What Grows There :: Hugh Conlon, Horticulturalist, Garden Advisor, and Photographer

Pest Alert- Boxwood Blight

Posted by Hugh on April 27th, 2012

Leaf Spotting and Black Stem Canker Symptoms (Photo credit: Craig Adkins, NC State Extension)

Boxwood blight is a disease caused by a fungus (Cylindrocladium pseudonaviculatum) which is a serious threat to most boxwood (Buxus) and related species such as sweetbox (Sarcococca spp.). To date there is no evidence of resistance to boxwood blight in any Buxus species.

Plant pathologists in England first identified the disease in the mid-1990’s. It was first detected in the United States in North Carolina and Ohio in late 2011. The disease is now confirmed in 8 additional states (CT, MA, MD, NY, OR, PA, RI, VA) and in British Columbia.

Initial symptoms of boxwood blight include leaf spotting, rapid leaf drop, blackened stem cankers, and severe dieback. Most boxwoods are not immediately killed by the disease, but the plant(s) will become badly defoliated and unsightly.

Boxwood blight is spread primarily by water (rain splash, irrigation, runoff, etc.), by sale or exchange of plants in the nursery/garden center trade, and through contaminated tools, equipment, boots, etc. The sticky spores infect pruners and other tools, clothing, tires, and, of course, boxwood leaves.

Disease spread is rapid, often in less than a week if weather is warm and humid. The disease is carried by splashing or wind driven rain. At this time there is no cure for boxwood blight. Here are some key steps for gardeners and landscapers to follow:

Know the primary symptoms of boxwood blight and check boxwoods in the garden regularly
Before purchasing inspect boxwoods at garden centers, and reject any with yellow spotted leaves and black stem cankers
During pruning, dip clippers into a 1-to-10 ratio of bleach to water or 70 percent ethanol solution for 1-2 minutes before moving on to a new plant.
Properly dispose of all diseased plants or clippings; do not compost them
Limit buying of new boxwoods and avoid combining new with existing plants
Take suspect boxwood plants and clippings to a county Extension office or to a full service retail garden center for proper diagnosis.

‘Fireworks’ Pennisetum

Posted by Hugh on April 23rd, 2012

'Fireworks' Pennisetum in late August at Dallas Arboretum

Fountain grasses are popular annual grasses that thrive in Southern Appalachian gardens (USDA zones 6 and 7). The new cultivar ‘Fireworks’ is totally spectacular. The mid-vein of the grass blade is burgundy red and edged in hot pink.

Fireworks displays three seasons of awesome foliage color before dying off in the fall. Red plume foxtail flowers bloom from mid-summer to early fall. Fireworks grows as a thick clump that is 3-6 feet tall and 1-3 feet wide. Utilize Fireworks as an accent plant in landscape beds and containers.

Fireworks flourishes in full to partial sunlight (6-hours minimum) in average well-drained, low acidic soils (pH 6.0 to 7.0. Weekly irrigation is a plus. It establishes rapidly and tolerates short summer dry spells.

The vibrant tri-colored grass blades intensify in the early autumn cooler temperatures. In the deep South (USDA zones 7-b to 9), foliage colors often fade under extreme sunlight and heat (see photo above).

Fireworks should be fertilized at planting time with a 5-6 month rated slow release fertilizer. Another alternative is to feed monthly with water soluble Miracle-Gro™, Schultz®, or a similar product according to package rates.

Grass blades turn brown in the first hard autumn freeze. Snow or ice embedded ornamental grasses can be quite beautiful. You may opt to shear them back in early fall as the old floral heads (plumes) often self-seed. Fireworks fountain grass is relatively disease and pest free and is deer resistant.

Pest Alert_ Downy Mildew on Annual Impatiens

Posted by Hugh on April 19th, 2012

Disease on Underside of Leaf (photo courtesy of Dr. Alan Windham)

In 2011 across Europe, seventy percent (70%) of the garden impatiens (Impatiens walleriana) were hit with downy mildew. Disease outbreaks were also reported in a few U.S. coastal cities last year. U.S. growers have been preparing for the possible outbreak in summer 2012. The potential for this disease can be catastrophic as the fungus can overwinter in the soil.

