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

Deadheading Rhododendron Flowers

Posted by Hugh on May 9th, 2016

Rhododendron days after flowering

After deadheading

Large-flowered rhododendrons (Rhododendron catawbiense) benefit from “deadheading”, the practice of removing the old spent flower heads. Deadheading encourages increased branching, which frequently results in more blooms the following spring. It also cleans up their scruffy appearance. Small-flowered types (including ‘PJM’, lepidote rhododendrons, and azaleas) do not benefit from deadheading and are mostly self-cleaning.

Deadhead your rhododendrons within three weeks after flowering. Vegetative buds have yet to emerge. Later, when the new soft vegetative shoots are emerging, they’re easily damaged. Deadhead early in the day when plants are fully turgid and not wilted. Deadheaded rhododendrons look better and it directs the plant’s energy into growing rather than setting seeds.

Flower stems are very sticky, so wear protective hand gloves. Gloves in turn also become sticky and results in bud breakage. You can use a sharp scissors or needle-nosed pruners to make clean, more precise cuts. Periodically, dip the shears in rubbing alcohol (ethanol) to remove the sticky debris.

Deadheading is beneficial to rhododendrons, but is not a necessary practice. It can be very time consuming. Otherwise, rhododendrons require little annual pruning. They eventually outgrow their space and/or crowd out nearby plants.

Avoid Spotted Spurge in Lawn And Garden Beds

Posted by Hugh on May 4th, 2016

Spotted Spurge becoms weedy mess

Spotted spurge (Euphorbia maculata) is a dreadful perennial weed that can quickly take over a lawn or garden bed. It often grows in poor compacted soils or in-between cracks in pavement. Once established, it is difficult to eliminate it from your garden.

Ideal temperature range for seed germination is 75 to 85 °F, and spurge will germinate in soil temperatures between 60 and 100 °F. Spotted spurge is most active from February through September.

Spotted spurge grows low to the ground like a carpet. Tiny leaves are dark green with reddish stems. Oval-shaped leaves (1/8 – 1/2 inch long) have a dark spot in the center. Plant is covered with fine hairs and its small pink flowers often go unnoticed. Its tap root grows long and deep. Seeds are prolific and are very hardy. Spotted spurge can grow back from either root pieces or seeds. It is a prolific seeder or can spread rapidly.

Hand-pulling in flower beds and vegetable gardens is no easy chore, and you must stay on top of it. The plant fans out from a main root, so yanking it rarely gets all the deep roots. Wear gloves as the milky sap can irritate your skin. Repeatedly weeding a bed eventually weakens the plant and eliminates it.

Glyphosate (Round-up® and similar trade name products) are very effective against spotted spurge. These herbicides are translocated, and eradicate all vegetation after multiple applications. It may re-grow from its deep taproot or from seeds dispersed from a neighbor’s yard. Stay vigilant and re-apply as needed. Contact weed killers become less effective as the plant’s foliage matures later in the year.

Barricade® (prodiamine) is a pre-emergent herbicide that stops weed seeds from germinating in your garden. It will not kill weed seedlings that have already sprouted.

Heavy mulching with several layers of newspaper or wood mulch is also an effective method of controlling spotted spurge.

In areas where spotted spurge is rampant, solarization may be your last resort. Blanket the weedy patch with plastic to kill off all vegetation. Allow 6-8 weeks to kill this tough weedy pest. Remove the plastic and seed lawn grass or garden bed in late summer or next spring.

Reviving A Severely Damaged Crape Myrtle In The Spring

Posted by Hugh on May 1st, 2016

Winter-injured Crape Myrtle

New basal shoots on Winter-Injured Crape myrtle

Crape myrtles (Lagerstroemia indica and hybrid cultivars) are rated as hardy perennials and semi-hardy shrubs or trees in USDA zone 6 and parts of zone 7. Since the year 2000, an average of 1 out of 3 winters has killed some crape myrtles to the ground. By late May new shoots emerge from the very hardy rootsystem below ground or from lower areas on the shrub or tree that were not injured by winter cold.

