Rhododendrons need extra attention to thrive in Chicago

May 10--I would like some tips on how to improve the health of my rhododendrons. They were planted three years ago and seem to be declining now.

-- Robert Hansen, Highland Park

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Most rhododendrons struggle in the typical neutral to alkaline soils in the Chicago area. They perform well in acidic soils (pH of 6.0 and lower) that are light, well-drained and rich in organic matter. Soils that are heavy clay and poorly drained will cause problems for rhododendrons. These plants have very fine roots, which require a consistent supply of oxygen, so standing water or water logged soils will drown them and lead to fatal root rot. They also do not like to dry out, so they need to be monitored during hot and dry periods. Rhododendrons that have leaves turning yellow with weak growth are probably suffering from poor soil conditions. Regular applications of sulfur in spring and fall can benefit rhododendrons that are growing in well-drained but alkaline soils by lowering the soil pH. A planting bed of rhododendrons and other acid-loving plants at the Garden is treated with sulfur at the rate of 4 pounds per 100 square feet in spring and fall. This is a very high rate of application that needs to be carefully done to avoid damaging other plants. A more conservative sulfur program would be to apply 2 pounds of sulfur per 100 square feet in spring and fall. Wear gloves and avoid getting any dust from the sulfur in your eyes when applying. Use pine bark mulch.

Deadhead your rhododendrons by pinching off or pruning the spent flowers as the color fades away. Removing spent flowers can help improve the next year's flower display. Be careful of new growth that may be beginning at the base of the flower clusters -- the later you deadhead, the more growth there will be. The new growth will be light green, sticky shoots that are easy to break off. Pinch off one-half of this new, green growth when it is at least 1 inch in length to increase flower production for the following year. It is best to monitor the plants' growth to determine the best time to pinch, as the weather each spring is different and will affect the plants' growth.

While you are deadheading the plants, prune out dead wood and gently shape as needed by cutting back any errant branches. In general, I find that rhododendrons and azaleas in the Chicago area do not need a lot of pruning to keep them looking nice. A good time to prune them is shortly after they have finished blooming. When pruning back evergreen rhododendrons, cut the branches to a point just above a lateral branch or just above a whorl of leaves. Azaleas, which are classified as rhododendrons, are treated similarly with my preference being to make cuts just above a lateral branch. You may need to cut above a lateral bud in some cases. These spring flowering plants form flower buds during the summer or fall. If you prune in the late summer, fall or winter, you may be trimming off the flowers for the following spring. One of the most adaptable and easiest of the genus to grow in the Chicago area is rhododendron PJM.

Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe.

If you have plant or gardening questions, contact the Chicago Botanic Garden's Plant Information Service at 847-835-0972.

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