Gardeners should purchase disease-free plants at garden shops this spring. They should inspect plants closely, and don’t buy those with yellowed or misformed leaves. They should not plant where impatiens were growing last summer. If other annuals, say snapdragons or zinnias, were infected last year, this form of downy mildew will not effect impatiens.

At this time downy mildew appears to be limited to seed-produced impatiens. Vegetatively-produced impatiens (from cuttings or tissue culture) appear to be clean. Some kinds of impatiens are not susceptible to downy mildew, such as SunPatiens and New Guinea impatiens.

Downy mildew is a systemic pathogen. Symptoms include a downy white coating on the underside of the leaves. Defoliation soon follows. Other clues include leaf stippling, chlorosis (yellowing), and downward curling of leaf margins. If plants become infected, there is no pesticide spray option available. Dig up and discard all contaminated impatiens immediately and never compost the diseased plants.

Permit me to offer a calming statement. If you are planning to or have already planted impatiens this year, enjoy the colorful array which impatiens do provide. U.S. greenhouse growers have been aware of the problem for almost a year and plants for sale should be disease-free. Stay informed about this disease.

Peruvian Lilies – A Gift Which Keeps On Giving

Posted by Hugh on April 16th, 2012

Peruvian lilies (Alstroemeria)

Peruvian lilies (Alstroemeria spp.), also called Lilies of the Nile), are not true lilies (Lilium spp.). Likely, you have purchased a bouquet of alstroemerias from the florist section of your local supermarket. In addition, many potted hybrid varieties have been introduced from Holland and elsewhere over the past decade.

Turns out that some Peruvian lilies are perennials, winter hardy in the Southern Appalachian Region (USDA hardiness zones 6 and 7). Perennial types are best suited for an east front border of the flower garden, exposed to mostly morning sunlight. In the deep South (zones 8 and 9), they prefer filtered (not direct) sunlight through nearby trees.

Afternoon sunlight is harsh in the summer on the leaves and reduces flower life. Mulch around plants in late fall in colder zones 5-b to 6-a climates or transplant back into a container and grow them in a cool garden room over the winter.

Peruvian lilies require little care after planting. Fertilize established plants in late winter with a handful of granular 10-10-10 around each plant. Irrigate Peruvian lilies weekly, particularly during summer dry spells.

Peruvian lilies comes in several flower colors including in white, yellow, salmon, pink and red. Perennial varieties average 18-36 inches in height in garden beds and in deck or patio containers. Cut flowers may last for 7 to 10 days.

Botrytis Disease in Marigolds Can Be Avoided

Posted by Hugh on April 10th, 2012

: Botrytis Disease in Marigolds

Marigolds (Tagetes spp.) have been long time favorite summer annuals in Southern Appalachian gardens (USDA zones 6 and 7). Marigolds come in three sizes: 18-36 inch tall “African” marigolds, 12-18 inch medians and 8 -12 inch “French” border types and in two color choices- orange and yellow. Begin planting in April or May after frost danger has passed. Figure in a six week period for establishment and marigolds possess exceptional drought and heat tolerance over the summer months.

Botrytis, also called “gray mold”, is a serious fungus disease. Flower petals turn brown and are often covered with gray, fuzzy masses. Brown target-like spots may also appear on the foliage.

Botrytis is most troublesome during periods of extended cloudy, humid, and wet weather conditions. Flowers may capture and hold water from rainfall and irrigation. Irrigation should be scheduled in the early morning hours. You should select marigold cultivars with flowers that shed and do not retain water droplets.

Sanitation or removing old (spent) and diseases flowers is key to reducing the spread of botrytis through the garden bed. Avoid wetting the flowers when watering and adequately space plants to promote good air circulation.

