I moved into a townhouse a couple of years ago where about 20-30 rhodos were used for landscaping. In front of my unit there are about 15, struggling it seems, in almost full summer sun. They are quite woody, and leaves on some have yellow edges or yellow veins. I want to help these guys and am planning on giving them a good top dressing. Any advice on what to top dress with (compost, fish compost, top soil/compost mix??? I imagine manure is too strong even in a blend). Also what else to do to help them. It seems the soil might not contain the right nutrients or be too basic. Thanks
Think bark mulch would help, maybe about 2-3" deep spread from near, not up against, the trunks and to the outside of the dripline, a foot or so anyways.
If they are anemic it may help to add some minerals. Basic Rhodo food may do the trick or consider adding something like kelp or seaweed fertilizer to begin to benefit the soil. Without a soil test its impossible to give an accurate diagnosis or treatment method.
If this is a condo with landscape maintenance hired out the contractor may be using preemergent herbicide which is causing the yellow leaf edges. Otherwise, common afflictions of rhododendrons in this region are inadequate irrigation, impeded drainage, root and stem rots, mildew and infertility - plants of all kinds (rhododendrons and others) not showing an assertive leaf coloring may be seriously deficient in nitrogen or another nutrient. The mildew often causes yellowish spots or patches on the upper sides of the leaves, with corresponding reddish and dusty-looking areas beneath. If you look closely or use magnification it can be seen that the "dust' is a fungal growth (the mildew). Infested plants lose leaves prematurely and in the worst cases become denuded and die. Different varieties of rhododendrons vary in susceptibility.
Generally , the large leaved rhodos don't do as well "in almost full summer sun" as the smaller leaved rhodos and the azaleas.