In general, crape myrtles grow very rapidly, assisted by adequate soil moisture and spring fertilizing. New shoots may also push out in upper parts of the plant, but are removed. It is best to severely cutback the plant or multi-branches near the base to invigorate numerous new shoots to pop out in the coming weeks

Repaired crape myrtle(s) bloom on new summer wood, so flowering is not lost, just delayed a few weeks.

Additional cultural tips: feed your shrub or tree crape myrtle immediately after pruning, using 10-10-10 or equivalent fertilizer. Lightly mulch around the base of the plant for a neat weed-free look. Irrigate only during long summer dry spells. When large numbers of new shoots form, you may opt to re-prune and eliminate all but 1, 3, 0r 5 shoots. By reducing numbers, the remaining shoots grow taller and still bloom.

Never, never prune or fertilize crape myrtles in fall and through most of the winter season.

Chinese (Kousa) Dogwood

Posted by Hugh on April 27th, 2016

Mature Kousa dogwood at Longwood Gardens decorated with Christmas lights

Mid-May Blooming Kousa dogwood

Chinese dogwood, aka kousa dogwood, (Cornus kousa) is a small 25 to 35 foot flowering tree (USDA hardiness zones 5-9). It grows in either full or partial sunlight (4-hours sunlight minimum). Depending where you garden, kousa dogwood begins blooming from late April or in May, almost two weeks after our native flowering dogwood (C. florida) has finished flowering.

Kousa dogwood is a tree for all seasons. New leaves emerge shortly before the tree flowers. The creamy white, four-pointed bracted flowers expand to 4 to 5 inches across. Pink blooming cultivars are a rare find. Aggregate green fruits, similar in shape to oversized blackberries (size of a nickel or larger), ripen orangey-red in September. Most birds do not relish them, but I’m told that monkeys love them.

Seedling kousa dogwoods exhibit wide variability in form, height, flower traits, leaf markings, and autumn leaf coloring. The 2-4 inches long summer leaves are generally dark green and glossy; variegated leaf cultivars are available. Leaf margins may be wavy or ruffled. Fall leaf colors varies from dull purple to crimson red. Bark mottling begins after 5-7 years as thin chips begin to flake off the main trunk and main branches. Some mature 50 plus year old trees are stunningly beautiful in the winter.

Young Kousa dogwoods often grow shrub-like. You can easily develop a tree framework by pruning; eliminate all root suckers and thin out inner branches to stimulate flowering.

Plant in a well-drained, moist, slightly acidic soil. Fertilized and mulch in early spring and irrigated during prolonged drought spells as this dogwood is only moderately drought tolerant.

Kousa dogwood possesses above average disease and pest resistance. Several hybrid dogwood cultivars are marketed with kousa as one parent. One novelty hybrid cultivar (C. nutalli x kousa) is Venus® which struts 6-7 inch wide white bracts; flower numbers are fewer, but are enormous in size.

‘Mojito’ Elephant Ear Is A Tropical Joy

Posted by Hugh on April 24th, 2016

Colocasia esculenta ‘Mojito’ at Biltmore Estates, Asheville, NC

Colorful Foliage of ‘Mojito’

‘Mojito’ elephant ear (Colocasia esculenta), aka taro plant, has become a new favorite in the Conlon garden where it is not dependably winter hardy (Zone 6b). According to Tony Avent at Plant Delights Nursery in Raleigh, NC, tubers are dependably winter hardy in zones 7b -10. ‘Mojito’ (pronounced “Mo-he-toe”) is a sport of Colocasia ‘Burgundy Stem’ x ‘Black Marble’. Its fancy leaves are medium green splashed with dark purple flecks. ‘Mojito’ is a thick-leaved, strong stemmed plant that stands up in summer downpours.

