Friday, October 31, 2008

Why I Would Never Plant A Hackberry Next to a Parking Lot

Wooly hackberry aphid, photo courtesy of (mine didn't turn out)
Years ago, many of America's street trees were from the genus Ulmus.  With the rise of Ophiostome ulmi, or Dutch Elm Disease, many Elm street trees were in need of replacement.  Celtis occidentallis, better known as the Common Hackberry, seemed like the tree for the job.

Walking around the University of Tennessee campus in Knoxville, next to many roads, parking meters, and parking lots one will notice an abundance of these graceful trees.  They provide nice filtered shade and grow relatively well in their locations.  Hackberries also have an attractive (if sometimes gnarly) growth habit, interesting warty bark, delicate yellow fall foliage, and fruits that birds and other wildlife can't resist.

So what's the problem?

This summer, the Asian Wooly Hackberry Aphid hit our area, and our hackberries, hard.  A hackberry in the very back corner of my back yard was so eaten up by the wooly aphids that it looked like it was covered with snow.  Many a time this summer I would be working in the front yard when a breeze would come from that direction and the wooly aphids would blow all over the place (like pollen, an insane amount of dandelion seeds, or again, like snow).  They would blow on my clothes and in my hair.  And while that is annoying, that's not the worst part.

Many may know that aphids excrete a substance called "honeydew".  Ants love this sticky substance because it is sweet.  When the wooly aphids are on a hackberry, they drop their honeydew all over anything below the tree.  One hackberry provides shade for a picnic area next the the UT Gardens, behind the vet clinic.  This summer, that area was glossy and even kind of sticky from the excretion.

Many cars parked under hackberries on the street will also become glossy and sticky from the honeydew.  Before a road trip, I noticed a friend's car had this honeydew on it.  He wanted to leave it because he likened it to a "cool paint job".  Remembering that a coworker informed me that honeydew will turn into a black mold, I strongly suggested he wash his car at the first gas station we come to.  As soon as the soapy water touched the car, a terrible smell enveloped the area that took more than half an hour of driving with the windows down to get rid of.

Don't get me wrong.  Hackberries are beautiful, soft, graceful trees.  They have great landscape value and their warty bark provides really great winter interest.  These trees would make a great addition for most landscape uses -- but not as a street tree where, if it gets hit with the Asian Wooly Hackberry Aphid, the cars below will need to be washed regularly.

Thursday, October 30, 2008

Cercidiphyllum 'Pendula'

Walking through the lower rooms of the University of Tennessee Gardens, there is a unique tree that never fails to grab my attention. Its blue-green leaves with silver backs fluttering softly in the breeze bring to mind memories of a waterfall or fountain. Cercidiphyllum japonicum 'Pendula', commonly known as the Weeping Katsura Tree, is an excellent specimen plant, especially for areas designed for calm and reflection. You would be hard-pressed not to feel more relaxed in the presence of its cool foliage color, graceful weeping habit, and gentle movement in the wind. It is pleasing to almost all the senses, especially since its leaves occasionally give off a fragrance likened to a cotton candy smell. Although this tree is deciduous, its light, shaggy bark and delicately hanging branches will still grab passerby attention in the winter months.

Michael Dirr in his Manual of Woody Landscape Plants mentions that this variety may grow 15 to 25 feet in height, but the specimen in the UT Gardens strikes me as being more around 30 feet tall. There is a similar variety called Cercidiphyllum magnificum 'Pendulum' (easy to confuse with 'Pendula') which could reach 50 feet in height. That variety has larger leaves, but according to Dirr may not weep as well as 'Pendula'.

'Pendula' would serve as a great specimen plant in the landscape for reasons already mentioned, and also because it is a fast grower that will fill in an area at the rate of one foot in height per year. One negative aspect to this plant is it would be a hassle to propagate one for yourself, unless you have the skill and tools to graft the weeping form onto a rootstock of the straight species.

In conclusion, this is a great plant any time of year, and would go great in an area of calm and reflection, a sensory garden, or especially near water.

silvery, blue-green foliage

habit makes me think of a hunch-back


Dirr, Michael A. Manual of Woody Landscape Plants. 207-208.

Tuesday, October 14, 2008

Canna 'Bengal Tiger'

A perennial that has always been a favorite of mine is Canna x generalis, commonly known as Canna, or the Canna Lily. It adds a strong, bold, very vertical element to a landscape, with its erect stems that reach five feet in height, and its big, two foot long leaves that reach for an upright position. Adding to the verticality of this plant are varieties specifically bred for the venetion such as, my favorite, ‘Bengal Tiger’.

