. Do not use aluminum sulphate, as it is known to be harmful to rhododendrons/azaleas. Never use mushroom compost or supersoil, since the salt content is too high for these plants.
The soil should be kept moist but never soggy. A layer of coarse sand, gravel or chunky pine bark above a clay soil can provide excellent drainage. If the subsoil is clay or hard pan, raise the planting bed at least 20-24 inches (60 cm).
Planting:
Most plants are sold in pots. Any pot or burlap should be removed. Before removing the plant from the pot, soak it in a pail of water. If the plant is pot-bound, inquire from the garden centre about the proper procedure. Set the root-ball one inch higher in the ground than it was in the pot; never plant it deeper than it had been growing. Backfill soil around the roots but do not pack too firmly. Make a little "saucer" of earth and allow the freely f10wing water to settle the soil around the roots. Apply a three inch mulch layer over the root system. Use pine-needles, oak-leaves or bark chips. Do not use peatmoss as a mulch, because it induces the plant to bring its roots higher (to the soil surface).
General Maintenance:
Keep the soil moist, especially during the growing months (May, June, July and August). After August IS, water moderately. Keep drier in September and October. Bark splitting could occur if lush growth freezes, but one heavy watering in November is beneficial. If there is too much rain in September and October (indicated by new tip growth on any plant) it is advisable to remove the mulch for some time but remember to replace it before freeze-up!
How well the plants survive the winter depends on their condition when the severe period hits. A plant which has lost many of its roots due to dryness has a hard time surviving the winter. A plant that was shaded by trees during summer and then is suddenly exposed to winter sun will dessicate readily. In such cases it is advisable to protect the plant with a screen each winter, not only the first few winters. Good maintenance depends on good observance.
Fertilizing:
This should be carried out in spring and early summer only. Nitrogen is better in a nitrate form and can be bought as ammonium sulphate. Most commercially available water-soluble fertilizers have the needed trace elements. Superphosphate can be fed right after petal fall to encourage good bud-setting. Avoid feeding nitrogen after the end of June. One feeding of Phostogen or potassium (last number on the fertilizer package label) around June 20 is of great help in hardening plants off.
Winter Protection:
Even the hardiest varieties benefit from protection during the first winter, since one cannot rely on snow cover.
Much more could be written, but space is limited. Many good books are on the market and any reliable garden centre should be able to supply good descriptions and assistance. My most important advice is this: do not buy unnamed plants since their hardiness is unknown.
Yours for better gardening,
Edeltraut Schmitz.
Please contact the OHS or the author if you wish to republish these articles.