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Updated 8 hours ago
Question: When should you fertilize azaleas and rhododendrons? Is it better to do it in the spring or the fall? What happens if you fertilize in the winter?
Answer: Azaleas and rhododendrons are very common landscape plants here in Pennsylvania, probably due to their evergreen nature and beautiful flowers. They make terrific foundation plants, and when grown in the right conditions, they're easy to care for.
In their native habitats, both azaleas and rhododendrons are understory shrubs, meaning they much prefer full to partial shade conditions, especially during the hot afternoon hours. They are not a good choice for areas of the landscape that receive full sun. When planted in full sun, azaleas and rhododendrons often suffer from infestations of an insect pest called lacebug. To grow the best azaleas and rhododendrons, site them properly.
Another thing these two plants have in common is their need for acidic soil. Azaleas and rhododendrons grow best in woodland areas with a soil pH between 4.5 and 5.5, so even when you're growing them as a landscape plant, this pH requirement is necessary. If your soil's pH is not optimal, these two plants will show it with leaves that turn yellow between their leaf veins (called interveinal chlorosis), slow growth, and reduced flowering.
Azaleas and rhododendrons, along with most other evergreens, require an acidic soil because an acidic pH level increases the availability of iron in the soil, a nutrient evergreen plants need a lot of to support their year-round foliage. Because of this, before fertilizing any azaleas and rhododendrons, I always recommend getting a soil test (available from the Penn State Extension Service; 724-837-1402 in Westmoreland County and 412-482-3476 in Allegheny County) to make sure the soil pH is optimum. The plants may not need to be fertilized; you may only need to adjust the pH to make them happy and healthy. Typically, elemental sulfur or ferrous sulfate is used to adjust the pH, but how much you'll need to use is dependant on the existing pH of your soil.
If you test your soil and the pH is optimal, and you're growing these plants in well-drained soil that's high in organic matter, fertilization is seldom necessary. But, if soil test results come back informing you that your rhododendrons and azaleas do, in fact, need to be fertilized, choose an acid-specific granular fertilizer, such as HollyTone, to do the job.
Fertilization of these plants is best done in the early spring, just before they flower and enter a period of active growth.
Fertilizing in the late summer or autumn generates new, late-season growth that's susceptible to damage from freezing winter temperatures because it does not have time to harden-off before frosts arrive.
Winter fertilization should be avoided as well. Since the plant is not in an active state of growth during the winter, most of the nutrients are likely to run off before they can be absorbed by the plant the following spring. It would likely be a waste of your money and time.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to firstname.lastname@example.org or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.
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