There were several good shots at analyzing Friday’s unhappy rhododendron. Mature leaf size can be determined by light levels, as both Lisa B and Tom & Paul suggested. Moving a plant from a low to high light environment could cause this change in leaf size. This rhododendron hasn’t been recently transplanted, however, so we can eliminate light levels as a cause. (And there was no other impediment to light, such as the presence of shading plants.)
Lack of nitrogen was mentioned as well; but a lack of nitrogen would have resulted in chlorosis in newer leaves as well as smaller leaf size. In this case, the new leaves are not chlorotic. (The chlorosis on the older leaves is probably a phosphate-induced iron or manganese deficiency.)
Foy alluded to issues with water…and indeed that’s what I believe is happening with this rhododendron. Plants that exhibit smaller mature leaves in subsequent years are often limited by water. Full turgor is needed to force leaves to expand fully; without this physical pressure from inside, leaves fail to expand and once cell walls have lignified, leaf expanion ceases.
Lack of sufficient water during leaf expansion could be related to irrigation, though in our wet spring climate this is rarely a factor. More likely is a problem with the roots themselves. Definitive diagnosis would require digging up the plant to find out whether its roots are still encased in clay and burlap (my guess) or if something else is restricting their ability to grow beyond the planting hole.
Was a sample taken to check for phytophthora root rot. May explain why the plant is unable to take up water.
Anthony, there hasn’t been any material diagnosis, so I don’t know about that. However, this is a sloped site with well-drained soil. So I don’t think it’s an issue of waterlogging.
Linda, you made me look more closely at the Catawba rhody in our south-facing/sloping front yard, and now I know why those new leaves are so small! Last summer in this part of the world was wickedly dry, starting in late June; everything in that part of the yard is so tough, including the rhody, that it didn’t occur to me to give them a drink until mid-July.
Would the lignification without enough water also be the reason that we see drought problems manifesting in the landscape a year or two after the drought (xylem and phloem cells never getting too large during drought, and then lignifying at small size, thus affecting future growth)? Or am I making something up?
I’m late to this discussion, but I often see problems with rhododendron soon after new mulch has been applied over the roots, even with long established plants. I recommend that the ground beneath rhodos not be disturbed, and that if any mulch is added that it be a bare minimum.
Deb, in theory the water issue should be reduced every year as roots continue to establish. What I see is rhodies (and other shrubs and trees) whose roots are impaired that suffer increasing drought stress: their crowns grow, but their roots don’t keep up. Thus the drought stress only gets worse.
Dave, mulch over rhody roots is not an issue. I have several in my landscape and they are all well mulched with coarse organic mulch. In their native environment
as understory shrubs, they are naturally well mulched. There is a great deal of published research demonstrating the benefits of organic mulches specifically on rhododendrons. (You do want to avoid thick layers of fine textured mulches like sawdust and compost, however.)
Yow — now I’ll have to do a little digging around and see what the root situation is in that bed.