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June 2008
guest feature
article
Penstemons for Green Roofs in the Midwest
By Brent Horvath, Nursery President and Grower
Graphics Courtesy
Brent Horvath At
Intrinsic Perennial Gardens (IPG), located in Hebron, Illinois, we list
about 900 different perennials for sale to the landscape and retail market
in the Chicagoland area, including Milwaukee and Madison. Over the
last six years, greenroof plant sales have become an increasing part of our
business. While sedums are still the primary plants used on many green
roofs, there are a great many penstemons that may work.
As you probably know, there are two main types of green roofs, extensive and
intensive. Extensive represents 3 to 5 inches of soil, where intensive
represents 6 or more inches. For this article I’m concentrating on an
extensive greenroof trial situation, since the plants mentioned can be
expected to do well in deeper soil. Of course, there are many other
variables when considering a green roof, including soil type and depth,
average yearly rainfall/precipitation or irrigation, and hardiness zones.
The primary greenroof soil used in the Midwest is a baked clay product
blended with peat and perlite; there are other mixes but this is the most
commonly used in the area. In northern Illinois our average yearly
rainfall is 30-35 inches. The majority of our green roofs here are in
USDA Hardiness Zone 5.
Since 2004, IPG has been testing around 200 plants for reliability in an
extensive greenroof situation, with more added every year. Lately, we
have been concentrating on U.S. native plants, which happen to include many
Penstemons. Other native genera we’ve tested and had success with have
been Allium, Aster, and many of the short grass prairie grasses like
Bouteloa, Schyzachrium, and Sporobolus. Next year we will introduce a
pallet of U.S. native plants specifically for an extensive roof in the
Midwest. One of the main differences here is the amount of rain and
winter wetness compared to the drier states east of the Rockies or slightly
more rainfall on the east coast. Our testing is done on mock up tables
with 3-4” of extensive soil.
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Intrinsic
Perennial Gardens greenroof plant trials,
photo of May 1, 2007 |
So far we have observed that the following
penstemons will work in the Midwest: Penstemon hirsutus and
var. pygmaeus have been favorites for me since they have not only
survived but seeded into some of the gaps between plants. This reseeding is
a major reason for the success of many of the Sedum species, too.
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Left:
Penstemon hirsutus; Right: P. arkansanus |
A nice companion for P. hirsutus is
P. arkansanus, above, which has a similar size around one foot tall,
strong upright stems, and white flowers. In addition, Penstemon
barbatus, below, has been grown from seed on a hydroseeded roof in
Chicago.
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Penstemon
barbatus: These plants have strong winter interest as well,
with their brown stems and seed heads holding on all winter. |
One of the surprises was Penstemon
grandiflorus. Besides thriving in only 3 inches of soil, it grew
over 2 feet tall and survived two winters in a part-shade, eastern exposure.
Another tall species that seems to be holding on in our trials so far is
P. tubiflorus (= P. tubaeflorus). This is closely related
to P. digitalis, which we have trialed as two cultivars, ‘Pink Dawn’
(a shorter pink flowered form) and ‘Huskers Red.’ These two also seem
very adaptable to greenroof conditions. Of the above species, P.
hirsutus, P. grandiflorus, P. digitalis, P. tubiflorus, and also
P. calycosus, are all native to Illinois (P. grandiflorus
rarely so), and so would be expected to potentially adapt to greenroof use
here within their native ranges.
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Penstemon
grandiflorus: Left: May 16, 2007; Right: July 9, 2007 |
Penstemon virens has a vivid blue
flower with curved stems reaching only 6 to 8 inches tall. This
species was planted in 2007 and looks good so far this winter. Among
the plants we lost over the last 6 years are the following: P. canescens,
which actually survived in the ground here; P. cobaea, P. linarioides, P.
ovatus, P. pinifolius, -- might work in Zone 6; P. smallii – also
might survive in Zone 6; P. tenuis and Penstemon virens.
I suspect winter wetness was the culprit in these losses.
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Penstemon
virens on May 16, 2007 |
In the future we will grow and test these
penstemons for survival on an extensive green roof in the Midwest: P.
amibiguus, P. angustifolius, P. barbatus var. coccineus, P.
cyananthus, P. gracilis, P. nitidus, P. pallidus (I expect it will
work), P. ovatus, P. procerus, P. secundiflorus, P. strictus and
P. tenuiflorus.
While the successful penstemons we’ve trialed may not all be Midwest
natives, I believe they are a good choice for green roofs since they are
U.S. natives, drought tolerant, have a wide range of colors, sizes and bloom
time, and will possibly attract wildlife, especially bees.
Reference books on green roofs: Look for
Green Roof Plants: A Resource and Planting Guide, by Edmund C. Snodgrass and Lucie L. Snodgrass, 2006 from Timber Press, Portland, OR.;
and
Planting Green Roofs and Living Walls, by Nigel Dunnett and Noël Kingsbury, 2004 from Timber Press, Portland, OR.
Brent Horvath is President and Grower of Intrinsic Perennial Gardens,
Inc. Brent says, "I grew up in the green industry working for my Mom
and Dad at their garden center, landscaping co., and florist. I
graduated with a B.S. from Oregon State University in 1991 and started
growing finished perennials for the wholesale trade. Today Intrinsic
Perennial Gardens, Inc. lists over 900 varieties and sells many more.
I select, breed and introduce new perennials to the trade, including Allium,
Geum, Pennisetum, Penstemon, Polemonium and Sedum. I have three plant
patents with three more pending."
Contact Brent at:
Intrinsic Perennial Gardens, Inc.
10702 Seaman rd
Hebron, IL 60034
brent@intrinsicperennialgardens.com
www.intrinsicperennialgardens.com
phone 800-648-2788 ext. 15
fax 815-648-2072
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