Q: The leaves of my rhododendrons look sickly, and there are few flowers. My soil tests neutral — is that the problem, and should I try to amend it? — Angela Rowe, Lagrange Park, Ill.
A: Wherever soil tests neutral to alkaline (i.e., yielding a pH reading higher than 7), rhododendrons will be hard to grow. They thrive in acidic soils (pH 6 and lower) that are light, well drained, and rich in organic matter. When the pH is too high, their leaves turn yellow. Your region has predominantly heavy clay soil with a pH of 7 and higher, and yet rhodies are flourishing only a few miles from you at both the Morton Arboretum and the Chicago Botanic Garden. There’s hope for your shrubs, but it doesn’t lie in trying to transform your native soil. Because the water in your area is neutral to slightly alkaline and clayey soil is hard to work, it’s nearly impossible to lower the pH either significantly or permanently by digging in organic matter (the conventional treatment). Your best bet is to mix a completely new medium for the shrubs to grow in and to pile it on top of the ground, as with a raised bed. Fortunately, rhododendrons have shallow roots and are therefore fairly easy to transplant.
At the Chicago Botanic Garden, an above-ground growing mix for rhododendrons is made from 50 percent well-rotted manure, 40 percent high-quality topsoil, and 10 percent shredded leaves. On top of that is spread a 5-inch layer of peat moss, and then it's all worked together. Morton Arboretum soil scientist Pat Kelsey recommends a mix that’s 80 percent organic matter (at least half of which should be slow-rotting composted bark, while the rest should be finer-textured compost, like leaf mold; mushroom compost is too rich). The remaining 20 percent of the mix should be washed builder’s sand. (Local sands are commonly alkaline if unwashed.) Either select a new site for your rhododendron bed or dig, lift, and set the plants to the side while you rebuild the old bed. Heap the growing mix in a mound at least 4 feet wide and about 12 inches high at the center, tapering to ground level at the edges. Plant the rhodies in the center of the mound. If you prefer a lower-profile bed, excavate 4 to 6 inches of the native soil. But beware of creating a basin that traps rainwater: rhodies must have good drainage. Avoid low-lying areas. After planting, mulch with 2 inches of composted bark or shredded leaves. Mulch again annually.
Exposure to winter winds and harsh western sun can scorch rhododendron leaves. Because cold weather kills their flower buds, less hardy varieties may continue to grow well but fail to bloom. Choose a location with an eastern or southern exposure: Rhododendrons need ample direct sun to flower well. Most important, provide a wind barrier — a hedgerow of hardy evergreens or dense deciduous shrubs or a screen of burlap stretched across wooden stakes. The rhododendrons that have proved hardiest in the Chicago area are small-leaved kinds similar to Rhododendron ‘PJM’. Recommended varieties are ‘Aglo,’ ‘Arctic Pearl,’ ‘Balta,’ ‘Laurie,’ ‘April Snow,’ ‘Llenroc,’ ‘Milestone,’ and ‘Molly Fordham.’ Among the hardiest large-leaved types are the Catawba hybrids, including ‘Elegans’. Now is a good time to visit public gardens to see which varieties appeal to you.