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

Katsura Trees For Urban Plantings

Posted by Hugh on August 11th, 2019

Hooray! American gardeners are finally becoming savvy about Katsura tree (Cercidiphyllum japonicum) as one of the finest landscape trees. This medium to tall growing tree (height depending on cultivar) possesses four-season appeal. Numerous varieties are now available. Small growing cultivars will be featured here.

Katsura tree is indigenous to Japan and China where some may reach an incredible 100 feet tall (USDA hardiness zones 4-8). In the U.S., it is a long-lived majestic 40-60 foot high tree. It exhibits a pyramidal form in its early years. Some aged specimens at some American botanical gardens are easily 75 feet tall.

The species is dioecious, e.g. separate male and female trees. The tiny flowers are mostly inconspicuous and small pods with numerous winged seeds are dispersed from female trees in late fall. Katsura tree is not invasive.

Its beautiful heart-shaped soft green summer foliage transforms to purple and gold shades in autumn. After the leaves shed in the fall the tree’s lovely symmetrical branching and scaly/shaggy bark is exposed. Often, the fall air holds a cinnamon scent reminiscent of cotton candy or a sugar-cookie.

‘Red Fox’ Katsura at Atlanta Botanical Gardens

Katsura Cultivars for Small Urban Landscapes:

‘Red Fox’ (‘Rotfuchs’) – exhibits new red spring foliage that becomes dark purple-green in summer.

‘Claim Jumper’ – a relatively new gold-leaved variety that grows 25-30 feet (8-10 m) and 20 feet wide. Leaves emerge with a pink blush and unfurl to soft, golden-yellow. Gold leaf color is lost by late summer.

‘Heronswood Globe’ – a small 15 to 20 feet tree with a rounded canopy, a good fit in a small urban landscape.

‘Raspberry’ – small 14 feet tall x 7 feet wide in 10 year; foliage turns a rich raspberry red fall color.

‘Tidal Wave’ – small weeping form that eventually grows to 20 feet and slightly wider

Katsura tree prefers a moist well-drained soil and full sun to part-shade. Its growth rate is equal to sugar or red maples. Seasonal soil moisture is most important. Newly planted 1-2 year old trees are particularly prone to leaf scorch in dry soils. Over the first 2-3 years following planting, supplemental irrigation should be provided.

Katsura are rarely troubled by serious pests or disease problems. Katsura tree and cultivars should be available at full-service garden centers, but sharp early shoppers are likely to buy up the inventory. Rare or new cultivars, some that are listed here, are sold by on-line specialty nurseries.

New Inkberry Varieties Substitute For Boxwoods

Posted by Hugh on August 6th, 2019

Inkberry holly (Ilex glabra) is a slow-growing evergreen shrub (USDA hardiness zones 5-9). This U.S. native hale from coastal Canada south to Florida, and west to Louisiana. Commonly, inkberry (the species) grows to 5-8 feet tall. It is at home in sandy woodlands and the edges of swamps and bogs where they form colonies via root suckers.

Two new varieties from Proven Winners® and Spring Meadow Nursery are Strongbox® and miniature Gem Box® , both growing only (24 – 36 inches tall and wide). Ideal for container planters! Strongbox shape is abit squat at the top than Gem Box. Utilize both as boxwood substitutes if you’re concerned about boxwood blight disease. Both varieties are densely branched with a low compact growth habit; they may lightly root sucker.

Inkberry hollies are undemanding., growing in average, medium to wet, acidic soils, and in full sun to part shade (4 hours minimum). It is one of few hollies that tolerates wet soils. Inkberries are dioecious, either male or female tiny white flowers. Black drupe (berry-like) fruits develop in the fall and persist through winter unless consumed by birds and other wildlife.

Inkberry hollies are popular hedging shrubs. Prune in early spring before new growth begins. Gem Box and Strongbox cultivars rarely need much pruning. Remove root suckers if undesired.

Inkberry is highly resistant to insect or disease problems, and winter burn. Leaf spotting occurs in wet summers. Spider mites may appear, especially during dry weather.

Fertilize in spring with an acidic water soluble fertilizer such as Holly-Tone® or Miracle-Gro® In high pH (alkaline) soils, foliar chlorosis (yellowing) may occur. Spider mites may appear, especially in dry conditions.

Mass or group several inkberry hollies as a shrub border or as a low foundation planting. Species inkberries will naturalize in moist woodland gardens or near streams or ponds.

