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

Why Mophead Hydrangeas Do Not Bloom

Posted by Hugh on March 18th, 2015

Hydrangea macrophylla at Morris Arboretum in Philadelphia

Hydrangea ‘After Midnight’ in Savannah, GA Garden

The introduction of Endless Summer® hydrangeas in the 1990’s got gardeners excited about growing mophead hydrangeas (Hydrangea macrophylla) again. Mophead hydrangeas bloom both on old wood in the spring and again on new wood in mid-summer. Flowers have either a blue/pink color depending on the pH of the soil. Endless Summer hydrangeas bloomed in cooler areas previously off limits for mopheads.

Bad Winter? Endless Summer hydrangea series are still very dependable and have spawned a number of new series. Still, abnormally cold weather in many areas may greatly decrease late spring/summer flower numbers. On the first day of spring ask yourself how the previous winter felt personally. If you rate the winter as bad, likely spring mophead bloom will be less.

Available sunlight. In the north (zones 4 – 5), mopheads can handle almost full day sun. In zones 6 and 7 reduce to 4-5 hours of mostly morning sunlight, and in zone 8 to 9 in day-long high shade. Less than optimum light levels means less flowers.

Wrong Cultivar. The marketplace is very confusing with many non-hardy cultivars still being sold.

Late Freezes. Hydrangea buds open very early. Late spring freezes frequently destroy or injure flower buds and new growth. Keep the crown (plant base) heavily mulch until mid-May in most areas.

Over-fertilizing hydrangeas may result in dark green foliage (a good thing) and fewer flowers (not so good). In USDA zones 4 and 5, do not fertilize past August 15th, as it may lead to winter injury.

Overwatering mopheads may push too much growth and fewer flowers. On hot summer days seeing mophead leaves wilting in mid-day is very common. Irrigate once weekly, 1 to 1 ½ inches of water, rather than a little bit every day.

Pruning mopheads in the fall is a mistake. Spring flowers are borne on the old wood.

Old-fashioned Star Magnolias Very Beautiful Start Of Spring

Posted by Hugh on March 15th, 2015

Star Magnolia in Northeast TN garden

Multi-petalled Star Magnolia

Star magnolia (Magnolia stellata) is a multi-branched shrub or small tree which is native to Japan (USDA hardiness zones 4 to 8). This early flowering magnolia is noted for its compact form. A mature tree form grows 15 to 25 feet tall, 10 to 20 feet wide, and exhibits an umbrella-like canopy. Each flower typically contains 12-18 narrow thick fleshy petals (referred to as “tepals”).

In southern U.S. climes, its white flowers emerge weeks before the leaves, often a week or two before the official start of spring. In a colder northern spring it may wait to bloom in mid-April. White fragrant blooms are often damaged by spring frosts, but the remaining buds wait to open; flowering may continue over 2-3 weeks.

Star magnolia is easily grown in an average, medium, well-drained soil in full to part sun (4-hours minimum). It prospers a rich organic soil. It is best sited sheltered away from high winds; avoid a southerly exposure where flower buds may open too early and get frost-killed. Star magnolia suffers from few serious disease or pest problems when properly sited.

Star magnolia stands as a handsome small landscape tree or large shrub near a patio or in front of an evergreen background. Plant several together as a hedgerow. Star magnolia is best planted in late winter into spring, either balled and burlapped (b&b) or container-grown.

