Admit it- Zone Envy
We all want to grow things that stretch the boundaries of our climate. Maybe just a little bit. If you ever want to meet an intense group in this realm hardy palm people are the best. They dare to grow a Trachycarpus in Zone 6 Pennsylvania- and damn they make it work. You can, and I have learned a lot from this group of intrepid gardeners. But understanding microclimates doesn’t just mean you can get away with something (and isn’t that the root of zone envy?) It makes us better gardeners. Without going into thermal diagrams and intricate wind flow patterns I would like to share my observations about some of my favorite ways of using microclimates to your advantage.
#1 The top of a slope
Cold air sinks. Don’t get all hot and bothered over your elevation. Being on the top of a hill in the Willamette Valley has serious benefits. And- I am not talking about huge mountain sides. Even small slopes create microclimates. These are called thermal belts. At our wholesale nursery there is a 10′ slope with a 45º grade. Just for shits and giggles I planted two of the same species of semi-hardy Grevilleas one at the top and one at the bottom. In December 2009 we plunged to 7ºF out there. And sure as shit the Grevillea at the top of the slope was UNDAMAGED and at the bottom? You guessed it. Dead as a door nail. Locate the highest part of your garden and then imagine cold air flowing like water down the slope because cold air is dense and heavy- it hugs the ground and it does flow like a liquid. Houses, fences, even hedges can interrupt this flow and there cold air collects. And as I have said, even small slopes have their benefits. If your garden is flat as a pancake, don’t worry- there are other microclimates that exist on the flat plane.
1000 year old stone walls
Yeah- those are in short supply in good ol Oregon but walls do have their benefits. And they have their drawbacks. Primarily south facing walls collect heat- even on seriously subfreezing days- solar wave lengths are amplified there and they can then radiate some of that heat back. This is good and bad. In my experience plants that require heat to harden off their wood for winter cold are ideally located here. A perfect example is Caesalpinia gillesii. It prefers summertime heat to harden off its woody structure which makes it hardier to ultimate cold in the winter. In spring and summer the reflected heat of a wall spurs it into bloom earlier and longer. A win win. And it goes safely and very dormant in winter. A lose lose however, is when this heat works against you. I’ve placed tender broad leaved evergreens against south facing walls in hopes of garnering added protection. Nope. What happens is that the reflected heat actually interferes with dormancy and then when a real arctic plunge sweeps in they have not hardened off adequately to withstand our lowest temperatures. For many borderline hardy plants, believe it or not- and I say believe it- full exposure is the best site. This lets frosts do their magic of inducing dormancy. And as a gardener I have come to respect the power of dormancy. You can call me a heretic, but I’ve tested and observed this for 25 years and it works. Of course even in this situation a plants ultimate tenderness to cold can be exceeded and it can be lost. But there are other microclimates you can seek for protection that do work miracles.
Get to know your sidewalks and asphalt.
Some of my most successful placement of semi-hardy plants has been adjacent to concrete sidewalks and the asphalt of the streets. Several things combine to make this a win win win. First the plant is in the open- preferably (but not exclusively- I’ll come back to that), that means it receives the full measure of dormancy. Then radiative heat from the hard surfaces raises the temperatures just a little- notice how snow melts first on black surfaces that collect heat as well as sidewalks? Thats heat working in your favor. The most surprising trait of this location is that roots are drawn under the paving – because it is perpetually moist from shading. Pavement is a great insulator and this protects the roots from freezing as well as not interrupting evapotranspiration (moisture loss that can desiccate a plant) during a freeze event. Even brick pavers and large rocks can create this effect. Its important to note that most of the days of our freeze events occur with clear sunny weather. And even long wave radiation from the weak winter sun can penetrate cloud cover and contribute to the ambient heat of paved surfaces. So- in the open the plant is exposed to dormancy inducing frosts but during arctic episodes the reflected latent heat of the pavement boosts the temperature. It works great. Some plants that I’ve grown successfully in this situation are: Woody Salvias, such as Salvia greggii and hybrids, Grevilleas, (which love this situation), Callistemons, Agaves and even (in a semi shady location) Gardenias- I had a tender cultivar live 10 years undamaged with its roots below pavers. Play around with this and see what you think.