Elephant ears are easy to grow. Your garden or container soil (media) should be well-drained, mildly acidic, organically rich. Soil pH should range between 5.6 and 7.0. Locate in direct or partial sunlight and keep soil moderately moist. Keep garden plants adequately mulched and away from prevailing winds.

Elephant ears are constant feeders and love a steady diet of water plus water-soluble fertilizer such as Miracle-Gro®, Jack’s Classic®, or Peters® every 2-3 weeks from planting time (mid-May here in zone 6b) to late August at the one-half strength rate. By late summer expect Mojito to reach 2-3 feet high and wide, less so on drier sites.

Disease and pest problems are rare if plant(s) are properly sited and cared for. Potential crown or stem rot problems are best avoided by growing in a well-drained soil (media) and by not overwatering.

In areas with moderate winters (zone 7) elephant ears should be heavily mulched during winter to protect the central tuber which will likely perish in temps in the lower teens. Spread 2-3 inches of loose non-packing mulch such as straw or oak leaves.

In zones 5 and 6 container plants or tubers should be dug up, bagged, and stored in a cool (unheated) attached home garage where winter temps should not drop below 40 ºF. Remove all leaves and stems and hold in a semi-dormant state. At the start of spring, repot and gradually reintroduce elephant ears and other sensitive semi-tropicals to rising outdoor temperatures.

Where to purchase: independent garden centers and mail-order nurseries like Plant Delights Nursery, Raleigh, NC.

Monitor And Manage Two-Spotted Spider Mites

Posted by Hugh on April 21st, 2016

Two spotted mite on henbit (magnified) (photo courtesy of Dr. Frank Hale, Univ. of TN)

Phlox -2-spotted mite damage (credit: Mt. Cuba Center)

Two-spotted spider mites are destructive pests that ravage a wide host of shrubs and garden plants, including certain house plants, annuals, perennials, herbs and vegetables. Many evergreens are also susceptible.

Two-spotted spider mites are warm season arachnids, not insects, and are exceptionally troublesome over a hot dry summer. They have a short lifecycle, from egg to adult can be completed in about seven days. High numbers can build up and get out of hand in less than a month.

Two-spotted spider mites have sucking mouth parts and most frequently feed on the underside of leaves, sucking out green sap which results in a mottled or flecking appearance. Severe infestations can cause leaves to drop off and massive webbing over plant foliage.

Monitor periodically to see if they are present and in what quantities. Take a clipboard with a piece of white or yellow paper. Briskly shake the flowers or foliage over the paper and inspect for mites. Adult mites are easy to spot; they will be moving and have 8 legs (insects have 6 legs). They’ll have two black spots on both sides of the abdomen. Also use a 20x hand lens for accurate identification.

A number of miticides are available but they must be applied correctly. Thorough coverage of all plant parts is essential. Many miticides are contact only, but some are translaminar, e.g. penetrate through the leaf tissue and remain active in the leaf.

Horticultural oil sprays can be used. Spray once every 7-10 days(no more) to maintain mite populations at low levels. Insecticidal soaps can also be applied. Professional miticides, not necessarily available to home gardeners, include abamectin (Avid 0.15 EC), acequinocyl (Shuttle O), bifenazate (Floramite 50 WP), etoxazole (TetraSan 5 WDG), spiromesifen (Forbid), and hexythiazox (Hexygon).

Never rely on the same miticide in the garden. Select two miticides for spider mite spray program that are different chemical classes.

Credit: Pesticide recommendations supplied by Dr. Frank Hale, University of Tennessee Entomologist, Nashville.