Little Bluestem – Beautiful And Environmentally Correct

Posted by Hugh on March 12th, 2015

Schizachyrium_scoparium 'StandingOvation' (photo courtesy of North Creek Nursery, Landenberg, PA

Schizachyrium_scoparium ‘StandingOvation’ (photo courtesy of North Creek Nurseries, Landenberg, PA

Little Blustem at Brooklyn Botanical Gardens in NY in October 2014

Little Bluestem (Schizachyrium scoparium) grows in old fields, meadows and prairies, and along roadsides across the United States and Eastern Canada (USDA hardiness zones 3-8). It forms dense clumps 2- 3 feet tall and 12 -18 inches wide. This warm season grass prefers a well-drained soil and full to part sun site. It looks its best in low nutrient soils; when fertilized or frequently irrigated, clumps tend to flop over. It grows easily from seed. It is very drought tolerant and survives over many years. A new seeding of little bluestem will be very small its first year as this native grass puts much of its energy into growing roots first.‘Standing Ovation’ is an outstanding vegetatively propagated cultivar introduced by North Creek Nursery in Landenberg, PA. It maintains a tight, upright habit throughout the entire season. Sturdy erect stems start out with a bluish cast; grass blades and stems turn fiery shades of yellow, orange, brown, red and purple in fall and winter.

Gardeners and landscapers should take advantage of its strong vertical presence of this unique cultivar. It grows 3-4 feet tall and only 6-12 inches wide. Leaf blades tend to be thicker than on other bluestem; disease and insect problems are rare and usually caused by poor site selection.

Plant little bluestems to edge a garden path, driveway or sunny flower border to catch textural contrasts. Little bluestem is highly deer-resistant. Several species of skipper butterfly caterpillars feed on the leaves and birds use it for cover and for nesting material.

Double Knockout® Roses

Posted by Hugh on March 9th, 2015

‘Pink Double Knockout’ roses at UT Gardens in Knoxville, TN

Knockout® roses continue to wow gardeners across the U.S. (USDA hardiness zones 5 to 9). The original Knock Out series grew 5 to 8 feet tall and 4 to 6 feet wide. Shorter growing Double Knockout® series make better fit in most urban gardens at 3 to 4 feet high and 3 to 4 feet wide. Double blossoms have the look of hybrid tea or floribunda roses. Colors range from red, pink, coral, and yellow.

All Double Knockout roses are bred for its resistance to common rose pests and diseases. They bloom heaviest in May and June, retreating in the heat of summer and finish with a flurry of color in late summer and fall. They hold up under rough weather and moderate drought.

Roses are best planted from spring thru mid-summer to permit roots adequate time to grow deeply before winter arrives. Double Knockout® roses should be planted 4 feet apart in a garden site that receives 6 to 8 hours of sun most days and in well-drained soil that ranges between pH 6.0 and pH 6.5. Roses need good air movement, so don’t crowd them.

Dig a wide hole to accommodate the roots from the container grown plant. Set the root ball slightly shallow in the hole. Backfill the hole with soil and water deeply. After planting, water regularly (usually weekly) until the plant becomes well established. Apply 2-3 inch depth of an organic mulch to conserve soil moisture and reduce weed infestation. Do not pile mulch around the rose trunk.

Fertilize Double Knockouts with a water-soluble fertilizer monthly from early spring until September 15 (no later). An alternate feeding method is to use special organic based rose foods sold at most garden centers.

In late winter or early spring prune roses when they are still dormant to remove all dead or damaged wood and to maintain the roses at a desired size.

Basic Hosta Facts

Posted by Hugh on March 8th, 2015

‘Golden Scepter’ Hosta in Kingwood Center gardens in Mansfield, Ohio

Hostas are the most popular shade perennial in the U.S. They prefer a moist, compost-rich, well-drained soil. In northerly areas (USDA hardiness zones 4-5), where summers are cooler, most hardy cultivars grow in full sun. In warmer zones those in full sun must be irrigated frequently.

Blue leaf cultivars look their best only in shady areas; the white waxy coating, responsible for their bluish foliage color, melts off quickly under high light. By late summer most of the leaf wax has weathered away even under shade.

While mostly free of diseases and pests, many cultivars are susceptible to foliar injury from slugs and snails. Deer are also a serious menace.

Hostas should be divided every 5 -6 years, usually in the early spring when emerging shoots poke through the ground or in early autumn as leaves decline. Exact timing is not critical. Dig up the entire clump and divide with a sharp knife. Each division should contain at least 3 eyes (growing points).

