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.

Established plants can tolerate up to ten days without water when temperatures are moderate. Young plants are much more moisture sensitive:  if not planted in a naturally damp site, moisture-retaining crystals should be added to the soil when first planting.

Providing the right amount of sunlight requires a balance with soil moisture. In regularly damp soil, ‘Vanilla Spice’ grows well in full sunlight. In the absence of damp soil, locate the plant either where it receives morning sun, very light shade or under very tall trees. Otherwise you may be constantly supplying extra water to keep the plant healthy.

Aesthetically, ‘Vanilla Spice’ can be used as a specimen, low hedge or within a mixed border. Bees are active when the shrub is in bloom, so best not to plant too close to entry doors.

Growth Rate

Fairly slow growing the first two years, Clethra ‘Vanilla Spice’ develops well in the third and fourth seasons. In 2010, I planted a 9″ (23 cm) tall container-grown shrub from a one-gallon pot. By 2014, the plant reached its mature size of 42″ (107 cm) high by 42″ (107 cm) wide. In a slightly sunnier location, I suspect the ultimate height would be 48″ (122 cm). Clethras grow by rhizomes, so if suckers develop, clip them off at ground level.

Physical Appearance

This is a gracious, refined shrub with leaves and flowers superior to the species. Leaves are a shiny, medium green that turn a pleasant shade of yellow in the fall. Branches grow to the ground on an open, vase-shaped plant.

Flowers are a pure white bottlebrush shape, unlike the pointed flowers of the species Clethra. Their fragrance is a light, spicy vanilla–just as the name suggests. It is a reliably prolific bloomer each August.

Cold Hardiness

Clethra ‘Vanilla Spice’ is certainly hardy to Zone 4. Although my shrub isn’t totally exposed to prevailing westerly winds, it withstood last winter’s brutally low temperatures with no damage or dieback. On the whole, ‘Vanilla Spice’ is a much easier and more reliable cultivar to grow in Zone 5 than the smaller Clethras such as ‘Hummingbird’ or ‘Sixteen Candles’. This plant is very late to leaf out in the spring, so don’t look for winterkill until late May.

Diseases

I haven’t observed any leaf fungus, cankers or blights.

Insects

Last year, a few leaves were damaged by a mystery chewing insect. Otherwise, this shrub isn’t bothered by insect pests.

Pruning and Fertilization

No pruning is required other than for possible suckers. If desired for aesthetic reasons, the plant can be pruned back by 20% after flowering. I typically apply a low-ratio, slow-release fertilizer early in the season, although Clethras don’t require fertilizer so much as the correct amount of sun and water.

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A Note on Powdery Mildew and Disease Management

Powdery Mildew

Powdery Mildew

“Powdery Mildew on Lilacs” continues to be one of the most read articles on Gardening in the Mud. However, anyone located in Zones 5 and 6 hoping to treat powdery mildew at this time of year has only one control option available–and even that control will only help contain spread of the disease for next year’s growing season:  collect all fallen leaves and either burn them or otherwise permanently dispose of them. Do not compost infected leaves. Powdery mildew fungus overwinters in the leaves, so you want to keep the fungus away from affected plants.

Regarding plant diseases more generally, there are four principles of plant disease management:

  1. Exclusion
  2. Avoidance
  3. Protection
  4. Eradication

Notice that plant pathologists use “disease management” rather than “cure”. That’s because there are no cures for plant diseases caused by biotic agents. This is such an important concept that I will repeat it:  There are no cures for plant diseases caused by biotic agents. Each new growing season requires repeating prevention strategies for susceptible plants.