Tall Stonecrops (Sedums)

Posted by Hugh on August 1st, 2019

Sedum ‘Autumn Joy’

Sedum ‘Matrona’ at NC Arboretum, Asheville, NC

Stonecrops (Hylotelephium spectabile) are tall clumping sedums. They are frequent seen in their habitat growing on rocks or stony ledges. Also written as Sedum spectabile, many varieties grow 1.5 to 2 feet high. Approximately 33 species of these drought-tolerant herbaceous perennials are found to Asia, Europe and North America (USDA hardiness zones 3-9).

Stonecrop make excellent plants for the late summer – fall garden. Their late season flowers attract butterflies and other pollinators. Foliage and spent floral heads persist well into the winter providing some architectural interest.

Stonecrops like it “mean and lean”; set plants in average well-drained soils and under full sun. Plants thrive in sandy to gravelly soils under moderate to low fertility. Under light shade or in rich organic soils, stonecrops produce weak floppy shoots. Also, they decline in wet soppy soils.

They are easy to propagate by divisions in spring or from stem cuttings in early summer. To build up a large collection cheaply, detached leaves can be rooted in coarse sand that eventually form new plants. Plants may be cut back in late spring to control height.

Generally, serious insect or disease problems are rare. Slugs, scales, mealybugs, nematodes, and aphids are occasional pests.

Sedums provide winter interest and food for birds. Plant them in front garden borders, containers, and rock gardens. Mass several together for a colorful show.

Popular varieties:

‘Autumn Joy’ (‘Herstfreude’) – a very popular variety; blooms emerge pink, gradually change to deep rose-red and finish coppery-rust. Flowers are variable in size, 3-6 inches across, flattened heads (cymes), typically 1.5 to 2 feet tall.

‘Autumn Fire’ – an improved ‘Autumn Joy’, slightly taller and sturdier, less floppy stems.

‘Brilliant’ – tiny, star-like, carmine flowers, 3-6 inch flattened blooms atop grayish-green stems; plants form upright to slightly spreading clumps to 18 inches in height.

‘Matrona’ – strong, shiny red stems hold large, pale pink flower heads which last well into the fall season; noted for its deeper gray foliage and strong, erect purple stems.

‘Neon’ – (perhaps a sport of ‘Brilliant’) typically grows in upright to slightly spreading clumps to 15-20 inches tall. It grows slightly shorter than Brilliant with more steadfast magenta flower color and slightly larger bloom heads.

July Blooming Plumleaf Azalea

Posted by Hugh on July 28th, 2019

Plumleaf azalea (Rhododendron prunifolium), a deciduous azalea native to Georgia and Alabama, is a particular summer favorite (USDA hardiness zones 6-9). It’s hardy even in Boston, NY City, and Philadelphia. Its bright orange-red blooms surprise in late July to early August. Compared to many fragrant spring blooming native species, flowers emit no scent. Its autumnal leaf color is uneventful.

New red blooming form (Photo by V. Abney)

This 5-8 feet tall and wide shrub is best protected from summer ‘s harsh afternoon sun. Planting on a north or east facing site and in mildly acidic, well-drained, compost rich soil is ideal. Plumleaf azalea appears to prospers calcareous clay soils found in the Eastern Tennessee and Western NC without pH correction.

Irrigate plants the first two years of establishment. Set the shrub into a wide, shallowly dug hole which has been generously amended with compost and/or peat; maintain 2-3 inches of a bark chip type mulch around the shrub base.

Plumleaf azaleas are generally sold at native plant on-line nurseries. Purchase and plant by late summer so that the fine shallow roots get deeply knitted into the soil before winter arrives.

Plumleaf azalea is the signature plant of Callaway Gardens in Pine Mountain, Georgia where I initially purchased the plant which has prospered in my northeast Tennessee garden the past 28 years ago. A new red flower form is becoming more available (see photo).

Plumleaf azaleas bloom on new current season’s wood. Prune this summer bloomer anytime after flowering to maintain desired height and spread.

Why My Hydrangeas Are Not Blooming

Posted by Hugh on July 23rd, 2019

Thankfully, this is not a very common problem that few gardeners run into. U.S. gardeners grow four (4) species of hydrangeas: smooth hydrangea (Hydrangea arborescens), oakleaf hydrangea (H. quercifolia), bigleaf hydrangea (H. macrophylla), and panicle hydrangea (H. paniculata).

