Oystershell Scale on Lilac

My neighbor’s lilacs, while showing signs of lilac blight earlier in the season, continue to experience massive dieback. Today, I decided to perform a closer examination, since the proximity of his plants puts my plants at risk.

Lilac dieback from oystershell scale

Lilac dieback from oystershell scale

To my surprise, his lilacs are covered with oystershell scale, and the infestation is so severe that it isn’t worth trying to save the shrubs. Certain types of scale, such as euonymous scale, are ubiquitous, but oystershell scale–especially when it has clearly been present for several years–is unusual among reasonably well tended suburban gardens. My guess is that he probably purchased inferior stock at one of the local big box stores. Combined with yearly bouts of powdery mildew, and a location with insufficient airflow, the stressed shrubs were doomed to an attack by opportunistic insects.

Oystershell scale is a member of the order Homoptera, family Diaspididae (armored scales) and genus Lepidosaphes ulmi. There are two races of oystershell scale, gray and brown. The gray race attacks lilac, ash, willow, poplar, and maple, among others. There is only one generation per year for the gray race.

Recent oystershell scale on lilac

Recent oystershell scale on lilac

If you’re not familiar with scale insects, there are two types:  soft-bodied and armored. They can be dispersed by wind, tools, or people. Armored scales are more difficult to control because they are only susceptible as crawlers (juveniles). Once they mature, they develop a protective layer that insecticides can’t penetrate or smother, and, likewise, the eggs are protected by the armored casing of the adults. Huge populations can develop rapidly. In the picture at right (I recommend enlarging twice), notice the hundreds of small “oyster”-shaped, bumpy white scales along the branch. Underneath each of these adult female scales will be anywhere from 40 to 150 eggs, and this is how the insect overwinters.

Older damage on trunk from oystershell scale

Older damage on trunk from oystershell scale

Scale insects only feed during the crawler stage, when the six-legged, pale yellow juveniles emerge for a few days to a couple of weeks and use their sucking mouthparts to consume plant sap. Signs of damage include reduced vigor, foliage that appears smaller than normal, and, in severe cases–such as the photo of the attacked lilac shrub shown above–whole branches die, never to recover.

Other than pruning out affected branches (assuming limited damage), there are no natural controls. Biological predators emerge too late to be effective. If damage is still somewhat contained, control crawlers with a systemic, such as acephate (Orthene), and follow the label instructions. Make your first application of insecticide when Spiraea x vanhouttei (the old-fashioned, cascading variety) has just finished blooming. Remove old scale casings by gently rubbing the infested bark with a small, plastic kitchen scrub brush.

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Invasive Species

Lythrum salicaria in a colorful border

Lythrum salicaria in a colorful border

Back in the 1980s, it was almost impossible to pick up a gardening book or horticultural publication that didn’t recommend planting species that are banned or restricted today as invasive plants. Some states have extensive lists of banned or controlled species, while other state lists are more moderate. Sometimes the lists make no sense unless you understand the particular economics of a state’s agriculture. The bans or restrictions may be specifically to protect grazing cattle or cranberry bogs or even wildlife restoration. More often, the listed plants are so aggressive that native species are being crowded out.

 For example, picture books thirty years ago, extolling English perennial borders, included photos and planting plans for Lythrum salicaria. Even the popular book “Bold Romantic Gardens”, by Oehme and van Sweden, claimed that Lythrum salicaria  was not invasive. Today we know that there is no such thing as a non-invasive Lythrum. It’s too bad, really, that Lythrum is so invasive, because very few perennials come close to Lythrum’s unique color, and only Liatris spicata comes close to Lythrum’s spiky floral presence in the garden.

Among vines, the Chicago Botanic Garden’s publication from the 1980s, “Vines for Northern Gardens”, recommended Five-leaf Akebia (Akebia quinata), Porcelain Vine (Ampelopsis brevipedunculata), and Oriental and American Bittersweet (Celastrus orbiculatus and Celastrus scandens, respectively). In some cases, the accompanying description suggested that these vines were aggressive, or that seed spreading might be problematic, but none were banned from most state commerce at the time.