In frigid northern areas, hostas should be mulched with a layer of finely shredded organic material to prevent frost heaving, particularly newly divided plants. Mulch is also reduces soil moisture evaporation; unfortunately it serves as an an ideal home for slugs.

Holes in leaf centers are a symptom that snails and slugs are present. Apply a commercial slug bait in early spring as shoots emerge to reduce their numbers. In the fall rake up and dispose off (do not compost) all hosta foliage to reduce slug and leaf-eating insect populations.

Depending on cultivar hosta’s funnel-shaped white or purple flowers arise equal to or several inches above the foliage. Yes, flowers are fragrant, but their scent is rarely overwhelming unless a fragrant cultivar is present in large numbers.

American Hosta Growers Organization offers different sizing standards for cultivars for the North and South*:

Type North South

Mini Up to 8 inches Up to 7 inches

Small 9-14 inches 8-11 inches

Medium 15-22 inches 12-18 inches

Large 23-29 inches 19-24 inches

Giant 30 inches and above 25 inches and above

* Interstate 80 divides the U.S. horizontally using North (above I-80) and South (below I-80)
Below Interstate 40 designated for the South. The area between these two Highways is designated Middle.

The Winterhazels Bloom In Late March Garden

Posted by Hugh on March 6th, 2015

Winterhazel (Corylopsis) at Atlanta Botanical Gardens

C. pauciflora ‘Red Leaf’ at UT Gardens, Knoxville, TN

Winterhazels (Corylopsis spp.), native to China and Japan, represent a group of winter flowering shrubs of varying heights and widths (USDA hardiness zones 5 to 8). Few U.S. gardeners know them. Their bright yellow colored flowers are larger and showier than witchhazels (Hamamelis spp.). Flowering begins as the witchhazels (they’re related) are finishing up in late February and March, many weeks before forsythias bloom.

The winterhazels are welcome additions to shrub borders or woodland spaces. There are eight or more species available from on-line nurseries. Spike winterhazel (C. spicata) and buttercup winterhazel (C. pauciflora) are low growing or spreading forms that fit well into urban and suburban gardens. The lemony yellow flowers have red-purple anthers and are slightly fragrant.

Spike winterhazel grows to 4-8 feet tall and to 6-10 feet wide. Individual flowers, only 3/8 inches long, comprise small drooping clusters (racemes to 2 inches long); clusters contain 6-12 flowers. After bloom, 4-inch long ovate to obovate leaves unfold colored bronze-purple and, within weeks, change to blue-green color. Tiny hairs on the leaf surface tend to collect dust over the summer, resulting in a blah yellowish-green fall color. Fruits are two-beaked capsules, each containing two small seeds.

Winterhazels prefer a compost-rich, well-drained, acidic soil in full sun to part shade. Winterhazels tolerate most garden soils except heavy clays. Plants favor shading from direct afternoon sun in southerly climate zones as well as shelter from high winds. Flower buds/flowers are susceptible to cold injury if winter temperatures are severe.

After flowering or by late spring, prune winterhazels as needed to hold them in check. At the same time feed with 10-10-10 or equivalent granular fertilizer. There are no serious insect or disease problems.

Rose Care Starts With A Good Planting Site

Posted by Hugh on March 4th, 2015

Great Roses Start With A Good Planting Site (UT Gardens, Knoxville, TN)

Roses should be planted 4 feet apart on a garden site that receives 6 to 8 hours of sun. The garden soil must be well-drained and pH between 6.0 and 6.5. Prevent disease problems by providing good air movement between plants and not crowd them.

Spring thru mid- summer is an ideal period to plant roses to allow their roots to grow deep before winter arrives. Dig a wide planting hole 2 to 3 times that of the container root ball. Tease (gently pull apart) root ball to spread the roots prior to setting and refilling the hole. Planting depth of the root ball should be same or slightly less than it grew in the container. Backfill the hole with soil and water thoroughly.