‘Bengal Tiger’ has almost neon yellow leaves with vertical lime green venation. Its most notable feature is definitely its foliage, which is big and bright. ‘Bengal Tiger’ will also flower profusely in our zone with highlighter orange flowers. Although this Canna has pretty flowers, other varieties are bred more for big flowers, which may be striped or speckled. These cultivars generally have smaller leaves and possibly no foliage variegation. If you like the looks of ‘Bengal Tiger’, another good foliage Canna is ‘Tropicana’ which also has large leaves and striped venation, but is a more muted bronzy color which would look nice next to a brick structure.

One thing that I do for any Canna if it has flowers is remove the spent blooms before they go to 'seed'. In my opinion, it cleans up the appearance although some people prefer to leave the seeds because of their interesting spherical appearance. There are benefits to removing the seed heads, such as encouraging faster flower production and saving energy that would be spent on making seeds for making bigger blooms. When pruning the flowers though, be careful not to prune so far back that you remove the next flower head!

This perennial will die back to the ground in the fall, but don't worry it will come back! Remove the dead growth in the fall so its not in the way of other plants that may not be done yet and wait for the Cannas to shoot back up next Spring!

Sources: Stills, Steven M. Manual of Herbacious Ornamental Plants. p. 161-162.

Friday, October 3, 2008

Verbena 'Homestead Purple'

Butterflies, like this swallowtail,
absolutely love 'Homestead Purple' verbena
Verbena canadensis, also known as "Clump Verbena", is valued for its color, habit, and durability. Round clusters of flowers on all clumping verbenas generally last from summer until fall, contrasting the almost spherical flower-shape with the very prostrate habit of the plant itself. Reaching 9" in height at the most, this plant can be a vigorous spreader. In fact, Clump Verbena propagates itself easily by air-layering. That is, its stems grow so low to the ground, that given the right conditions the stems will start to grow new roots and spread even farther as a new plant. This plant does best in a well-drained soil and a full-sun site, but keep in mind that when this plant is happy, it can become incredibly difficult to manage because it will spread so vigorously. If planted in perhaps partial-sun, the plant should live but be less vigorous and produce fewer flowers. Also, if planted in soil that does not drain well, Clump Verbena becomes susceptible to powdery mildew. Be aware of what hardiness zone you are in when planting Clump Verbena, because it is only hardy as a perennial in zones 6 - 10. If planted any farther north from our East Tennessee location, Clump Verbena is considered an annual.

The story of Verbena canadensis 'Homestead Purple' is one worth mentioning. It begins when two horticulturalists from the University of Georgia by the names of Allan Armitage and Michael Dirr (sound familiar?  You may see these names on your book shelf) were driving back to their university from Atlanta, GA. At one point, a brilliant purple in a yard they were passing happened to catch their eyes. They went to investigate what particular variety it was, but the homeowner only knew that it had always been growing in her yard. She gave them some cuttings which were later named 'Homestead Purple'. Today it is believed that 'Homestead Purple' is a cross between the traditional Verbena canadensis and some unknown variety that may not even exist anymore.

The cultivar of Verbena canadensis 'Homestead Purple' is valued in today's market for its low spreading habit, rich purple flowers, and spring / early summer blooms that will last until frost. This specific cultivar is located in at least three areas of the University of Tennessee Gardens. Not only is 'Homestead Purple' valued for its long bloom period, but the coarse texture of its evergreen foliage can definitely add winter interest to your perennial garden. Also, if you want to attract wildlife such as butterflies, this plant is an excellent selection. One suggestion for 'Homestead Purple' is that it should be planted near taller, more shrubby plants. This way, if it grows vigorously, it will not suffocate other smaller plants and you won't have to worry about cutting it back so often.

Update (2013): 'Homestead Purple' is easily propagated by rooting cuttings, division, or just pulling up stem sections that have rooted by layering.  March of last year we planted 3 very small rooted cuttings of 'Homestead Purple' around the Knoxville Botanical Garden & Arboretum's new "Every Child Outdoors" Youth Vegetable Garden.  Each plant was just around 3 inches tall.  By May, each plant was so large many visitors believed the Verbana that were spilling out of the beds must have been there for years (We had only just built the garden!).

'Homestead Purple' is an impressive, reliable perennial that can't be beat in a "transition zone" garden.

North Creek Nurseries
Stills, Steven M. Manual of Herbaceous Ornamental Plants.

Photo from: