Yes, grafted roses in Prince Edward Island need winter protection. The job is simple: bury the base of each bush under 10 to 12 inches of bagged soil once hard freezes have set in, which usually happens late in the fall. Plastic wraps and sealed foam cones cause more harm than good. For everything that comes before winter, see the main guide to growing roses in Prince Edward Island. What follows covers timing, depth and the mistakes that cost gardeners their plants.
Do roses need winter protection in Prince Edward Island?
Natural Resources Canada places Charlottetown, the reference city for the province, in Canadian hardiness zone 5b. That is the Canadian system, and it does not convert to a USDA zone. The Canadian map weighs seven climate variables, including snow depth and winter rainfall, where the USDA map uses one, the average coldest night. NRCan says there is no simple conversion between them, and that Canadian zone 4a alone overlaps nine USDA zones, from 3a to 7a. A US gardener comparing notes with an Islander should compare winters, not zone numbers.
For a rose, zone 5b means winter cold deep and long enough to kill the canes of a tender plant. It can also kill the bud union, the swollen knot where the named variety was grafted onto a tougher rootstock. A gardener in the mild Southeast can leave roses standing all winter and never think about it. Here, grafted hybrid teas, floribundas, grandifloras and many climbers need protection every year.
The lowest temperature is only part of the risk. The island is exposed to wind off the surrounding water. Winter also swings between thaw and refreeze, and thaws wake plants up just before the next hard cold. Snow is an excellent natural blanket for a rose, but wind strips it off open beds and leaves the base of the plant bare. A bush in the lee of a house or a fence may come through under snow alone, while the same variety in the middle of a lawn does not.
The answer also depends on the plant. Hardy shrub roses growing on their own roots often survive on snow cover with no help, and if the canes die back, new growth comes from the roots. A grafted rose that loses its union has nothing left worth saving. Young plants in their first winter deserve protection whatever their type, because their roots have not yet spread out and settled in.
When to start, and what triggers it
Environment and Climate Change Canada’s 30-year averages for the Charlottetown A station put the first fall frost at about October 18. The mean last spring frost is May 17, and the average frost-free period is about 153 days. Both dates are averages, so in roughly half of years the fall frost arrives earlier and in roughly half of years the spring frost comes later. They are also Charlottetown numbers, and a garden in a northern or upland district can run weeks off them.
October 18 is not the day to mound. A first frost only blackens the soft growth and the last of the blooms. The roots are still warm and the plant is still moving sugars around. Soil heaped over the base at that point holds warmth against the canes and can keep buds active, and any new growth that results is exactly what the cold will kill.
How do you tell roses are ready to be mounded?
The trigger is a run of hard freezes. Watch for nights that fall well below freezing and days that barely climb above it, repeated for a week or more. The leaves will have blackened and dropped, and the top inch or two of soil will feel firm and crusty. A trowel pushed in at the edge of the bed meets resistance instead of sliding through. That is the plant telling you it has gone dormant.
Do the shopping early, even though the mounding comes late. Buy the soil in October and store the bags in a shed or garage. Bags left outside can freeze into solid blocks by the time you need them.
Do hard freezes arrive at the same time across the island?
No. A province is not one climate. Exposed coastal gardens, sheltered inland ones and higher ground each reach their hard freezes on a different schedule, and the gap can be a couple of weeks. Use Charlottetown’s dates as a rough guide and let your own bed decide. If the soil is still soft and the roses still have green leaves, wait. Mounding a week late hurts a dormant plant far less than mounding a week early hurts an active one.
How to mound, and how deep
The graft union is what you are burying, and the canes come second. That knot at the base of a grafted rose is the most valuable and most vulnerable part of the plant. If it survives under soil, the rose can rebuild from it in spring even when every cane above ground dies. Mound 10 to 12 inches of bagged soil over the graft union, shaped into a cone that covers the whole crown and runs out several inches beyond the base of the plant.
What should you mound roses with?
Use bagged topsoil, garden soil or a compost-based planting mix. Plan on about a cubic foot per bush. It should be loose and free of stones. Peat-heavy mixes work, and soil that drains well matters more than richness, since a mound that stays soggy and then freezes does more harm than good. Before you start, rake fallen leaves away from the base of the plant and clear any that are spotted or yellowed. Black spot spores overwinter on dead leaves, and you do not want to bury them against the crown.