Plant diseases caused by abiotic agents, such as insufficient sunlight, too much or too little water or fertilizer, insufficient airflow around plants, or transferring a fungus by handling wet plants, can be addressed by altering the circumstances leading to plant disease. Although the plant disease will still not be cured, rapid improvement in health is likely. For biotic diseases, Avoidance and Protection are the most commonly used individual management strategies. Avoidance includes purchasing a disease-resistant variety of lilac (avoiding varieties more susceptible to powdery mildew). Protection includes a chemical spray program for the lilac (chemicals encompass any product from sulfur to horticultural oil to regulated fungicides). Otherwise, there is Exclusion (don’t grow lilacs) or Eradication (remove all existing lilacs that are susceptible to powdery mildew).

Although I’ve used lilacs and powdery mildew to illustrate the different disease management options, depending on the disease, all four options may not be available. For example, virus-stricken plants need to be eradicated–there is no treatment that will improve the plant’s health. Also, if you’re living in certain Canadian provinces, fungicides may not be an option (although you can still use a horticultural oil treatment in the case of lilacs). Mostly, however, we can choose among different strategies based on trade-offs between cost, aesthetics, time commitment to caring for particular plants, and personal preferences.

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Sport or Sucker?

Rose with yellow sport

Rose with yellow sport

This bicolor rose in my neighbor’s garden has a shoot that is producing a single yellow rose. The majority of flowers on the plant are a combination of orange and yellow, so this particular shoot could simply be sporting back to its yellow rose parent. The plant could also be creating a “sport” that represents a genetic change in the plant material, in which case the shoot with the yellow rose could be propagated as a cutting to see if a new, all-yellow rose bush could be developed, different from the parent. Since this particular shoot isn’t growing from any of the main stems, it could also be a sucker from the rootstock onto which the rose is grafted. However, since most rootstocks produce either red or white flowers on suckers, my guess is that this shoot represents some sort of sport.

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Plant of the Month: Asiatic Hybrid Lilies

Pink Asiatic Hybrid Lily

Pink Asiatic Hybrid Lily

In between early summer and high summer, among the best perennials to join June-blooming roses are the Asiatic hybrid lilies. Stately and strong, they provide a welcome linear accent in the garden. The multitude of colors and patterns makes it almost impossible not to find a suitable selection to enhance any color scheme or garden plan. Upward facing, outward facing or down facing, the petals are typically smooth-edged and may be recurved. For greatest visual impact, plant in groups of at least three. Continue reading

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Review of Jack’s Classic Petunia FeED Fertilizer

Jack's Classic Petunia FeED

Jack’s Classic Petunia FeED

Earlier this summer, while replenishing my fertilizer supplies at a local nursery, I came upon Jack’s Classic Petunia FeED. Made by the reliable JR Peters Inc, long known for its excellent assortment of indoor plant fertilizers, the clever name suggests petunia nutrients including chelated (ED) iron (Fe). The full label informs you that not only does the product contain chelated iron, it also contains an assortment of micronutrients, including chelated manganese, zinc and copper. Chelated minerals are more expensive to produce, but they are more readily available to the plant. Continue reading

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Tomato Hornworm Parent?

Sphinx Moth

Sphinx Moth

Before you are able to appreciate that glamorous butterfly or majestic moth, there is a voracious caterpillar (larva) that needs copious amounts of food before it can molt into maturity. As a gardener, it’s important to be able to identify insect visitors to know whether to be on guard rather than simply admire.

A few days ago, this rather large sphinx moth was positioned on one of the deck posts right behind my trailing petunias. Being nocturnal, she decided to stay all day, so I had plenty of photographic opportunities, but no real macro photographic ability without disturbing her. Sometime after 9:00 p.m. that night she flew off. Continue reading

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Crab Tree Farm

 

Entry to Crab Tree Farm

Entry to Crab Tree Farm

Light years ago, when I was a young girl, I learned to play tennis just up the road from Crab Tree Farm. Gazing out the car window, on the way to my lessons, the farm had a storybook quality–like a Tasha Tudor illustration come to life.  So I was thrilled when The Garden Conservancy announced that one of the gardens available for viewing on Open Days this year was Crab Tree Farm. At last I would discover what lay beyond the roadside buildings and fields. Continue reading

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