An alternative is to apply fungicide sprays as soon as disease symptoms appear. Among the pesticides registered for use are Chipco 26019® (iprodione), Daconil® (chlorothalonil), and either Cleary’s 3336® or Domain FL® (thiophanate-methyl).

Bloomerang™ Lilac For Non-Stop Flowering

Posted by Hugh on April 8th, 2012

Young 'Boomerang' Lilac in mid-April Garden

In most Northeast and Midwestern states, lilacs (Syringa spp.) are the classic spring-flowering shrub, treasured for their beautiful and fragrant flowers. Lilacs are vigorously growing shrubs and are utilized in the landscape as single specimens or as multiple hedging shrubs for privacy. Further south, in the warmer Southern Appalachian region (USDA zones 6 and 7), most lilac cultivars tend to disappoint because of low flower numbers and their susceptibility to foliar powdery mildew in late summer.

Bloomerang™ is a hybrid cross of four lilac species (Syringa patula x S. macrophylla x S. meyeri x S. juliana). It grows bushy and compact at 5 feet tall and 6 feet wide. Boomerang grows vigorously, fits into small garden spaces, and forms a lovely low growing hedge.

Clusters of sweet-scented flowers open deep purple that densely cover the branches in April and bloom off and on until October frost. Flower clusters fade to pink and fall without any need for deadheading. Although the long re-blooming trait is never showy, you and your gardening friends will likely take notice.

Boomerang is best pruned immediately following spring bloom, to form a dense shrub with more branches and flowers. This lilac is deer resistant and its small dark green leafy foliage is moderately resistant to powdery mildew.

Lilacs grow in full to partial sunny locations and in a low acidic to slightly alkaline soil pH. Lilacs benefit from annual feeding containing minor nutrients such as Miracle-Gro™, Miracid™, Hollytone™ or Schultz® fertilizers. Follow package label directions.

Re-blooming Iris For Twice (and More) Flowering

Posted by Hugh on April 6th, 2012

Re-bloomer Iris germanica 'Sugar Blues'

Gardeners who desire more flowering from garden iris (Iris spp.) should be planting re-blooming types. Some cultivars may re-bloom up to three times in one growing season. The re-blooming trait is found both in bearded and beardless irises (I. germanica).

Re-bloomers need to be well established for one year, sometimes two, to bloom multi-seasonly. Some re-bloomers are sporadic, and a short list of dependable varieties is included below.

Information on growing tall bearded iris is found in an earlier blog (April 1, 2012). Re-bloomers multiply faster than one-time bloomers, and need to be divided every two to three years to avoid overcrowding. Some cultivars need to be well established before they re-bloom; lift and divide only part of the clump and leave the remaining portion undisturbed to re-bloom. Also, clipping off the foliage too early in late summer reduces re-bloom potential.

Re-bloomers are heavy feeders and benefit from two feedings, once in the spring, nearing the end of the regular flowering period, and a second time in mid- August, the start of the 2nd or 3 rd re-blooming period. Fertilize with a low nitrogen analysis, such as 5-10-5. Re-blooming cultivars need frequent irrigation during dry summer periods compared to standard irises.

A baker’s dozen cultivars include: Best Bet, Bountiful Harvest, Clarence, Hemstiched, Immortality, Jennifer Rebecca, Mother Earth, Pagan Goddess, Pagan Dance, Autumn Tryst, Champagne Elegance, Orange Harvest, Rosalie Figge.

Complete Care of Tall Bearded Iris

Posted by Hugh on April 4th, 2012

Tall bearded Iris-- Queen of the Perennial Garden

Most gardeners plant tall bearded iris from late August through October to insure good root development before winter sets in. Spring planting is another option as iris may be purchased in containers. Bare root rhizomes need to soak overnight in water before planting. Bearded irises prefer a sunny location and a slightly acidic well-drained soil. Work in plenty of compost into the garden soil prior to planting and lightly mulch iris 1-2 inches after planting. Irrigate the bed every 5-7 days if natural precipitation is light; once the first new center leaves appear, iris rhizome(s) have rooted.

