Wednesday, July 28, 2010


More and more people are living with zone denial. Typically, to most gardeners zone denial means growing plants outside of their recommended zones. Who hasn't grown (or at least tried) hybrid tea, grandiflora, or floribunda roses here in WI? Very few of them are hardy enough to be grown in zone 5, most teas really prefer zone 7. But gardeners do it anyway because they're beautiful.

There are a number of factors affecting a plant's hardiness. One is provenance. Where a plant is from generally affects where it will grow. A simplistic way of looking at it is that tropical plants need tropical climates and plants from the north don't do as well in heat. This gets a little messier when you consider other factors like elevation. A plant may be from a fairly warm area (say equivalent to zone 8) but may range to higher elevations (maybe 12,000' up a mountain) where it is much colder. Selections collected from these higher elevations may survive zone 6 or even 5.

Sometimes doing research into a family of plants that is commonly available as non-hardy will provide an obscure species that is hardy to our area. Everybody knows Croton (at least by sight if not by name) with its colorful foliage found for sale as a houseplant at every lowes, home depot, and walmart or grown as a landscape plant in Florida. Croton alabamensis is a species that is hardy to at least zone 6, possibly zone 5. Another case is Cyrtomium fortunei, which can be grown in zone 4 with ease even though other Cyrtomium species require warmer temperatures.

Another big factor affecting hardiness is growing conditions. This may seem obvious, but so very many gardeners miss it. Every plant has different needs. While we can make broad recommendations and put together a garden with plants that have similar needs, those needs still must be met. Woodland plants don't thrive in heavy clay or full sun, and no gardener should expect them to. Spending an extra $5 on a bag of compost, mulch, or fertilizer might mean the difference between success and failure.

Even placement in the garden can make a difference. Sometimes moving a plant as little as 2 feet from its original spot will show vast improvements in plant growth and hardiness. This can be due to many reasons. Soil might be a slighty different pH, there might be more or less soil moisture, light exposure might vary slighty, and the microclimate might be drastically different. Wind and light exposure plays a big role in affecting both winter and summer conditions. You can really capitalize on a spot that's cooler in summer and warmer in winter when it comes to stretching your zone.

Some plants can be potted up and overwintered in the garage if supplied with a little supplemental water in the winter. Other plants can be screened from the wind. Winter mulch can help tremendously protecting plants from freeze/thaw cycles, and you can try building a cage around the plant and filling it with compost to get non-hardy plants through the winter. Or you can try ugly white rose cones, but I find them the least effective method of winter protection. And they're ugly.

I live with zone denial for a number of reasons. I think most new plants are hardier than they are given credit for. There are some exceptions, Coreopsis 'Limerock Ruby' being sold as zone 5 without extensive trialing comes to mind. But I think for the most part most nurseries are responsible enough to err on the side of caution.

I've found a number of new plants to be hardier than previously thought. When I first got into the industry, Agastache rupestris was widely sold as being hardy only to zone 6. I can say with certainty that it is easily hardy to zone 5, and probably to zone 4 if provided the right conditions. Other Agastache varieties have followed suit, with 'Raspberry Summer', 'Summer Love', 'Purple Haze', and 'Black Adder' surviving in my (and many other) gardens beautifully.

I've also been growing Black Mondo Grass, Ophiopogon planiscapes 'Nigrescens' for a few years now and it is starting to spread. This is another zone 6 plant.

Helleborus 'Hot Flash' in its 4th season. Not big and vigorous, but starting to do well. Listed as zone 7 upon release, now listed as zone 6, 5 with protection. I do no protection other than not removing the maple leaves that fall on it.

Sometimes I like to push zone limits just because I think a plant looks cool. It might provide a texture or color not readily available to us in colder zones. Or it might just be my horrible plant addiction. Either way, I planted Cunninghamia lanceolata 'Glauca' in spring of 2008. This is a neat evergreen conifer with broad, wicked-looking but soft, blue needles that is zone 6/7 borderline hardy. I planted it in my south butterfly garden and other than some pretty good winter-burn it has survived pretty well. I have taken pity on it though and have moved it to a pot with some miniature hostas that will be overwintered in the garage.

There is also another form of zone denial that many of us in the industry have to deal with. The "this plant didn't do well for me, so it must not be hardy" denial. This usually comes about from improper cultural practices, and not providing the plants with the proper requirements. And some people just can't grow certain plants. I've killed ridiculously easy to grow species for no apparent reason. At the risk of my gardening reputation, I'll admit to killing Gooseneck Loosestrife. I'm also very good at killing ferns.