South facing aspect
Lets face it we are far to the north. Almost exactly half way to the Arctic Circle in fact. The sun angle is low in winter but on south facing slopes the maximum amount of solar insolation means that these situations can be a few precious degrees warmer. Located on a slope- or near asphalt and the the two previous micro microclimate benefits can double up. Spend time in your garden in winter- outside during the coldest episodes. Notice where the ground thaws first, where snow disappears first, where soil remains frozen and frost lingers. These simple observations are eye opening. I can tie into this location the benefits of overhead protection. Tree branches- especially evergreens- but to some extent even deciduous branches interrupt heat escaping on our coldest nights. When the canopy is high enough- you have two effects. It still gets cold enough to induce dormancy but long wave radiation escape (heat loss) is interrupted by the branches and it adds a respectable amount of protection. Located on the south side of a tree canopy plants that are tender can sail through with minimal damage. Just remember the phrase woodlands are warmer. This goes for the overhead protection of porches and even eaves. So as a gardener while you spend a lot of time staring down at the dirt it is equally as important to look up as well. One plant that really benefits from siting in this situation is Edgeworthia- which as gardeners know can have flower buds frozen in early or late freezes. Overhead protection really works to minimize damage.
Your own personal Ice Box
The opposite of a southern aspect is called an open north exposure. The north side of a house, a hedge, an unwelcome 6 story building can cause shade. There frost and freezing temperatures linger. The soil can become more and more frozen- and the low sun angle means that there is little if any thawing. This situation can create up to a half a zone colder conditions. Locate hardy plants here- perennials, really tough shrubs- in the summer this is a great site for Hostas, Hydrangeas and Rhododendrons.
That god forsaken East Wind.
Our arctic episodes come to us in two ways directly from the north and in the Portland Metro area from the east. Arctic air collects east of the Cascades and is funneled- and as you know quite violently through the Columbia River Gorge. (This is called a Bora wind). Portland is on the direct receiving end of that pressure gradient. So its important to locate plants that can be damaged by the intensely cold and extremely dry arctic blast. Notably, broad leaved evergreens- (New Zealand Pittosporums come to mind) I’ve tried to cover many plants with Remay, sheets, even Scooby Doo sleeping bags to protect them from this wind. To be honest the Remay and sheets simply blow away in the gale- no matter how hard you fasten them. (Scooby Doo sleeping bags simply crush the plant). So, in this case its location, location, location. A western wall or west side of a hedge interrupts the main focus of a gale. And be aware that these winds are sneaky. They swirl around and between houses, fences and hedges creating eddies- pay close attention during our next East Wind Episode and you’ll see what I mean. So a west facing wall works well but also, I’ve found locating tender plants in the midst of cold hardier shrubs offers very good protection from wind as well. Its all about the mix and the protection of friends. They can keep the full force of the wind from frying your plant. This works amazingly well with such plants as Abutilons, Drimys, Phormiums even semi-hardy Scheffleras.
As long as we are discussing cold its important to understand what snow cover does. It can be good and it can be VERY bad. You may have heard that snow insulates plants from cold. Yes, and no. 1′ of snow has some insulating power. 2″ (which is what we are likely to get) offers little protection. In fact it can be very bad. Just an inch of snow can excel the long wave escape of heat from the surface that means that our temperatures with just a skiff of snow can be MUCH MUCH colder when its in place. So, if you are praying for snow to protect your plants- think twice, just a little can cause over night lows to plummet with no real benefit and real risk. If a plant is covered with snow- do not remove it. Unless branches are breaking- then god help us all- Snow on leaves interrupts moisture loss and its kind of pretty too.
Keep it simple
I hope I have offered some suggestions that will make you as a gardener aware of your microclimates. It pays to play around with it and experiment. Keep your eyes open and really pay attention to weather as it happens. This is not just for tender plants but also for most others. Get to know the weather in your garden. And grow kick ass plants.