Crop Rotation Group
Miscellaneous
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Soil
Moist, well-drained soil enriched with plenty of compost or other organic matter, with an acidic pH below 6.0.
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Position
Part shade.
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Frost tolerant
Cold tolerance varies with cultivar and type of rhododendron grown. Many cultivars are hardy to -20°F (-29°C).
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Feeding
Boosting soil fertility results in bigger, better rhododendron flowers. Feed in spring and late summer with a shrub fertilizer blended for acid-loving plants. Keep the root zone mulched with an organic mulch year-round to keep the shallow roots moist.
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Companions
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Spacing
Single Plants: 4' 11" (1.50m) each way (minimum)
Rows: 4' 11" (1.50m) with 4' 11" (1.50m) row gap (minimum)
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Sow and Plant
Set out purchased plants in spring or early summer. Amend the planting hole with plenty of compost, leaf mold, or other acidic organic matter. Set the root ball high, so it is barely covered with soil. Disturb the roots as little as possible when transplanting rhododendrons. Water thoroughly, and then regularly through the first season. Cover the root zone with an organic mulch to keep the soil lightly moist at all times. Spacing varies with the cultivar grown. Even the smallest rhododendrons grow more than 3 feet (1 m) high and wide, so space them 5 feet (1.5 m) apart. Large rhododendrons are typically grown as single specimen plants. Check plant tags for a plant’s mature width. Smaller rhododendrons can be grown in a 14-inch (35 cm) pot for a year or so, but eventually will need to be transplanted to a garden.
Our Garden Planning Tool can produce a personalized calendar of when to sow, plant and harvest for your area.
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Notes
Rhododendrons are stately evergreen shrubs that adapt to partial shade, and put on a vibrant show of flowers in late spring to early summer. Transplant container-grown plants with care, and do not let the soil dry out. Rhodies sometime fail to transplant, but using smallish plants reduces risk of failure. Also see Azaleas, closely related plants that bloom earlier than rhododendrons. Technically, azalea is a type of rhododendron that blooms in spring, has funnel-shaped flowers, with stamens limited to five. Rhododendrons are larger plants that bloom in early summer, with dense flower clusters at the ends of branches, and thick, leathery leaves. A few cultivars like ‘PJM’ show characteristics of both groups. Rhododendrons seldom need pruning, but any trimming should be done in summer, after the plants bloom. Rhododendrons bloom only on old wood, so winter or spring pruning removes latent buds.
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Harvesting
A single rhododendron stem makes a dazzling and long-lasting cut flower.
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Troubleshooting
Rhododendrons need moist soil, but can develop root rot problems when water is excessive. Lace bugs or whiteflies can be controlled with oil sprays. Avoid planting R. ponticum, a spreading rhododendron that is invasive in many parts of Europe.
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