There are five reasons why some hydrangeas don’t bloom:

1. Type of hydrangea… Bigleaf hydrangeas, those that produce blue or pink flowers, are the most troublesome. Lots of cultivars are available, some that are very winter hardy and others not so much. If winter temps in your region are extremely low, plants may die back to the snow line or to the ground. Bigleaf hydrangeas bloom on both old (last year’s) and new (current season) wood. Loss of the old wood results in your hydrangea blooming only in late summer on current season’s wood. No flowers may mean that the variety that is not reliably hardy in your zone.

2. Not enough sunlight… The site should receive 3-6 hours of direct sun daily. Panicle hydrangeas require the most sunlight and oakleaf hydrangeas and smooth hydrangeas as little as 2 and 3 hours daily respectively. As a rule, more sunlight means more flowers.

3. Pruning at the wrong time… Generally, hydrangeas don’t need pruning other than to control plant size and to remove dead wood. Pruning time is most important. H. macrophylla and H. quercifolia plants are pruned in early summer or in late winter; H. arborescens and H. paniculata are pruned in late winter to early spring before new growth begins. Don’t cut plants back too far. If your hydrangea won’t bloom, skip pruning any time. You may be overpruning.

4. Too much fertilizer… Overfeeding your hydrangeas particularly with too much nitrogen (N) or not enough phosphorus (P) in the soil. High nitrogen levels produce lush green growth, but little to no blooms. Phosphorus is responsible for the flowering and fruiting in most plants.

5. Deer… If you live in an area with deer, their feeding may prevent your hydrangeas from blooming. One solution is to construct a 6 foot (or higher) fence around your garden.

Five Shrubs Make Wonderful Small Trees

Posted by Hugh on July 18th, 2019

Multi-trunk rose of Sharon tree

With some aggressively growing shrubs, it frequently comes down to you (and your pruners) versus the plant.

Technique: a sharp pair of hand pruners along with 3-4 times annually to train a large unruly shrub into a behaved small tree.

Here is a list of 5 great shrub candidate that may become a great small tree:

Harlequin glorybower (Clerodendrum trichotomum) frequently grows as an unwieldy shrub that is 10 – 15 feet in height and spread and lacks symmetry. Through judicious pruning, you can create a beautiful 10-12 foot specimen tree that blooms in September. (USDA hardiness zones 6b-10).

Ninebark (Physocarpus opulifolius ‘Diabolo’ is a vigorous ninebark cultivar with deep burgundy maple-like foliage and an upright, spreading habit. ‘Diablo’ninebark also boasts pink-tinged, 2- inch wide white flower clusters in late May or June. (USDA hardiness zone (3-7).

Rose of Sharon (Hibiscus syriacus) is an old-fashioned summer flowering shrub which is easily converts into a small single or multi-trunk lawn tree. Flower colors range from snow white, pink, red and purples. (zones 5-8).

PG hydrangea (Hydrangea paniculata ‘Grandiflora’) is an old-fashioned shrub that bears lacy creamy white flowers starting in late July. Its dark green foliage is highly disease and insect resistant. (zones 3-9).

Shoal’s Creek chaste tree (Vitex agnus-castus ‘Shoal Creek’) is a southern favorite that blooms reliably from late June into August. Many cultivars are available; mid-size 12 foot ‘Shoal Creek’ has proven landscape reliability. (Zones 6-9).

Some basic tips: All 5 plants perform well in average well-drained acidic soils and in full to partial sun. All bloom best in full sun. Major pruning should be done in late winter after the severity of the winter weather has been evaluated. Secondary cuts, including root suckers, can be performed in any season to develop and maintain its tree form. Trees may also be maintained in large containers.

Crusader™ Cockspur Hawthorn

Posted by Hugh on July 13th, 2019

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Crusader cockspur hawthorn (photo source unknown)

Cockspur thorn (Crataegus crus-galli) is sometimes seen as a dense, low-branched, broad-rounded tree to 25-35 feet with horizontal branching armed with numerous large 1.5 to 3 inch long thorns. Lower branches often sweep near to the ground. It is also often seen as a tall, flat-topped shrub. This native hawthorn grows naturally from Quebec to North Carolina and Kansas (USDA hardiness zones 3-7).

Obovate to oblong-obovate dark green 2- 3” long leaves turn to a striking red/orange in the fall. The 3 inch wide white corymb flowers bloom in May over period of 7-10 days. Flowers emit an unpleasant peppery odor. Flowers are followed by rounded fruits (3/8” diameter) that ripen to deep red in fall (September-October) and typically persist to late fall. The tiny fruits (called “haws”) are edible, but the amount of pulp is not much and is best left for the birds.