Apply 2-3 inch layer of loose organic mulch to conserve soil moisture and to reduce potential weed infestations. Do not pile mulch around base of the rose trunk. Irrigate roses regularly (usually weekly) until the plant(s) become well established, usually within 4-6 months. Provide an average of one inch of water per week from natural rainfall and irrigation.

Fertilize roses with a water-soluble fertilizer monthly from early spring (March) until September 15 (and no later in USDA hardiness zones 6 and 7). Use a water soluble fertilizer such as Miracle-Gro™, Nature’s Source™ or Rose-tone™ according to package directions. Another feeding alternative is a slow release fertilizer specifically for roses in early spring. Or apply a granular fertilizer such as 10-10-10 or equivalent in early spring and a second time in late June.

Prune the dormant rose plant in late winter to early spring. Remove all dead or damaged wood; take out weak and tangling interior branches for better air circulation. Every 2 or 3 years remove about one third of the old branches to stimulate new growth and to maintain the rose at a desired height and spread.

Spicebush – The Native Plant And The Butterfly

Posted by Hugh on March 1st, 2015

Bright Red Spicebush Fruits In Fall

U.S. native spicebush (Lindera benzoin) is a beautiful landscape plant (USDA hardiness zones 4-9) for an open or woodland garden. Spicebush matures into a 12-15 feet tall and wide large shrub or small tree over many years.

Light green deciduous leaves, 3 to 5 inches long, are delightfully fragrant when-crushed. Leaves is the preferred food for the larvae of the Spicebush swallowtail butterfly (Papilio troilus).

This dioecious shrub contributes a great deal of pleasure: aromatic foliage, showy autumn fruits, and lemony yellow fall color. The tiny 1/5 inch wide pale yellow flowers open in early spring and go mostly unnoticed during a time when showier shrubs and trees are also blooming. Female plants are ladened with 1/2 inch diameter berries that turn from green to yellow to glossy red. Fall red ripe berries quickly disappear, devoured by numerous birds and 4-legged fauna. Fall color rating varies from one year to the next and may be brightly yellow in good years.

Spicebush thrives in moist, well-drained, acidic soil and in full to partial shade. Initial growth rate is slow and gradually picks up in succeeding years. A multi-year old spicebush will naturalize in a moist or dry woodland site. Warning: spicebush tends to root sucker.

The spicebush swallowtail butterfly is endemic to the eastern US and southern Ontario. They are among the largest butterflies in the world. Spicebush swallowtail adults (butterflies) nourish themselves on nectar filled garden flowers and lay her eggs on spicebush foliage. Eggs hatch and the brown caterpillar larvae feed on the spicebush leaves.

Fairy Wings (Epimediums) Carefree Shady Groundcovers

Posted by Hugh on February 26th, 2015

Epimedium perralderianum

Fairy wings, aka barrenwort (Epimedium spp.) thrive in shady spots. Fairy wings continue to be a plant collector’s dream. There are over 70 cultivars to choose and new hybrid selections coming along every year. No two cultivars are alike in flowering, foliage size and color. Species will vary from 6 to 20 inches in height and 10 to 24 inches in spread.

Fairy wings, depending on species, may be evergreen or deciduous. Seasonal leaf colors are also variable. Some begin chartreuse, turn dark green in summer, and finish in autumn shades of red. Others may develop a red or silvery tint in the summer.

Dainty flowers appear in early spring and are rarely hurt by cold morning temps. Flowering interval lasts 3, sometimes 4 weeks long and attract early arriving butterflies and hummingbirds. Deer don’t bother fairy wings.

Fairy wings grow in average soil. It may take 2-3 years for you to decide that fairy wings are for you. They start out slow. Provide them with a half day sunlight, preferably in the morning or in day-long partial shade. Feed slow-release fertilizer at planting and each spring; or feed monthly from spring to mid-August with a water-soluble plant food.