Remove the bloom stalks after flowering and cut the foliage down to 3-4 inches in late fall. Irises need dividing every 3-4 years to prevent overcrowding. Right after Labor Day is a fine time to divide and/or transplant in the Southern Appalachian region (USDA zones 6 and 7). Avoid the stressful hot days of summer. Plant each rhizome shallow, barely beneath the surface of the soil.

Irises grow best in raised beds, particularly if the garden soil is heavy clay. Add superphosphate (0-45-0) to the soil before planting. A good low-nitrogen fertilizer such as 6-12-12 can be used early in the spring. Do not mulch or overwater.

Tall bearded rises are susceptible to a number of disease and pest issues. Remove and discard (do not compost) all dead leaves and bloom stalks by late fall so not to overwinter various diseases and pests.

In the spring treat leaf spot diseases with Daconil™ fungicide. To manage iris borers, aphids and beetle grubs, choose a systemic insecticide like Merit™ (imidacloprid).

Evergreen Kousa Dogwood? – Depends on Where You Garden

Posted by Hugh on March 28th, 2012

Chinese dogwood in flower

Chinese (kousa) dogwood (Cornus kousa) is a small spring flowering dogwood. Kousa blooms about 2 – 3 weeks after our native flowering dogwood (C. florida) has finished. Lustrous dark green leaves begin to emerge a week or two before the lovely pointed 3- inch white bracted flowers. Fall leaf color is purplish red and not quite as vibrant as C. florida. Its patchy exfoliating bark is a winter asset which gets better as the tree ages. The fleshy raspberry fruits are not a popular food source with birds and four legged critters in the fall.

Evergreen kousa dogwood (Cornus kousa var. angustata) is a variant also listed as Cornus angustata. Angustata forms a shorter tree with narrow pale green evergreen foliage is reliably evergreen in USDA zones 7-b and 8, and takes-on a purple wintry tint in zones 6-b and 7-a. The tree is deciduous in zone 6-a and further north. Protection from winter winds may retain the autumn foliage longer. Angustata mature tree height averages 20-25% smaller than the species.

Plant in part shade and in well-drained compost rich soil in southerly climes of zones 7-b and 8 and in full sun further north. Angustata is only moderately tolerant of summer heat and humidity and should be irrigated during periods of extreme drought.

Two evergreen cultivars are:

Empress of China®, introduced by John Elsley, flowers abundantly in mid to late May.
‘First Choice’, a selection from JC Raulston Arboretum in Raleigh, NC. has good flowering and growth rate.

Very Tall Stake For A Weeping Blue Atlas Cedar

Posted by Hugh on March 26th, 2012

Weeping Blue Atlas on Poor Site

Weeping Blue Atlas Cedar (Cedrus atlantica ‘Glauca Pendula’) is “gravity prone”. This strongly weeping tree just doesn’t happen without staking in its formative years. At planting position a sturdy 12-16 foot metal or plastic stake or rod close to the trunk at planting time.

The staking and tying procedure is neither complicated nor time consuming. When the spring growth flush has finished and while the leader is still soft enough (flexible), bend the leader upward and tie to the stake or trellis. Use soft jute or cotton twine, and not wire twist-ems® or plastic ties. After 6-8 months, loosen the ties so they won’t girdle the tender main shoot (leader).

The new shoots emerge in mid to late April. The needles have fully elongated by late May. Staking is best performed anytime from late June thru mid July in the Southern Appalachian region (USDA hardiness zones 6 and 7). The shoots should be slightly hardened but still flexible. If too stiff, the main shoot (leader) may not bend to straighten up vertically and leave a crook.

How many years you continue to stake ‘Pendula’ depends on the mature height desired. Most gardeners train to 5-8 feet, but I encourage you to set a higher goal between 12-16 feet.

Another idea is to train Blue Atlas cedar to an arbor or trellis at a garden entry (see photo below).

Staking when young provides an early foundation for a well-shaped mature tree. Plants can be alot like people. Youngsters left to their own devices become unruly adults.