‘Cruzam’ is commonly sold under the name of Crusader™ as a thornless plant with a slightly more compact shape. What distinguishes the thornless Crusader form is the absence of thorns from the species. In my opinion the species should never be planted in any public grounds and residential safety considerations. Crusader is a welcome alternative. Thornless tree makes plant general maintenance such as pruning alot. Crusader is rated a superior landscape tree.

Cockspur hawthorn performs best if grown in full sunlight and any average well-drained soil. It is very adaptable to both dry and moist growing conditions. It is highly tolerant of urban pollution and will even thrive in inner city environments. The tree needs occasional maintenance and is best pruned in late winter once the threat of extreme cold has passed. It has no significant negative characteristics

Pests and diseases are a problem with most hawthorns. Plants are susceptible to cedar-hawthorn rust and fireblight. Other potential diseases include fungal leaf spots, powdery mildew, cankers, apple scab, leaf blight and twig blight. Potential insect pests include aphids, borers, caterpillars, lacebugs, leafminers and scale.

Small flowering landscape tree for lawns or open woodland areas. Notwithstanding disease susceptibility, a well-maintained tree can be ornamentally attractive. On species plants, thorns are a drawback for pedestrian areas or areas with small children. Plants may be effective when grown as barrier plants on property borders or in hedgerows.

Perennial Favorites For The Summer Garden

Posted by Hugh on July 8th, 2019

If someone was to create the Garden Performance Hall of Fame for summer flowering perennials, I would nominate this list of ten (10). I have had experience growing many of them in my Northeast Tennessee garden (USDA hardiness zone 6-b). In a few instances, I have selected a variety that grows in friends’ or a nearby arboretum or botanical garden.

‘Zagreb’ Coreopsis (C. verticillata)– awesome lemon yellow small daisy like flowers on this mounding long blooming perennial. Other favorites are Uptick® series; ‘Route 66’. If you want something different, try tall growing (4-6 feet high) C. triptera ‘Gold Standard’ which blooms from August into October.

Shasta Daisy (Leucanthemum x superbum)– lots of pure white daisy flowers on 2-4 feet sturdy stems. Favorite cultivars: ‘Snowcap’; ‘Becky’; ‘Whoops-A-Daisy’.

Russian Sage (Perovskia atriplicifolia) – Unique silvery contrasting foliage with long lasting summer to fall dusty blue flowers. ‘Denim N Blue’; ‘Little Spire’.

‘May Night’ (Mainacht) Salvia (S. nemorosa)– deep blue flower spikes in spring; prune (deadhead) the entire plant down to a 4-6 inch mounded shape and feed with a nitrogen-based fertilizer to promote new growth and re-bloom within 4-5 weeks. Other favorites: ‘East Friesland’; ‘Caradonna’.

Garden Phlox (Phlox paniculata)– some great disease resistant varieties are available such as ‘David’, ‘Jeana’, ‘Glamour Girl’.

Purple Coneflowers (Echinacea purpurea) – summer full of colorful blooms will fill your garden. Favorites include: PowWow® Wild Berry series, Sombrero® series, Cheyenne Spirit’, ‘White Swan’.

Black eyed Susan (Rudbeckia fulgida) – ‘American Gold Rush’, ‘Goldstrum’, ‘Little Goldstar’

Daylilies (Hemerocallis spp.)– so many great varieties – ‘Happy Returns’, Fooled Me’, ‘Hyperion’, ‘Strawberry Candy’, ‘Red Volunteer’ are all- time personal favorites.

Sedums (Sedum spp.) – late summer to fall blooming varieties such as ‘Autumn Joy’; ‘Autumn Fire’; ‘Matrona’; and lots more.

Coral Bells (Heuchera x) – wonderful accent plant for foliage color; favorites include ‘Caramel’, ‘Citronelle’, ‘Sweet Tea’, and many more.

Six Flowering Perennials For Downtown Areas

Posted by Hugh on July 3rd, 2019

Rudbeckias in Downtown Pittsburgh, PA

‘Becky’ Shasta Daisy

Here is what some downtown areas in the Southeast and Midwest U.S. were planting this spring:

Rudbeckia lacinata ‘Herbstonne’ aka ‘Autumn Sun’ is a long-blooming butterfly favorite with clusters of bright yellow ray flowers with large green cones from mid to late summer. Leaves are large, glossy, deep green, deeply cut along the stems.

