Overwintering an outdoor Zone 6 vertical garden

Boxwood and Heuchera 'Hercules' late November
Admittedly, this may be an answer to a question that no one has asked, but after four years of tinkering, I think vertical gardens (or vg's) can and should be a component of a zone 6 landscape design.

The first step in creating a vertical garden that will overwinter in Zone 6 is thoughtful plant selection.

I approach outdoor vg plant selection the same way I select for a standard planting composition: a combination of zone hardy, herbaceous and evergreen perennials and shrubs.

Boise may not get tremendously cold, but the dull, flat grey light of winter seems indeterminable as it drags into early "spring". As a countermeasure to the winter blahs, I make it a rule that at least half the plants I've chosen need to contribute winter interest either though dormant foliage (Pennisetum, Echinacea etc.), or evergreen or semi-evergreen foliage (Heuchera, Lamium, Hellebore, etc.). Hostas , for example, are terrific in the growing season, but they offer little in the way for winter interest.

As any gardener worth their Felcos knows, you need to select zone appropriate plants for your growing region. In horizontal applications, you have a little wiggle room; I've had pretty good luck with Zone 7 Black Mondo grass for instance. In a vertical application, you're better off sticking with a plant that is actually a zone or two hardier than what you'd normally choose.

Evergreen/semi-evergreen plants that have done well for me in a vg application include:

Boxwood (Buxus microphylla var. koreana 'Wintergreen")
Heuchera (quite a range of hardiness among the new cultivars. 'Stormy Seas' and 'Hercules' do very well.)
Winter Creeper (Euonymus fortunei)

The second (and equally important) step for successfully overwintering a vertical garden in our  Zone 6 is scheduling consistent, weekly irrigation through the winter.

Winter desiccation can be a little rough on a high desert garden but can wipe out even the toughest plants in a vertical garden.  A vg can't take advantage of winter precipitation without sophisticated rooftop water harvesting (more on that another time), so you're left to puzzle out how to get water to your vg long after your sprinklers have been winterized.

The solution I've come up with for the time being is to "t" off the 1/4" line to our ice maker behind the fridge. Once a week, if the outside temperature is at least five degrees above freezing,  I run 50' of 1/4" line out to our vg's and irrigate for about five minutes.

Oh, and try to remember to make sure the irrigation lines on your vg are free draining and no longer connected to your outside irrigation system. Frozen, busted irrigation pipes and fittings can really take the fun out of vertical gardening. Good luck...

UPDATE 12-3-13

January 2013 ended up being the 5th coldest in Boise since 1865, with temperatures 10-24 degrees lower than average. Needless to say, irrigating our vertical gardens was a bit challenging. Most of the Heuchera and the Hostas woke back up in the spring, but we lost many of our newly planted Boxwoods from the unrelenting and extremely dry cold (lack of a developed root system?).

A typical winter in Boise isn't nearly as extreme, and with occasional irrigation this is what you can typically expect over the course of 10 months:

The first photo shows the vg module in all its late summer glory: Heucheras, Hostas, Winter Creeper Euonymus, Lamium and a single Blue Star Juniper.

After a couple hard freezes in November, the herbaceous Hostas turn to jelly but the rest of the plants still look pretty good. By February though, everything but the Euonymous starts to look pretty tired and mopey (a pretty accurate description of myself in late winter).

Finally, as our northern hemisphere leans forward again to the warmth of the sun, the vertical garden comes back to life.

This year I've been working on selecting tough natives and climate adapted species that could survive through the winter with little or no care. In May I was commissioned to build my first "xeric" vertical garden.

The first pic shows the wall right after it was installed at the end of May. The second shows growth after the first six weeks. 

Hemerocallis, Nepeta, Artemisia, Yucca, Sedum and Stachys byzantina 'Helene Von Stein'.
Stay tuned as I'll be posting a mid-winter update to show how the vg is holding up.