Why Can’t I Grow this?

Liza Drozdov – Halton Master Gardener

Garden dreams meet Ontario reality.

Rehmannia angulata

Many gardeners want to grow every plant they fall in love with, regardless of the inconvenience of geography. And many of us try. And fail. Sometimes we’ll try and fail again. (The rule is you’ve got to fail three times before you give up.)

There are a number of reasons plants won’t grow, some of which you can control and some you just can’t. That’s when you really have no option but to accept reality. But a little knowledge can make acceptance a little less painful, or it can inspire you to give it another try–armed with more information.

            We’ve all heard the maxim right plant, right place. And, as a start, it’s good advice. When you read a plant label, it gives you the basic info: what colour the flower is, how tall and wide it grows, how much light it requires, and what zone it wants to be in. In principal, that should be enough…but it’s not.


When it comes to herbaceous perennials (those that die down to the ground in winter) the zone listed on the label isn’t necessarily helpful information. The hardiness index–the basis for plant zones–was developed in the 60’s by Agriculture Canada, for the benefit of the fruit industry. It’s really designed for woody trees and shrubs–those that remain exposed to cold temperatures and desiccating winds all winter, as well as the risks of freeze and thaw.

            The plant hardiness index is a very sophisticated and complex system that takes into consideration seven climate variables: minimum temperature, frost-free period, winter precipitation, summer precipitation, snow cover and depth, maximum wind gusts and maximum temperature. The zone maps are always evolving, as a reflection of climate change.

            Climate zones are basically the ‘average weather’ in your area–and that average is dependent on several factors: average frost-free periods, the dates of first and last frosts, temperature (degree days of cold), precipitation (how much moisture falls in the form of rain or snow), and PE or potential evapotranspiration. (PE is the amount of water that could potentially evaporate or transpire from surface vegetation. Simply put, it’s an index of heat from the sun.)

            Averages are helpful, but every year can have unique and often extreme conditions that will affect plants’ survival, and with climate change it seems like all bets are off in terms of weather.

            Zone maps don’t necessarily apply to herbaceous (non-woody) plants, so you need to be flexible in how you apply them. And–most important to someone shopping at a plant nursery–most plant tags are based on temperature-and sunlight-only models…and don’t take into consideration other factors that determine a plant’s survival, like moisture and soil conditions.

            Once a plant dies down in the winter, there’s nothing left of it above soil– and once the ground freezes, it stays a constant temperature–0 degrees Celsius. Herbaceous perennial plants don’t feel wind chill. If you are careful to protect the crown and root zone of your perennials with a thick layer of leaves, your plants will stay cozy under their winter blanket. Their temperature remains constant and they aren’t subject to cycles of freeze and thaw, which is fatal to many plants. Many gardeners in northern parts of the province have better success with some slightly tender perennials because they have reliable snow cover all season–unlike those of us in the south that end up with bare soil most of the winter.

            So remember that perennial plants were never rigorously tested across a wide spectrum of conditions then assigned to a specific zone. Hardiness zones, when applied to herbaceous perennials, are often a “best guess”, based on the region where they are native. But often–and especially with new cultivars and introductions, growers just don’t know. Nobody ever asked the plant, so it’s worth a try.


Mecanopsis ‘Lingholm’

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.

Erymgium ‘Jade Frost’

            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.


This is probably the easiest requirement for gardeners to provide. We all know how to water! We have irrigation systems or use soaker hoses, we deploy sprinklers and watering cans as needed, and I’ve even created lined beds to simulate bogs. But plants’ moisture requirements can be more complicated that we realize.

  Drought tolerant is a common term we use all the time, and we understand it means the plant will survive without additional watering. But it can also mean the plant actively resents water, especially in winter. I’d bet as many plants die from being too wet than from being too dry. Just as we tend to overfeed our plants by placing them in rich soil they don’t appreciate, we can overwater plants. We can also water them too often and too shallowly, so they don’t develop the deep root systems they need to withstand drought and winter cold.


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.


Digitalis ferrunginea ‘Gigantea’

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.

            The general rule of thumb in gardening is to mimic a plant’s native habitat when you put it into your garden–or that of its species if it’s a cultivar. But do a bit of research and try to give it all the light, moisture and soil requirements it needs to flourish. Then you really can put the right plant into the right place.

Hydrangeas and Your Soil

You want blue mophead hydrangeas, but your soil is alkaline…and after a year, that lovely blue plant you bought is now pink. You might have read that you can amend your soil and water your hydrangea with a solution of aluminum sulphate. The idea is that this will lower soil pH which and turn your hydrangeas back to blue. Using Aluminum Sulphate will definitely acidify your soil and have the desired effect, but I wouldn’t recommend it. A prolonged diet of Aluminum sulphate can also be toxic to plants and inhibit their growth. The long-term effects of this treatment aren’t yet known, but it’s highly likely it can damage soil and the microorganisms living within it, not to mention what it might do to local watersheds. You can try to plant your hydrangea in a mix of ericaceous (acidic) compost, like that made from pink bark and needles, and mulch annually with the same, but you’ll need to keep it up forever. Or you can learn to love pink.

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