Clip back or mow over at 4-5 inch height setting in March as winter weather begins to moderate and before flowers appear. Removal of older foliage permits easier viewing of emerging flowers. Fairy wings have almost no pest or disease problems.

Fairy wings are excellent ground covers for planting under trees or fronting flower borders with the taller shade-loving perennials such as Lenten roses (Helleborus spp.), Variegated Solomon seal (Polygonatum), brunneras, astilbes, and ferns in the rear.

Thinking about starting a fairy wings collection? Start with these five:

Red barrenwort (E. × rubrum) – reddish summer semi-evergreen foliage; bicolor flowers.
E. perralderianum – yellow spring flowers; spiny-edged evergreen leaves, tinged red in the fall
Young’s barrenwort (E. × youngianum) – deciduous green spring foliage starts reddish green, green in summer, ending red in fall; pinkish white flowers.
‘Sulphureum’ barrenwort (E. × versicolor) – mottled red foliage becomes evergreen; primrose yellow flowers.
Longspur barrenwort (E. grandiflorum) – light green heart-shaped deciduous foliage; large white flowers.

Pest Alert: Japanese Maple Scale Becoming A Serious Problem

Posted by Hugh on February 24th, 2015

Japanese maple scale (photo by Dr. Amy Fulcher, University of Tennessee Horticulurist)

Japanese maple scale is a new pest in Tennessee and surrounding states. It infests many more plant species than just Japanese maple. The insect’s small size and ability to blend in naturally with tree bark makes it challenging to detect until populations are high and it has infested large landscape and neighborhoods.

Japanese maple scale (JMS) is a small, oystershell-shaped, armored scale (photo on left). The waxy coating over the body is white, but the female, eggs and crawlers (the immature stage) are lavender in color. Scales are most commonly found on bark but can be found on leaves.

JMS life cycle isn’t completely understood. The insect overwinters on trunks and branches as a small but visible immature scale that matures in spring. Females lay about 25 eggs under their body. Eggs are estimated to develop in April and May, with egg hatch and crawler emergence likely to occur in mid-May in central Tennessee. Crawler hatch coincides with bloom of Syringa reticulata ‘Ivory Silk’ and Hydrangea quercifolia. JMS has two generations a year, a second egg hatch begins around August 1. JMS has extended crawler hatch, which causes the first and second generations to overlap.

JMS has an extremely wide host range that includes Acer, Amelanchier, Camellia, Carpinus, Cercis, Cladrastis, Cornus, Cotoneaster, Euonymus, Fraxinus, Gledistia, Ilex, Itea, Ligustrum, Magnolia, Malus, Prunus, Pyracantha, Pyrus, Salix, Stewartia, Styrax, Syringa, Tilia, Ulmus, Zelkova and others. JMS has been identified along the East Coast of the U.S., Virginia, Kentucky and Tennessee.

The small, white adults are difficult to see and often blend in with light-colored bark or lenticels. Focus at the base of the plant near the soil line to approximately 8 inches high. On standard-form trees, look on the trunk and scaffold branches, in particular at the branch collar. JMS is easier to see in the dormant season when foliage is not hindering the view and the waxy coating appears brighter. JMS is often hidden within the protected interior of dense plants.

The protective waxy coating over the crawlers limits the effectiveness of contact sprays. JMS is especially difficult to control in Ilex and other heavily sheared plants with a dense canopy. A contact insecticide spray must effectively penetrate the canopy to control pest.

Thorough applications of horticultural or superior oil during the dormant period in late winter have been successful. Insecticide sprays should be used to target each flush of crawlers during the spring and summer. Recommended insecticides include the insect growth regulators pyriproxyfen (Distance) or buprofezin (Talus 40SC) and the neonicotinoid clothianidin (Arena 50 WDG). Horticultural oil (at 0.5 to 1 percent) may be tank-mixed with Distance or Talus 40SC for improved control.