Garden Notes : Rhododendron time
By Abigail Higgins
Published: June 5, 2008
Welcome to June, the apex of the spring garden: roses, poppies, irises, flowering shrubs - the lot. The mature rhododendrons at the Polly Hill Arboretum defy belief. The colorful rhododendron and azalea collections are spectacular this year, along with clematis, peonies, and much more. They will impress just about anyone, not only plant lovers (who may purchase all plant material propagated and grown by the Arboretum at the now-open Visitor Center.) If you have friends or family visiting the Island in the next couple of weeks, make the Polly Hill Arboretum a stop on the tour of Island attractions.
My own rhododendrons and azaleas are a fairly pedestrian collection but for the most part look pretty good this spring. The exception is an Exbury azalea hybrid, the extravagant, flamboyant 'Gibraltar.' Though covered in clarion-call clusters of grapefruit-sized orange red flowers, it is plagued and disfigured by "apple galls," actually Exobasidium gall. I had mentioned these, in passing, in a previous column when I learned they are considered edible. While unsightly, the condition is not considered serious. Nevertheless, it is a symptom of something about my plant that must be unbalanced.
The Maryland Cooperative Extension web page on Exobasidium spp. corroborates this: "While very noticeable, these galls will not threaten the health of the plant. This problem is more common during cool and wet spring weather. The first symptoms are swollen or puffy portions on newly expanding leaves, shoots, buds, or flowers. The galls range in color from green to pink or red, depending on the part of the plant infected. As these galls age they develop a white surface growth which is a layer of reproductive spores. Eventually the infected tissue will turn brown and shrivel up into hard galls. Management: Fungicide sprays are not effective for the control of this disease. Prevention strategies involve hand picking the galls off before they develop the white surface growth to reduce the incidence of disease next season.
Cutworm problems
I have had heavy losses to cutworms this spring. It is said that cool springs, as with the Exobasidium above, favor their prevalence. But I now tend to think that all factors contribute, and in all years cutworms are a menace somewhere. Did I plant my cabbage seedlings in the wrong phase of the moon? Was I otherwise occupied, or just plain tired and lazy about checking after dark? How does one place individual collars on two seed rows of leeks?
I know that Bacillus thuringiensis controls chewing caterpillars, but I believed that the latest losses would be the end of it - ha ha. The end result: spending more time than I really have, sifting through the soil around the destroyed plants to find the culprits. When I spot, against the dull dirt, the pale sheen of the cutworm - belly distended with its meal of my seedling - it is an easy matter to make a collection for the hens. But in the end, revenge, though sweet, is more time-consuming than prevention.
Photo by Susan Safford
Defoliating caterpillars: Just a part of nature?
Island tree owners are probably holding their collective breath, hoping the caterpillars of gypsy moth, winter moth, and spring and fall cankerworm will pass over them. The weekly Landscape Message of UMass Extension umassgreeninfo.org mentions only a few hotspots for the Cape and Islands district. Caterpillar defoliation in Chilmark between the North Road and Vineyard Sound is occurring.
There is much we do not know or understand about "outbreak" insect cycles. Remember the first law of ecology: all things are interconnected. Ecological conflicts stem from dilemmas of control, or attempts at control. On the one hand it is a given that these inevitable cycles are complex parts of nature, whose own intricacy is never - cannot be - fully understood by humans. In future we can learn from the work of entomologists and ecologists how these interactions are triggered, or what elicits them. But, on the other hand, in the present there is an anguished homeowner with perhaps less than a handful of irreplaceable mature trees on the property, determined at all costs to protect them.
Except for areas of lawn, our place is "woodlot," firewood kept "on the hoof," where six, 60-plus year-old oaks have recently left us, victims of caterpillars but perhaps ultimately of their own maturity. In addition to these full-sized, mature oaks we have somewhere around a dozen smaller, dead-or-dying oaks and sassafras. A couple of them exhibit dieback of the tops or the characteristic trunk swellings typical of wood decay fungus infection, likely Ganoderma or Fomes.
These symptoms of slow-acting fungal infection were evident prior to the caterpillar infestations of the past three seasons, due perhaps to how thickly the trees are growing on thin soil. It is no surprise that the trees' death has been hastened by the caterpillar defoliation. But the point is that there is always something at work in woodlands to carry on the processes of death and renewal:
A tree is harmed, perhaps by lightning or wind. Insects or fungi gain entrance to its body and create housing or food for wildlife or birds. Though the tree does have defense mechanisms, over time it fails and falls, leaving a gap in the forest canopy, which becomes an opportunity for another cycle: its own offspring or another species. Or maybe an understory habitat takes over for a while....
That would be the story in some Forest Primeval untouched by humans. But at our place usually the tree is felled, becomes firewood, or maybe even usable lumber. The twigs and small branches go on the brush pile where in an amazingly quick process they break down into humus that is returned to the soil of the property.
I value Chris Luley's slim handbook, "Wood Decay Fungi," of the Visual Identification Series, in helping me identify mushrooms I find growing on trees. It was written to provide tree care professionals with a guide to assessing the prospects of trees that have symptoms of fungal infection. Interestingly, some fungi are medicinal; used in the treatment of serious diseases they sell for very high prices. Others simply make for delicious eating or curious collectibles. I picked it up at a conference I attended a few years back, but it should still be available. Look for it under "Wood Decay Fungi; Common to Urban Living Trees in the Northeast and Central United States," Christopher J. Luley, Ph.D. Urban Forestry LLC, 2005, 61 ppg.
From the Polly Hill Arboretum schedule of events: Saturday June 14, Robert Thorson on New England Stone Walls. Call 508-693-9426 for more information.
