Three of my last four columns have been about pests or invasive plants; how depressing! So I decided to write about something more enjoyable this month. Family friends recently moved into a house with a number of mature azalea plants and they asked for some advice on how to maintain them. This led me to review information on rhododendrons and azaleas and decided that would be a more pleasant topic for this month’s column.
Rhododendrons and azaleas are some of the first plants to bloom in the spring along with daffodils and crocuses; but there are two reasons to talk about them in the fall. The first is that late summer and early fall are great times to plant them for a beautiful bloom next spring. The second reason is that flower buds are formed the fall prior to spring blooming. For this reason the plants should not be pruned in the fall.
Azaleas and rhododendrons are colorful, showy woody plants that belong to the genus Rhododendron. There are over 800 species within the genus but not all are hardy in Minnesota. When we talk about rhododendron and azalea hardiness we are referring to the hardiness of the flower buds. Those that are hardy in Minnesota will do well with minimal attention, if they are planted carefully in the proper site.
Azaleas are deciduous and require full to partial sun. Azaleas bloom before they leaf out and often before the snow is completely melted away. Most rhododendrons are evergreen and can tolerate more shade. Evergreen rhododendrons do need some protection from the winter sun so that their leaves do not suffer winter burn. Both are very sensitive to extreme summer heat so they should not be planted with exposure to the hot southern or late afternoon western sun. The best location for them is where they can receive the eastern sun in the morning and early afternoon. Be sure to allow for plenty of room as some of the shrubs can get 6-8 ft. in height and width. Because of their shallow, fibrous roots they dry out easily and may require supplemental watering in the summer. For this reason they do best in sandy or loamy soil with good drainage. In moisture holding claylike soil they may develop root rot. Acidic soil with a PH of 4.0 to 5.5 is what they need to thrive. If your soil is not naturally acid add peat, sulfur or ferrous sulfate to the soil when planting and fertilize once or twice a year with an acidic fertilizer. Do not plant too deep, 4”-5” is sufficient, and back fill the hole with organic matter such as compost. In addition to adding nutrients to the soil, this will help with drainage.
Hallelujah! There are few pests that bother the rhododendrons and azalea; although we will probably find one in the future. In hot humid weather plants may develop powdery mildew, but while it may be unattractive it is not usually a threat to the plant’s health. The biggest enemy of rhododendrons and azaleas is a harsh winter which could possibly kill the flower buds, the least hardy part of the shrub. But if this should happen one winter it will not affect the vegetative growth or the ability of the plant to set buds for the next year.
The University of Minnesota recommends some rhododendrons and azaleas that are ‘bud hardy’ in our state.
Rhododendron P.J.M. – an evergreen rhododendron with small dark green leaves and lavender pink flowers which bloom in late April or early May and is bud hardy to -35°.
Rhododendron vasiyi (Pinkshell azalea) – pale pink flowers blooms in early to mid- May and the buds are hardy to -35° to -40°.
Rhododendron x kosteranum (mollis azalea) – blooms in late May with red, yellow or orange blossoms, the buds of which are hardy to -20° to -25°. This shrub can grow to between 6-8 ft.
Rhododendron prinophyllum (Roseshell azalea) – a smaller rhododendron at about 3 ft. and bud hardy to -40°. It blooms in late May with white to rose pink flowers.
In the early 1980s, the University developed, at the Landscape Arboretum, a series of Minnesota hardy azaleas called the Northern Lights™. All of the Northern Lights™ azaleas are hardy to -30° to -45° and come in a variety of flower colors.
- Pink Lights – grow to 8 ft. and has a sweet floral scent
- Rosy Lights – also grows to 8 ft. and bears an abundance of deep rose pink blossoms
- White Lights – sets many light pink buds which open to white flowers with yellow splotches. It is a little smaller at around 5 ft.
- Spicy Lights – hot salmon colored blossoms with a light aroma on a 6-8 ft. shrub
- Orchid Lights – This is a smaller variety in both shrub and flower. The 3 ft. plant produces 1 1/2” orchid colored flowers.
- Golden Lights – golden yellow blossoms show on a 4 ft. shrub which has a greater resistance to mildew than the other varieties.
- Northern Hi-Lights – the flowers on this 4-5 ft. shrub are a creamy white with a yellow upper petal.
My rhododendron usually blooms in late April and many years the blossoms have been snow covered. I watch it very closely because when it finally blooms I know that spring has arrived!