In the highly respected tome from The Garden Club of America, “Plants That Merit Attention, Volume I–Trees”, published in 1984, Phellodendron amurense (Amur or Chinese Cork Tree) is listed as a “handsome, tough, small shade tree”. To the credit of the authors, a final descriptive sentence suggests:  “A grafted male clone, with no pods to be removed, might be considered best for city planting.” As of 2015, the State of Wisconsin has banned the female tree entirely.

If you live in the United States, I encourage you not only to check your own state’s list of invasive species, but also to check out the lists of some other states. I think you may be surprised by some of the plants that are listed. Indiana, Illinois, Iowa, Delaware, Missouri, Maryland, Michigan, New Jersey, and Nebraska have especially fine websites for invasives. Not only do these state sites list the plants, and how to identify them, they also explain why the plant has been listed as posing a threat. When checking the lists, note that for many species with varietals, certain varieties are still acceptable for planting. The allowed varieties are usually seedless or nearly seedless.

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Eye Candy for Gardeners

Rosa Morden Snowbeauty

Rosa Morden Snowbeauty

This photo seems to perfectly capture one reason gardeners have such a long-standing love affair with the rose. I can’t think of another flower as delicate and elegant when first opening as a rose. The Morden Snowbeauty rose shows just the faintest hint of pink before opening to pure white. Aren’t roses fantastic?

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Bristly Roseslug

Bristly Roseslug leaf damage

Bristly Roseslug leaf damage

It never ceases to amaze me how all it takes is one rose in a garden in order for the Bristly Roseslug to find it and attack. The roseslug is the transparent (when young) or green (when older) larval caterpillar of a sawfly (wasp), Endelomyia aethiops. The larvae are no more than one-half inch in length and, if you’re lucky, can be found  while eating leaves and promptly squished. Younger larvae skeletonize the leaves, as seen in the photo at left, while older larvae chew holes in the leaves, often along the leaf edges. Continue reading

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Four-Lined Plant Bug

Four-Lined Plant Bug nymph

Four-Lined Plant Bug nymph

For the past two years I’ve had problems with Four-Lined Plant Bugs. As shown in the photo at left (click to enlarge), the nymphs are black and red, so they’re easy to spot, even though they’re only one-quarter inch in length. They have voracious appetites, and with their chewing-sucking mouthparts they can leave numerous small holes on tender leaves. The plant tissue surrounding the holes dies from the toxins injected by the bugs and appears as a series of brown, shot-hole marks or the holes can blend together into more of a blotch. Continue reading

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Plant of the Month: Alchemilla mollis ‘Thriller’

Alchemilla mollis (Lady’s Mantle) is probably a well-known plant to many readers here. A perennial border classic, it is celebrated for its magnificent scalloped foliage and its delicate chartreuse-yellow flowers rising on wands above the main clump of leaves. It’s a great edging plant and an attractive visual filler among other flowers.

Alchemilla mollis 'Thriller'

Alchemilla mollis ‘Thriller’

The photo at left is the ‘Thriller’ variety in its second year, grown from one-gallon pots. I’ve grown both the standard Alchemilla and the ‘Thriller’ variety and, on balance, I prefer ‘Thriller’. Descriptions of both varieties can vary in publications or on the internet, but, in my experience, ‘Thriller’ is a larger plant whose leaves are distinctly green rather than gray-green. Continue reading

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Final Update on Clethra alnifolia ‘Vanilla Spice’

Clethra 'Vanilla Spice'

Clethra ‘Vanilla Spice’

The Clethra ‘Vanilla Spice’ that began as a garden trial four years ago has matured into a lovely, medium-sized shrub. While not easily found at retailers, this is a highly desirable shrub well worth locating and planting.

Location Requirements

This cultivar grows well in acidic or alkaline soil. The natural environment for Clethras is woody swampland, so they all require constantly moist soil; however, once established, an inch of rainfall per week is adequate to grow ‘Vanilla Spice’. If weekly rainfall is unreliable, choose a location with easy access to supplemental watering. Continue reading

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