Mite Webbing on Marigolds in Late Summer

Japanese Holly Fern

Posted by Hugh on April 18th, 2016

Japanese holly fern (Cyrtomium) In Woodland Area at Biltmore Estate in Early February

Japanese Holly Fern (Cyrtomium falcatum ‘Rochfordianum’) is a handsome, evergreen fern for gardens in USDA hardiness zones 6-11. It earns its name from its stiffly erect holly-like fronds. Individual fronds are vase-shaped, 1 to 2 ½ feet long with glossy, leathery, serrated, holly-like compound leaflets (pinnae). The slender, arching fronds arise from erect, scaly rhizomes just above the soil surface.

Spore masses are light green when young and dark at maturity; they cling to the backs of leafy fronds. Japanese holly fern is evergreen in frost-free growing areas, but tends to lose its fronds in colder climates. It also tolerates more sun than many other woodland ferns.

It is best grown in compost-rich, well-drained soil with medium moisture and in part shade to full shade. Annual mulching around holly fern is beneficial; it does not ask for much feeding. It tolerates dry air, low light and gas fumes in the home better than most ferns. Extreme summer dry spells or too much sun will singe pinnae.

Holly fern develops into an effective groundcover and can be grouped to edge a woodland or shady border. It performs well in containers either planted by itself or mixed with other woodsy perennials. Holly fern also makes a great indoor plant in the colder parts of the U.S. Holly fern is not troubled by disease or pest problems. Problems with root rots or leaf spotting may indicate a poor site. Deer and rabbits usually stay away from the sharp needle foliage.

‘Palibin’ Lilac A Good Choice For Small Landscapes

Posted by Hugh on April 15th, 2016

Tree form of Syringa meyeri ‘Palibin’

Dwarf Korean lilac (Syringa x meyeri ‘Palibin’) forms a dense multi-stemmed deciduous shrub, 4 to 5 feet high and 5 to 6 feet wide, with rounded top or canopy (USDA hardiness zones 3-7). It branches low to the ground.

Flower buds form which are distinctively violet in color. In mid-spring this diminutive lilac is covered by billowy panicles of fragrant purple flowers. Trusse are excellent for cutting and enjoying inside the home for their color and fragrance. Spring foliage first arises burgundy and becomes dark green all summer long.

Palibin lilac maintains its neat and tidy compact form. Plant in full sun and a well-drained soil for best flowering annually. Initial growth rate is slow and under proper care, specifically annual feeding and pruning, will prosper for 25 to 30 years. It then may need to be renovated by pruning.

Compared to other lilac species (Syringa spp.), Palibin leaves are smaller and the overall shrub offers a fine texture appearance. The small pointed leaves do not develop an appreciable fall color. Smooth gray bark and fruits (seed capsules) offer no ornamental value.

This low maintenance shrub should be grown primarily in full sun. It is very adaptable to both dry and moist sites, but not in standing water. It is not sensitive to soil type or pH and leaves tolerate urban inner city conditions. Wait to prune a month after spring flowering to avoid removing spring flowers. Palibin is a good choice for attracting butterflies.

Dwarf Korean lilac can be utilized as a single specimen shrub or group as short hedges or privacy screens. It also may be nurtured in a large container for several years. Foliage is highly powdery mildew resistant on landscape sites with good air movement. As the above photo depicts, nurseries are now training Palibin lilacs into small compact trees.

Update: Prevention / Treatment Options For Rose Rosette Disease

Posted by Hugh on April 13th, 2016

Rose Rosette Symptoms (photo by Dr. Alan Windham, Plant Pathologist, Univ. of Tennessee, Nashville

Rose rosette is a devastating disease of most species of garden roses, including hybrid and shrub roses. Rose rosette disease has spread from the Rocky Mountains to the East Coast over several decades and is poised to obliterate the rose industry. To date, there is no known effective treatment.

Rose rosette is caused by an Eriophyid mite called Phyllocoptes fructiphilus, which transmits the virus. Symptoms of the virus include: (1) excessive thorn production, (2) leaf distortion, and (3) excessive branch development, known as “witches broom” that will eventually kill the plant.

