On Saturday, May 23, the Polly Hill Arboretum (PHA) visitor center opens for the season, with daily hours from 9:30 am to 4 pm. Staff members lead tours every day at 2 pm. The grounds are open year-round, from sunrise to sunset. The Arboretum's plant sale area also opens for the season this Saturday with a choice selection of unusual trees, shrubs, and perennials. Arboretum staff have chosen and grown plants that are noted for their superior ornamental qualities and their adaptability to Island conditions. Staff will be on hand to answer your questions. More information about other PHA activities is available at pollyhillarboretum.org.
Knitting nicely with its neighbors, rhododendron 'Delaware Valley White' displays an attractive, layered habit. Photo by Susan Safford
May: Green goddess
The past two weeks have seen the yearly dramatic color shift in the landscape, from neutral to green and flowery. Suddenly the world is brimming with colors, scent, and birdsong. The down-Island towns are practically in summer mode, while the more rural Island landscape, dominated by oak woodland, tarries somewhat. By now we know how many flower-heads the lilac has, how much new growth the Japanese maple put out, and whether the peonies and iris will have the desired (or any) number of flower stems.
Brute invaders
We may also have observed uninvited plants: I am talking about the brute invaders, invasive exotics. Seeds or roots of oriental bittersweet may arrive in construction fill used in various ways around homes or those of neighbors. Mugwort has an affinity for nursery-raised pots and root-balls. Ruderal areas like roadsides, drainage areas, or back lot-lines host rubbishy plants like Japanese knotweed, Norway, and sycamore maples, weed honeysuckles, and autumn and Russian olive. These plants have the capacity to muscle out native plants and designed ones alike, in the landscape. To view the Massachusetts Prohibited Plant List, go to mass.gov/agr/ farmproducts/proposed_ prohibited_plant_list_v12-12-05.htm. For images, click on the common names.
One of the fastest increasing invasive plants around the Vineyard is garlic mustard, Alliaria petiolata, stands of which I see nearly every day. It just springs up. Though it is innocuous looking, with its fresh green leaves and topknot of small white flowers, prolifically spreading alliaria secretes a substance that interferes with the mycorrhizae that trees need to grow, and is a threat to woodlands. Even plants that were originally desired and not on the Prohibited Plant List, like wisteria, can get out of hand. I have known a couple of properties where considerable area was "lost" to the jungly tangle of its tree-like vines that covered the ground and woods.
It is possible for Oriental bittersweet or wisteria to deform a host plant inside of a season unless its stranglehold is discovered and removed. The seedlings of Norway and sycamore maples, lodging near the roots of intentionally planted trees and shrubs, grow at a much faster rate and starve the desired plant. The gardener's only real resort is to learn to recognize these brute invaders and watch for them. The good news is that patrolling your grounds early and regularly in spring offers the best chance to spot and weed them out while they are small. Be careful also in what you buy to plant.
Rhododendrons and azaleas
Rhododendron and azalea time is here once again. Were it not for Martha's Vineyard's deer herd, this would be a perfect place to enjoy the two colorful members of the Ericaceae. Even so, it is still almost perfect. We have acid soils, adequate rainfall, and wooded landscapes in which rhododendrons thrive and look appropriate. (Azaleas, the plants with smaller flowers and stature that are often deciduous, are also botanically rhododendrons, along with the larger plants with the leathery, evergreen leaves and large flower trusses.)
The white azalea (photo) pictured is the durable 'Delaware Valley White.' There are three here, purchased as twig-like plants with about a dozen leaves apiece, from one of the conservation seedling sales for next to nothing, about 25 years ago. They would have acquired far greater size without the attentive pruning of the "unpaid gardeners" (my ironic attempt at gracious acceptance of the deer and other wildlife we share the garden with). Now they are large enough that I myself occasionally prune them, trying to emphasize the layered, horizontal look of azaleas that I find appealing.
In selecting a rhododendron or azalea, choose one whose ultimate size fits the space you have to plant. The pot tag should give an indication of mature size at 10 years. Many modern rhododendrons have been bred to grow lower and wider than some of the towering older varieties. I think it also looks good when shrubs in a foundation planting or shrub border knit together over time. The 'Delaware Valley White' illustrated has knitted nicely with Gaylussacia frondosa (dangleberry) and rhododendron 'Bali,' which it fronts.
If the location is in a prepared border, dig the planting hole wider than deep and place so that the top of the root ball sits a little higher than the surrounding soil. Rhododendrons and azaleas have fine, shallow fibrous roots, easily smothered if planted too deeply. Mulch evenly after planting (no mulch volcanoes). For an unimproved site, dig the hole much wider than deep, set the root ball high, and replace the soil with humus-rich, acidic compost.
Young rhododendron plants may be leggy. "The Pruning Book," by Lee Reich (The Taunton Press, 1999, 234 pages) contains good pointers for pruning them. Rhododendrons, as well as azaleas, might benefit from pinching out the central, terminal growth buds (flowering buds are much fatter) in each leaf cluster. For mature plants that outgrow their space, Mr. Reich recommends preparing them a year before surgery with mulch and a good supply of moisture and food. It may have to be done gradually, cutting back a few old branches each year.
