Why Can’t I Grow this?

We’re all careful about minimum temperatures. When weather turns cold we rush to bring plants inside for winter, we take cuttings as insurance, we store them as dormant bulbs and tubers, or we treat them as annuals and let them die then replace them next year. It’s what we gardeners do in the frozen north.

            But we need to be aware that zones on plant tags generally refer to ‘how much cold can this plant take’–not necessarily how much heat. Many garden plants fail not because they get too cold, but because our weather is too hot and humid for them. The iconic blue Himalayan poppy–Meconopsis spp–thrives in northern Scotland and mountains of China and Tibet. But it dies quickly in my garden, and not because it’s too cold in winter. I can give it the woodland soil, moisture and light requirements it needs, but it just refuses tolerate the heat and humidity of an Ontario summer.

            Lupins are another example. They thrive in coastal areas where there’s plenty of moisture in the soil and the air and where it’s cool. But in most of Ontario they are at best short-lived perennials or even annuals (and aphid magnets). Alpine plants also hate our summers. Sure, we can give them full sun that mimics the exposed mountaintops where they originate, but they won’t thank you for the reflected heat radiating off your rockery, 30 degree Celsius temperatures or humidity. They don’t get that kind of weather high in the Alps.

            Many other perennials might not immediately turn up their toes when our July and August heat waves hit. Instead they’ll suffer from mildew or fungal diseases, become vulnerable to insect attack, or just drop leaves from stress because they can’t keep up with transpiration loss.

In their native habitats, plants have evolved to grow in specific soil types: rich or low-nutrition, wet or well-drained, acid or alkaline, clay or sand or even stone and gravel. Most perennial garden plants aren’t that fussy and they’ll manage wherever you plant them, assuming they have enough moisture and light. But others are very specific. Some even symbiotic relationships with specific microorganisms and fungi in the soil, and without them they won’t survive.

            Not all plants appreciate a rich, moist soil that’s full of nutrients. Well-meaning gardeners will treat all plants equally; they’ll assume all perennials really want rich, moist soil. Sure, the label says dry, well-drained soil but isn’t that just…mean? Surely everyone likes cake!

            But the plant doesn’t appreciate it one bit; it fails to perform or gets sappy and rots off. Some plants really do want to be grown hard and lean. They want shallow, dry or rocky soil and they are the better for it.

            You might not even know exactly what’s going on in your soil, especially if it’s a new garden. Beyond the obvious–clay or sand, what’s a few feet down? Is there an impenetrable layer of hardpan? An underground stream that’s acting like a sump? You won’t necessarily know until your plants tell you, by disappearing.

            One of my friends in Niagara had several flourishing evergreen trees growing almost 20 feet tall in her garden, then one windy day they all just…blew over. That’s when she discovered bedrock was only about four feet below the soil surface. The trees’ roots had spread laterally and found plenty of nutrition and moisture to thrive for years, but were so shallow that the trees couldn’t withstand the wind once the trees grew tall enough to feel its force.

            You can, to a great degree, mimic the soil conditions some plants absolutely require, but you need to respect what those needs are. If you don’t, then prepare yourself for the reality that the plant will either dramatically die straight off, or just slowly fade away.

            In a mixed border planting, in full sun, we might place several plants together that look good–the tag says they all want full sun, right? But what if some are from a wetland and some from a prairie and some from a mountaintop or even a beach? Not all of those plants will appreciate the lovely, rich loam you’ve created in your border and many will resent the regular watering you give them. Full sun just isn’t enough information. We must take soil conditions into consideration.

Terms like part shade and full sun can be very confusing. Is partial shade the same as partial sun? And how dark is shade, really? (Not to mention the complicating factor of dry soil that goes along with shade from trees or house eaves.) We think we understand—part shade is 3-6 hours of sunlight day and full sun is a minimum of 6 hours of direct sunlight a day, right? Not necessarily. It depends.   

Morning shade is completely different from afternoon shade, so when those 3-6 hours of sunlight occur in your garden could mean all the difference in your plant’s success or failure. Morning sun is easier on plants than that later in the day, at least partly because of additional radiant heat that develops as the day goes on. The hottest part of the day is always in the hours after noon. Your plant may prefer to be protected from strong afternoon sun, or it may not get flower well if all it gets is in the morning hours. Blossoms might bleach and fade and foliage wither in afternoon sun, while the same plant would do well with different light exposure.

            Light also changes throughout the growing season, in both its quality and intensity. As we approach the Solstice, the Northern Hemisphere’s tilt toward the sun is at its maximum. Days grow longer as the sun moves toward its highest elevation in the sky until June 21, which makes the sun more intense. Then, for the rest of the growing season sunlight gradually weakens and the days grow shorter as we come to the Equinox.

            We need to also consider the intensity of light. Our full sun in Ontario is not the same as full sun at different latitude, (or altitude as in the case of alpine plants), which is something we need to bear in mind. The sun’s rays near the equator hit the Earth more directly–at close to a ninety-degree angle, which concentrates the solar radiation in a smaller area. At more northern latitudes, even though we do have more hours of daylight in summer, the angle of the sun is more oblique and solar energy is less focussed than at the equator, so the light is less intense.

            Many world-famous gardens, like those in the UK and much of Europe, have a cool, temperate climate as well as less intense sunlight. But here in Ontario not only do we have some extremes of heat and cold–since we are at lower latitude, our summer sunlight is brighter, relative to the UK. (We’re on the same latitude as south of France, but without the temperature-moderating influence of the Mediterranean and the Gulf Stream.)

            The strength of sunlight is an important factor when designing gardens and especially in selecting flower colour. Gardens at more northern latitudes have a less intense quality of light, which makes pastel colours glow. But those same colours, when used in a stronger light, are less vibrant and can seem to disappear. Hot colours may look strident in the north, but they are perfectly suited to tropical light.

Local growing conditions, like those in your garden, are also influenced by altitude, air drainage, height of water table, proximity to bodies of water, local topography–and that’s just as a start. You should also take into consideration individual factors such as exposure: is your garden south or north facing? Are you in a built-up, urban space or out in the country? Do you need to be aware of drier rain shadow under house eaves or large trees, or can you perhaps make use of low-lying wet areas like swales or where your downspouts and sump pump drains?

            Is there any way you can make use of the microclimates in your garden to cheat a zone or two? Maybe you can take advantage of radiant heat from a house?

If you are able to provide the soil, light and moisture conditions a tender plant needs, then temperature might be one you can experiment with. You can plant in a microclimate and mulch the crown with dry leaves, and even a plastic sheet over that to keep out winter wet (if that’s what you plant needs), and you’ll be able to successfully grow many plants that ‘shouldn’t’ survive in Ontario.

            I grow several tender plants, leaving them in the ground over winter, and they do fine. (Though, full disclosure, I’ve lost many as well.) Try planting some from a zone higher than your own and see how they fare. If you are able to take advantage of microclimates in your garden–areas that are protected from wind, or with perhaps some warmth radiating off a house wall or a southern exposure, you could even jump two zones. I’m in Zone 6b and I successfully grew a Gunnera manicata (from Brazil, Zone 8) for years in my garden until I decided to move it–and it decided to die.