U.S. plant scientists have joined a multi-state comprehensive project to develop a management plan. USDA has extended a $3.3 million research grant to a 17-member group headed by David Byrne of Texas A&M University to fight rose rosette. “We are anxious for us to develop options for managing this virus”, says Dr. Gary Knox, Professor of Environmental Horticulture and Extension Specialist at the UF/IFAS North Florida Research and Education Center. Rose production is a $400 million annual business in the U.S.

A nursery might not know it has the disease and sell rose plants to unsuspecting customers. Months later the disease shows up. The major issue is being able to detect the virus before it shows up. A Florida team is developing techniques to detect low levels of the virus in the plant. The goal is to detect the virus in non-symptomatic plants utilizing a rapid field-based assay. They are trying to develop a field-based detection system to find the virus early.

Scientists are looking at new compounds for preventing or managing the disease. The team is treating plants with compounds that would potentially help plants better defend themselves against the virus and are trying to reduce the severity of the symptoms.

Finally, plant breeders are attempting to develop a new generation of roses that are highly resistant to the rose rosette virus.

Information credit: Edited article from Greenhouse Grower Magazine, March 2016

New Coreopsis Cultivars All The Rage

Posted by Hugh on April 12th, 2016

Coreopsis verticillata at Kingwood Center, Mansfield, Ohio

Coreopsis ‘Jethro Tull’ (photo from ItSaul Plants, Inc.)

Threadleaf coreopsis (Coreopsis verticillata), also called tickseed, is a popular summer flowering perennial. Coreopsis are native to the U.S. and produces hundreds of pale-yellow blooms. Depending on cultivar, plant grows 12 to 18 inches tall and 2 feet wide. Some gardeners even plant old-time favorites like ‘Zagreb’ and ‘Moonbeam’ as annuals (USDA hardiness zones 4 – 9).

Big flower tickseed (C. grandiflora) produces larger flowers on taller plants, up to 3 feet high. Flowers are bright yellow and daisy-like. Many cultivars re-bloom a second and even third time if mowed back after a bloom cycle has finished. They tolerate hot, humid summers, but garden soil should not be allowed to dry out. Height and spread vary by cultivar and level of care (irrigation + fertilizer).

Coreopsis are one of the easiest perennials to grow. For best show plant 3 or more of one cultivar for a sea of color in a sunny front garden border or in containers. Set out plants anytime from late April through September. Plant 12 to 18 inches apart in a sunny spot and in well-drained soil. Plant the crown just at or above the soil surface.

Removing spent flowers encourage re-blooming, prevents reseeding, and gives the flower bed a neater look. Yellow to gold flowers attract numerous nectar seeking bees and butterflies. Leaving the seedpods on plants will attract large numbers of goldfinches.

Divide clumps every 3 years for optimum plant performance. Aphids may be occasionally pests. Powdery and downy mildew on foliage in garden areas with poor air circulation; some cultivars are rated better for mildew resistance.

Recent Coreopsis Introductions:
‘Cosmic Eye’- daisy blooms with wine-red petals, tipped in soft yellow and surrounding an orangey yellow button eye; grows 18 inch high by 24 inch wide. Introduced by Darrell Probst and is reliably hardy in zones 4 – 9 with winter snow cover.

‘Jethro Tull’ – compact grower, 18 inches high by 24 inches wide with dark-green leaves and blanketed with single golden-yellow daisies with a ring of tubular petals; hybrid introduced by ItSaul Plants in Georgia.

‘Mercury Rising’ – another Darryl Probst introduction large velvety-wine daisy flowers with contrasting gold-orange button center, appearing in succession from mid summer to mid autumn. Some flowers may appear frosted in creamy-white. Cut plants by half in mid-July if plants begin to flop. (hardy in zones 4 – 9 with winter snow cover).

‘Route 66’ – threadleaf coreopsis discovered in 2005 in a Pennsylvania garden; plants grow 24-28 inches tall with bright yellow petals with large red eye; long blooming period from late June until mid October.