The most common reason most plants die in my experience is that their specific soil conditions are not met. Some plants resent being too wet in the winter and will simply rot away to nothing. The new Echinacea hybrids are a good example. Gaillardia is another example that needs good drainage. I've had Gaillardia 'Frenzy' for 3 years now, and it does very well in a fairly well drained clay-loam. Alpine/rock garden plants like Dianthus licinata or Delosperma species can't be too wet in spring or they will get crown rot. Rhododendrons are widely planted here, despite needing a moist but well-drained acidic soil. Most of them fail in our heavy alkaline pH clay. I've seen a number of huge beautiful Rhododendrons (primarily PJM, but others as well) in the north-central sand counties of WI. Some plants need organic soils that are consistently moist, like Lobelia cardinalis and Lobelia siphilitica. Providing the right soil conditions can make all the difference in the world.

Don't be afraid to try new plants, or even new ways to use old plants. Don't be afraid to try a plant again just because it dies once. If you do some research, it's amazing what can be done in our gardens.

Wednesday, July 21, 2010

Allium Summer Beauty

The genus Allium is a hugely diverse group of plants, many of which make great garden subjects. Ufortunately, most gardeners are only familiar with the giant globe onions like 'Globemaster', or the weedy species like A. azureum or A. sphaerocephalon. (As a side note A. azureum pulls very easily and the beutiful blue flowers make it worth including in any sunny garden).

The last few years have seen a number of new introductions for Allium (I'll write about more of the in the next 2 months), and one of my favorites has been A. lusitanicum 'Summer Beauty'. This form has nice pink flowers starting in July and lasting into August. The foliage remains a beautiful emerald green all season, despite neglect, and is deer and rabbit resistant. It quickly forms a clump 24-30" wide, and flowers at 30". Summer Beauty seems to tolerate heavy soil fairly well, but I would still amend clay soils with compost. Use it in any sunny garden as a textural contrast with Echinacea, Monarda, Coreopsis, Roses, etc. It makes a great nectar plant for bees and butterflies.

Wednesday, July 14, 2010

Shasta Daisies

I'm not a big fan of Shasta Daisies. Don't get me wrong, I don't dislike them. They have their place in the perennial garden as long blooming bee and butterfly magnets that provide a bright and cheery spot of white and yellow in many sizes and shapes. There have been some interesting introductions in the last few years in the way of light yellow flowers, and wildly formed double flowers. But for the most part they're just common and unexciting old fashioned perennials you see everyday. Aren't They?

Maybe not, with the introduction of Leucanthemum 'Paladin' this year it was once again proven that even a perennial that's been around as long as the old fashioned Shasta Daisy, selective breeding can take the plant to the next level.

'Paladin' is a nice mid-height daisy to 20" tall with large double white flowers that cover the plant so completely you almost can't see the foliage. The stems are very sturdy, and shouldn't require any staking unless the plant is overfed. Shastas don't need much in the way of fertilizer, so lay off for most of the season. My plant has been blooming for about 2 weeks now, and is showing no sign of slowing down. I've been deadheading to promote more blooms, but you can just as easily wait until the first bloom cycle is done and shear it back to promote a second flush of blooms. Most varieties of Leucanthemum have an unpleasant (horrid is the word I prefer) fragrance but 'Paladin' has no scent at all, which is a big bonus to me.

'Victorian Secret' is a new variety for 2011 which I had the pleasure to view in Ohio recently. It is similar to 'Paladin' in all respects except it has a shorter 14" habit and a nice citrus scent. I'm told there are more varieties in the pipeline, including some nice yellow varieties. All with pleasant or no fragrance. I'm happy to see daisies getting a makeover, it's a plant that deserves some excitement.

Thursday, July 1, 2010

Aralia cordata 'Sun King'

I like tropical textures in the garden, especially when gardening in the shade. It's important to make an impression with textural contrast in the shade garden, since it's difficult to make a huge impact with flower color. Aralia 'Sun King' gives you that great huge-foliage tropical look, as well as bright color due to the yellow to chartreuse foliage. Individual leaves should get nearly 3' long x 2.5' wide. Big racemes of white flowers in mid-late summer are followed by purple berries. Other species of Aralia attract honeybees, and this one should be no different. Grown with some morning sun in a good organic rich soil, 'Sun King' will make an awesome accent plant. There isn't a lot of information out there on this plant, and a lot of it is conflicting. I've seen zone ratings anywhere from 3 to 6. Also, mature sizes range from 3-9' tall x 3-10' wide. So, what's the truth of the matter? I think hardiness will be at least zone 5, which is the rating Asiatica gave it in their catalog a few years ago. Aralia cordata is known to be hardy to zone 4, this selection could possibly be a little less hardy. Time will tell. Size? I think the 3' x 3' that Terra Nova is giving it is a bit on the small side. I would guess it will easily hit 6' tall and 7' wide, given that it's a selection of A. cordata which can grow to 9'. Be sure to give this baby some room in your shade garden!