Tie the canes together loosely with twine so the wind cannot whip them against the mound. Trim only what would thrash around, such as unusually long canes. Leave the real pruning for spring, when you can see which wood is alive. Many canes above the mound will die back over winter, and that is normal.
Why not use soil from around the plant?
Scraping soil from beside the bush looks free and convenient, but it digs into the shallow roots that feed the plant. It also leaves a trench around the base, and a trench collects meltwater and freezes into a ring of ice right where the roots sit. Soil from the bed can also carry black spot spores and other fungi, which bagged soil is much less likely to do. Bagged soil comes from outside the bed and leaves the roots undisturbed. You can also lift it out in spring and spread it somewhere useful, with no trench left to fill.
When does the mound come off?
In spring, pull the mound back in stages once the soil has thawed and buds begin to swell. Do not clear it all at once. The mean last frost at the Charlottetown A station is May 17, and roughly half of all years bring a frost after that. Keep a bag or two of soil nearby so you can push some back over the base if a hard frost is forecast after the buds have started growing.
What does not work in this climate
Common winter mistakes involve covering a rose with something that looks protective but changes the conditions around the plant for the worse. Three of them are widespread.
Why is plastic wrapping bad for roses?
Plastic traps moisture and blocks airflow. On a bright day in January or March, the sun warms the air inside the wrap well above the temperature outside. The canes wake up, then freeze again when the sun goes down. Condensation collects on the bark, and a wet, closed space is where canker and gray mold take hold. If you want a windbreak, use burlap, which lets air through. Skip the plastic entirely.
Do rose cones work if you leave them sealed?
A foam cone with the lid taped shut behaves like plastic wrap in a smaller package. It heats up in winter sun, holds humidity and sets up the same wake-and-freeze cycle. It also makes a tidy shelter for mice and voles, which can chew bark and cane bases all winter, usually unseen. A cone can help when it sits over a soil mound, with a vent hole cut in it and a stone on top to hold it down. In that setup the soil does the insulating and the cone only keeps wind and snow off.
Is mulch a problem if you pile it up early?
Straw, wood chips or shredded leaves piled around the base while the ground is still warm do the same thing as early soil mounding. They insulate warm soil against the cold, which keeps the plant growing and the roots active. They also give voles a cozy, hidden place to feed. Mulch belongs on top of the soil mound after the ground has frozen, and only where wind is a problem and snow will not stay put. Leaves alone make a poor mound on their own, because they mat down into a wet layer that does not drain.
Checking this against your own county
The numbers above come from a single reference city and a single weather station, so treat them as a starting point. The Government of Prince Edward Island agriculture pages are the place to ask about local conditions and winter damage. You can also look up your own municipality using the Plant Hardiness Zone by Municipality tool on Canada’s Plant Hardiness Site, run by Natural Resources Canada.
Elevation and exposure change the picture even within one county. Cold air drains downhill and settles in hollows, and ridges take the wind. In your own yard, note which spots lose their snow first, and put your most valuable roses where snow stays.
Common questions
Can I mound roses with leaves instead of soil?
Soil is the better base. Leaves mat down, hold water and can carry black spot spores. If you want extra insulation on an exposed site, add shredded leaves or straw on top of the soil mound once the ground has frozen.
Do climbing roses need protection too?
Grafted climbers with tender canes do. Hardy climbers often come through on their own, but any climber with a graft union should have its base mounded like a bush. Bending canes down and covering them with soil is possible on some types, though stiff canes can snap, so check the specific plant before attempting it.
Should I prune roses in the fall before mounding?
Only lightly. Trim canes that are long enough to whip in the wind and risk loosening the plant. Hard pruning in fall can open fresh wounds just as the cold arrives, so wait for spring, when you can see which wood survived.
What about roses growing in pots?
Pots freeze solid and from all sides, which is harder on roots than the ground. Move them to an unheated shed or garage once the plant is dormant, or sink the pot into a garden bed and mound over it. Give the roots a little water now and then so they do not dry out completely.