Black Eyed Susan (Rudbeckia fulgida var. fulgida) is a native species form that grows 2-3 feet tall and is long flowering. Blooms are slightly more resistant to Septoria Leafspot disease. Removing the old flowers regularly promotes some re-blooming up to frost (zones 4-9). Available in 2019 is Glitters Like Gold PPAF rudbeckia which exhibits better Septoria resistance.

‘Becky’ Shasta Daisy (Leucanthemum superbum) is a heat tolerant selection that is a flowering machine. The glossy dark green stems rise to 36 inches high and are topped with large white daisies in early summer. ‘Becky’ grows vigorously and handles summer heat better than most shastas. Flowers stand upright on sturdy stems; blooms up to 8 weeks if spent flowers are quickly deadheaded.

May Night Salvia (Salvia x sylvestris ‘May Night’ (Mainacht’)) blooms prolifically with dark purple-blue flowers (zones 4-8). This vigorous perennial exhibits excellent cold and heat tolerance. Blooming in late spring with a profusion of flower spikes, it re-blooms through the summer when deadheaded.

Hairy alumroot (Heuchera villosa) is the toughest coral bells that I know of. Some seed strains like ‘Autumn Bride’ and ‘Brownies’ are available for large urban plantings. Tiny flowers appear in late summer and contribute to the overall landscape appearance.

‘Northwind’ switchgrass (Panicum virgatum ‘Northwind’) is a 4-5 foot tall and 2-3 feet wide upright ornamental grass (zones 2-9). Dark green foliage is supported on a rigid vertical framework. In September this vigorous ornamental grass is topped by attractive infloresences in September.

Sedum ‘Autumn Joy’ and ‘Matrona’ are two topnotch varieties of border sedums (Sedum telephium) that grow 18 to 24 inches tall and wide (zones 4-9). Flower heads form in mid-summer and gradually open into 3-4 inch wide dusty-pink flower heads from August onward, eventually turning bronzy-red in October. Performs in average soil and is highly drought tolerant.

Heuchera villosa ‘Brownies’

Panicum ‘Northwind’ in mid-summer

Three High Performance Coneflower Series

Posted by Hugh on June 28th, 2019

‘PowWow Wild Berry’

Coneflowers (Echinacea purpurea) are very popular perennials (USDA hardiness zones 4-9). Their colorful flowers will attract a multitude of beneficial pollinators to your garden. Some cultivar series come in a myriad of colors.

For the best and biggest flowers, coneflowers perform at their best in full sun and in well-drained soils. After planting echinaceas grow off quickly, are moderately drought tolerant, and stand up to heavy summer rain, high humidity, and moderate winds.

Three of the top performing series are:

Cheyenne Spirit® (2013 AAS Trial Winner) requires minimal care ― no deadheading, staking or excessive watering. Cheyenne Spirit comes in myriad of colors – white, yellow, orange, red and purple, sometimes all on one plant! Plants are somewhat bushy and upright, measuring 30-36 inches tall.

PowWow® Wild Berry™ was selected an All-America Selection (AAS) Award in 2010. Plants form a mid-sized mound of coarse dark-green leaves and large daisy flowers with magenta-pink petals surrounding an orange-brown central cone. ‘Pow Wow White’ is equally beautiful. Plants are well-branched and bloom all season. It requires no deadheading and flowers are nonstop. Its compact size, 18-24 inches high and 12-16 inches wide, is an added plus.

Sombrero® series also grows compact, is free branching, early blooming, and come in a wide range of colors including purple, red, coral, pink, orange, yellow and white. Blooms measure 2.5 to 3 inches across. Sombrero Salsa Red and Sombrero Adobe Orange is the earliest to start flowering (around mid-June). Hot Coral grows 24 inches tall. Salsa Red is the shortest, only 16 inches tall. ‘Sangrita Red’ (above) grows 16-18 inches tall with scarlet-red petals. Adobe Orange is taller at about 20 inches with larger flowers. Their compact size makes them idea for containers on decks and patios.

Fill your flower vases this summer with long lasting coneflowers. Native to North America, coneflowers are great pollinator plants in gardens when planted in a perennial bed or as part of a landscape border or and in large containers and window boxes. Their floral seed heads feed birds all fall and winter. Coneflowers are also deer resistant.