Green giants | Ottawa Start

7 • POSTED ON August 16, 2005
Green giants
Posted by ottawastart Subscribe to the OttawaStart RSS Feed for instant news updates.

August is well underway and in the garden that means the days of the “green giants” have arrived; the time of year when tall perennials have reached maturity and start to flower.

Our gardening column is published every Tuesday on OttawaStart. Read previous columns here.

Firstly, I should extend a warm “thank you” to my better half for taking care of last week’s column. I have to admit that creating memories in your garden is not something that would have occurred to me as a topic for a column, but I guess it takes a woman to come up with such a great idea…

August is well underway and in the garden that means the days of the “green giants” have arrived; the time of year when tall perennials have reached maturity and start to flower. I should add “much to my delight”, as I love to walk around plants that are my height or taller. There is no doubt in my mind that this says something about my psyche, but what the heck. I thoroughly enjoy perusing through my own little “jungle”, fully expecting to walk into some exotic animal every time I turn a corner. In reality, the “exotic” wildlife in our garden consists of our dog Emma, a tiny bunny that has recently taken up residence under our bay window, and chipmunk Alvin who has been living under our deck (and sometimes in our basement) for the past two years. However, imagination is something I have never been short of and wouldn’t you agree that - at a quick glance - there is a slight resemblance between a black Labrador Retriever and a fierce and frightening panther? Hmm, maybe I should just stick to telling you something about tall perennials rather than exposing my deepest thoughts.

As a rule of thumb, most tall perennials bloom later in the season. The reason for this is simple; it takes them a while to reach their mature height and develop flowers. In most herbaceous border designs you will find the tallest of the perennials situated in the back of the border. Obviously, you do not want them to hide the medium and small perennials, but that is not the only reason for positioning them in the back. Their long growing season – certainly if summers are as hot and dry as the one we are currently experiencing – causes their lower leaves to dry out and die off while the top part of the plant is still growing. By placing them behind other plants you can hide this cosmetic imperfection.

If – like me – you prefer to utilize a tall plant in a more solitary position in your garden, make sure to keep them well-watered throughout the season and apply a sufficient layer of mulch around the base of the plant to help retain moisture in the soil. Also, spend some time on research to find the right plant to meet your garden’s specific conditions. For example, if your garden is exposed to the sun for most of the day, you may want to look at ornamental grasses, which are often better equipped to handle hot, dry conditions than most tall “leafy” perennials. One of my biggest mistakes ever was to plant a clump of tall, fall-flowering Asters in an extremely sunny – and, unfortunately, rather prominent - position in the garden. The idea was to use the plants to hide the compost bin that was positioned behind them. Idea A-plus, execution D-minus! Not only did half the leaves normally bid their farewell before mid-July; the remaining leaves on the plant got covered in powdery mildew because there was not get enough airflow around it. What I ended up with were a few flowers on top of a major disaster…

Over the years, I have had much better results with these “green giants”:

Eupatorium maculatum Joe Pye weed. Native to the Eastern and Central U.S. this perennial – a member of the Aster family - can grow to a staggering 6 or 7 feet provided you plant it in a sunny or partly shaded position in moist, rich soil. I have always found this plant particularly easy to grow; it needs no staking (its stems are so sturdy that most people mistake it for a shrub rather than a perennial) and in my experience, it is not prone to disease and it is perfectly hardy in the Ottawa area.

Alcea rosea or Hollyhock. Technically, this beautiful, old-fashioned cottage garden plant is a perennial, albeit a short-lived one. More often, it behaves as a biennial but because it self-seeds prolifically, you will never be without it once you have planted them. Earlier this season, I moved four plants from our back garden to the front; all of which by now have reached six feet (and are still growing). In windy locations, staking is required, although they are sturdier than you think.

Miscanthus sinensis or Maiden Grass. Off the top of my head, I would say there are about 20 cultivars of Miscanthus sinensis available at our local garden centers, out of which “Gracillimus” and “Zebrinus” are good choices (to name but two). On average, the plant reaches a height of 6 to 7 feet and a width of 3 to 4 feet. Unlike bamboo, with which it is sometimes confused, Miscanthus does not spread but instead forms a clump (and therefore never becomes a nuisance in your garden). Although mainly grown for its foliage, in hot summers Miscanthus will produce fronds. It is advisable to cut it back in spring rather than fall, thus allowing the dead leaves to serve as winter protection. Technically, this native of Asia is hardy in Ottawa, although I would recommend giving it some protection during its first winter. Once established, it should be fine (just remember to leave it alone until spring).

Cimicifuga racemosa (black cohosh, black snakeroot or bugbane). A shade perennial that prefers rich, moist soil, this native of Canada and the U.S. was used by aboriginal people to assist in treating kidney disease, rheumatism and woman’s reproductive health. The plant itself reaches about four feet in height, but the fragrant flowers that appear in August grow to about seven feet. If you have a shady garden, Cimicifuga is a great choice for accompanying perennials such as Hosta and Astilbe.

Next week, more “green giants”.

Until then, happy gardening.

René Trim

Do you have a gardening question for René? Send it to us, and he'll try to answer it in his next column.
Click here to send us an e-mail...

About René
René Trim was educated and trained in The Netherlands, before moving to Canada in 2000. After studying the differences in climate and season faced by Ontario gardeners he established Trim Garden Design and Rescue in May 2002. A gardener since the 1980’s, he has a wealth of knowledge and experience, a vast library of reference materials from which to draw answers to various garden situations as well as an enthusiastic and professional staff.