Garden Notes: Letting go of ailing black oaks

Garden Notes: Letting go of ailing black oaks

October glory: dahlias backed by regal asparagus foliage in Carol Brush’s West Tisbury garden.

As many Islanders have become aware, certain black oaks, Quercus velutina, are hosting a tiny insect that is proving to be highly destructive to numbers of these trees. The insect doing the damage is a tiny cynipid wasp, Bassettia ceropteroides. There is more information, with links, from Polly Hill Arboretum at pollyhillarboretum.org/science/cynipid-wasp-information/

I have been frequently asked my opinion about the black oak situation this year. As usual I have a slightly different, contrarian, pro-life perspective, which attempts to employ up-stream thinking and discern less-proximate causes.

According to Michael Dirr in the “Manual of Woody Landscape Plants,” Q. velutina “makes best growth on moist, rich, well-drained, acid soils but is often found on poor, dry, sandy, or heavy clay hillsides.” Do the latter sound like Island conditions?

Furthermore, this tree is an earlier-stage species in the deciduous forest succession, lacking the durable wood and hardiness of some other Island oaks. Taken together, these two factors add up to trees that come to the end of their life span after a relatively short cycle. They then fall prey to one or more pathologies; their fate paves the way for species in the next stage of forest succession, such as white oak, beech, and hickory.

The trees most damaged appear to me to have been stressed, in one way or another, mostly by age or unfortunate growing sites, often given an assist by human intervention. This includes mutilation for power-line trimming or compaction from roads and other activities. Many troubled trees have been carved out of their forest environment and turned instead into lawn trees, with their root runs deprived of their preferred cover of pucker brush, fallen leaves, and leaf mold, and fungal/bacterial balance altered to benefit lawn, not tree.

For trees absolutely critical to the landscape, there is an off-label use of a systemic pesticide that can be infused into the oak. It makes the entire tree toxic to the communities of life-forms that live off that tree, such as caterpillars, and consequently to the life-forms that live off the life-forms, such as birds. It is worth remembering that oaks as a group support more life-forms than almost any other genus, according to Douglas Tallamy, in his work, “Bringing Nature Home,” (Timber Press, 2009).

My opinion for landscape trees that are not critical is to relinquish them and plant two good replacement trees for each one lost. (My replacements here include oxydendrum, magnolia, and styrax.) Polly Hill Arboretum exists as a resource for those who would appreciate expert, site-specific advice for what to plant.

Gardening on slope

A technique that seems especially advantageous if your garden is blessed with sunny slope — where some of our black oaks used to be — is recommended by Gertrude Jekyll, the 20th-century British garden designer. (If my garden is “blessed,” it must be because practically our whole place is slope.) She recommended training later-blooming perennials forward, as arching sprays, to cover the remains of plants that have previously gone by in the border. Chrysanthemums, natural leaners, are known to be amenable to this sort of training, as many will know who have seen the displays of Japanese chrysanthemum cascades, a stunning art and flower show category of its own.

Other natural leaners to try on sloping sites might be platycodon, perovskia, and asters. Additional subjects well suited to slopes include those whose natural preference is for well-drained or even dry soil. Lavenders, stachys (anything with grey/silver leaves, for that matter), daylilies, agaves, sedums, salvias, nepeta, grasses, and all the rock garden plants, should do well, given sun and good drainage.

In the garden

Lawn repair is on the minds of many as cooler conditions and, we presume, more regular rainfall make scarifying and re-seeding timely. If the lawn is small, weed out and remove by hand any of the growths of crab grass that have smothered out the better stuff, leaving dead patches. The rakings that are produced from combing over the lawn are deluxe material for compost piles, producing heat and breaking down readily.

Dahlias in early fall are at their most beautiful and prolific. You may notice that many, which have been double all summer, now appear more single and show yellow pollen centers. Pollinators are avid for this stage and nighttime finds flowers with sleeping bumblebees still nuzzling them.

When frost hits, let the tubers remain in the ground for a while, as long as two weeks, to drive the energies down into them for good keeping qualities. “Tubers dug too early are still ‘green’ and will not store,” according to the folks at Swan Island Dahlias.

Further storage advice: Use a storage medium such as slightly dampened peat moss, sand, or sawdust/shavings. Tubers should be stored in crates or cardboard boxes. Line the containers with 10–12 sheets of newspaper, for which the MV Times is perfect. Start with the packing medium in the bottom and layer tubers and medium until the container is full. Store in a cool, dry area, 40–50°F.

Clean up and cut back in both vegetable and ornamental gardens. Compost is almost always in short supply and using it always seems to involve prioritizing: use on places/plants that need it most. Cover-crop open areas of the vegetable garden. Rake up leaves regularly and compost them. Mulch may be applied to portions of beds that are dormant or areas that need protection. My own preference is first to put down a layer of low-number organic soilfood (i.e. fertilizer) to assist the soil organisms that process the organic matter you plan to lay.

Coming up

Friday and Saturday, October 4 and 5: Living Local/Harvest Festival: Check the paper for details.

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