Gardening Help FAQs

Here are answers to some of the most common questions we receive about garden plants. You will find concise information on general gardening techniques as well as plant selection and care. For detailed information on specific plant pests and problems refer to our Common Garden Pests and Problems page.

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Horticulture Questions and Answers

How do I grow azaleas and rhododendrons?

There are few plants with such spectacular floral display in the spring as the azaleas and rhododendrons. Azaleas and rhododendrons are cultivated all over the world and are represented by some 900 species of evergreen, semi-evergreen or deciduous shrubs. Only 30 of these species are native to North America. One of these, Rhododendron prinophyllum, (the mountain azalea), is a somewhat fragrant, deciduous Missouri native which is extremely hardy, as evidenced by it's northern distribution well into Canada.

Overall, the majority of species in this group prefer to growing in mild, humid climates with cool summers. This climate is characteristic of the southeastern Gulf, and Appalachian Mountains as well as the Pacific Northwest. Most Azaleas and rhododendrons are not well adapted to areas like St. Louis with our temperature extremes in summer and winter. However, breeding programs are continuing to produce more adaptable hybrids.

Because the Midwest is not a mild climate, plant performance will vary from year to year. Generally, the best success is when these plants are grown in protected areas away from prevailing southwest winds and direct afternoon sun. Wind protection can be enhanced by locating plants next to structures, (the east side is best), on north or east-facing slopes or, by planting them in the shelter of evergreens. Azaleas and rhododendrons are not shade plants despite their protective requirements, but neither do they like the intense sunlight of the early afternoon, mid-summer sun. For best results, choose a site which will deliver morning sun and avoid the common temptation of planting them in a southern exposure against the building foundation. This location becomes extremely hot in summer and faces the brunt of winter winds. If the tips of your azaleas or rhododendrons leaves turn brown, these are the two most common reasons why.

Azaleas and rhododendrons prefer acid soils (with a pH of 4.5 to 5.5), and soils high in organic matter. If you don't know your soil pH, you can take a sample of your soil to the Center for Home Gardening at the Botanical Garden to have the pH of your soil tested.

The planting hole should be dug 12 inches deep. Clay soils should be amended with plenty of Canadian peat, oak or pine leaf compost and sand or expanded clay. If your soil needs to be acidified, add agricultural sulfur to the soil. Applied at the rate of 2.5 pounds per 100 square feet, sulphur will lower the pH from 6.5 to 5.5. Iron sulfate can also be used. Ask for application rates when you have your soil pH tested. Check the pH each year to ensure that it remains in the preferred right range of acidity. Supplemental annual sulfur applications and acid reacting fertilizers like ammonium nitrate or ammonium sulfate will help keep the pH low.

Correct planting is also critical, because these plants do not grow well in boggy, wet soils. Under these conditions, many will be victims of Phytopthora root rot, a fungal disease of the roots and stem especially prevalent in wet sites. The best approach is to plant the soil ball half-way above ground level and then mound soil around the root ball. This practice keeps the plant high and will ensure good drainage. Follow with a 6-inch layer of oak or pine-leaf mulch and don't fertilize until May, after the flowers have finished blooming.

Rhododendron hybrids which have performed well at the Botanical Garden include: 'Roseum Elegans' (lavender-pink), 'English Roseum' (lavender), 'Nova Zembla' (red), 'Catawbiense Album' (white), and 'P.J.M.' (lavender-pink). Evergreen azaleas that have performed well include: 'Hino Crimson' (brilliant red), 'Hinodegiri' (bright red), and 'Purple Splendour' (deep violet). Deciduous azaleas include: 'Homebush', 'Gibraltar', and 'Fire Ball'.