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Azaleas and Rhododendrons

Photo that's half rhododendron and half azalea

Within the large genus of Rhododendron, azalea and rhododendron refer to two subgroups of shrubs prized for their showy flowers. While there are differences in flower details between the two subgroups, for northern gardeners the most important difference is more obvious: azaleas are deciduous (losing their leaves for winter), while rhododendrons are broadleaf evergreens.

Both subgroups have very shallow roots that tend to dry out quickly, and need consistently moist, organically rich, acidic, well-drained soil. Bark or pine needle mulch (2–4”) is recommended to retain moisture.

Above ground, they differ more. Azaleas need full to partial sun to flower well, while rhododendrons tolerate somewhat more shade. During winter the larger-leafed rhododendrons need protection from winter sun to prevent leaf burn; the smaller-leafed varieties (such as PJM) have more tolerance to winter sun.

Broadly speaking, the bloom sequence of hardy varieties can be ­generally categorized as follows for the Twin Cities area:

  • Early (late April into May): Small-leafed rhododendrons, ­including PJM, its relatives, and the Korean rhododendrons
  • Mid (May into June): U of M Lights series and Exbury azaleas and ­larger-leaved rhododendrons
  • Late (June–July): Weston series azalea hybrids
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