Yes, roses in the Northwest Territories need winter protection, and grafted roses need it most. The method that works is simple. Once hard freezes have set in, usually in early to mid October, bury the base of each plant under about a foot of loose bagged soil. Plastic wrap, sealed cones, and mulch piled on warm ground all do damage, and the sections below explain why.
Do roses need winter protection in Northwest Territories?
Natural Resources Canada places Yellowknife in zone 1b on the Canadian plant hardiness map. That is the Canadian system, and it does not convert to a USDA zone. The Canadian map is built from seven climate variables, while the USDA map uses winter lows alone. Zone 1b also describes one city. Communities farther north or at higher elevation can run colder.
Environment and Climate Change Canada’s climate normals for the Yellowknife A station put the average last spring frost at May 25 and the average first fall frost at September 18. That leaves about 115 frost-free days. Both dates are midpoints. In one year out of two the last spring frost falls later than May 25, and in one year out of two the first fall frost arrives before September 18.
A short growing season and a long, deep winter put roses at the edge of their range. Winter cold is only part of the danger. Roses also suffer when a mid-winter thaw wakes the canes and the next freeze catches them wet. Dry, drying wind pulls moisture out of canes that are frozen and cannot replace it. Snow is an excellent insulator, but it drifts, so one bush may sit under three feet of it while its neighbor stands bare.
The weak point is the graft union, the knobby swelling at the base where the named rose was joined to a hardier rootstock. If the union freezes, the top dies and the rootstock sends up suckers. Those suckers bloom, but not with the rose you paid for.
Few if any places in the territory are mild enough to skip protection on a grafted hybrid tea, floribunda, or grandiflora. A few very hardy, own-root shrub roses can sometimes get by on snow cover alone, and even they lose cane tips in a thin-snow winter. Which ones are worth trying is covered on the rose varieties for Northwest Territories page. For the wider picture of getting roses established in the first place, see growing roses in Northwest Territories.
When to start, and what triggers it
A run of hard freezes sets the timing. The calendar does not. Mound too early and the plant, still taking in warmth, keeps growing under the soil. It pushes soft shoots that cannot survive the cold ahead. Mound too late and you are chipping at frozen ground with a shovel.
For Yellowknife’s climate, the average first fall frost of September 18 is only the opening act. A frost that day blackens the tender leaves, but the rose is not yet dormant. The signal to mound is several nights in a row well below freezing, with daytime highs that no longer climb much above it. In practice that usually arrives in early to mid October. Communities north of Yellowknife or at higher elevation get there sooner, and a milder fall pushes the date back.
Before that, give the plant a few weeks to settle down on its own. From late August, stop fertilizing and stop deadheading, which tells the plant to quit making new growth. Let the hips form. Leave the canes uncut for now, because pruning at this time of year invites new shoots. Spring cutting belongs on the rose pruning time page.
Gather your materials ahead of the first hard freeze, because bagged soil is much harder to find, and to move, once the snow flies. A short list is enough:
- Bagged topsoil or garden soil, roughly two to three bags per bush
- Twine for tying canes loosely together
- Spruce boughs or clean straw for a top layer
- A bucket or wheelbarrow to carry soil to the bed
Keep the bags in a shed or garage so they stay dry and loose. Soil that is frozen into a block will not pack around a crown.
How to mound, and how deep
The goal is to bury the graft union, which sits at the base of the plant just above the roots. The canes themselves do not need burying. Pile loose soil over the base of the plant until the mound stands 10 to 12 inches tall, high enough to cover the union with room to spare. In a deep-freeze climate, that depth also keeps the temperature at the crown from swinging between thaw and freeze.
Why bagged soil beats soil dug from around the plant
Use bagged soil brought in from elsewhere. Digging it from around the plant strips soil off the shallow feeder roots and leaves a trench that fills with meltwater and freezes around the crown. Soil dug from a rose bed can also carry fungal spores from the leaves that fell there. Bagged soil is usually looser and cleaner, and kept dry in a shed it holds little water. It is unlikely to bring disease from last summer’s leaf spot. Fill the mound from the bag and keep the surrounding bed level.
Handling the canes and the top layer
Tall canes whip in the wind and loosen the plant at the base. Tie them together loosely with twine, and trim the tallest to roughly knee height if they stand well above the mound. Do not cut them all the way down in fall. Leave the heavier pruning for spring, when you can see which canes survived.
Once the mound has frozen on the surface, lay spruce boughs or clean straw over the top and around the sides. This layer catches blowing snow and holds it in place, which is the insulation you cannot buy. Do not lay boughs on warm ground and call it done. The soil mound is the protection, and the boughs are the blanket on top of it. If you skip the mound, the graft union will be left exposed to the coldest air, and that is where the plant is most likely to die.
In spring the mound comes off gradually. Pull the boughs first, then remove the soil in stages as the ground thaws and the danger of a hard frost passes. Remove it all at once on a warm April day and a late freeze can still burn the new buds. Frost can arrive late, and May 25 is only the midpoint of the last-frost window at Yellowknife.
What does not work in this climate
Three popular habits can cause winter losses of their own. Each one looks protective, and each does something different to the plant.
| Practice | What it does to the plant | What to do instead |
|---|---|---|
| Wrapping canes in plastic | Traps moisture and heat on sunny days, then freezes hard at night. Canes sweat, then split, and the bark rots under the film. | Tie canes loosely and leave them open to air |
| Rose cones left sealed | The inside warms in March sun and the plant breaks dormancy early. Sealed tops also hold condensation against the canes. | Use soil as the main layer, and vent any cone on mild days |
| Mulch piled on warm ground | Keeps the soil warm and damp, so the crown keeps growing and can rot. Mice and voles nest in it. | Wait until the ground surface has frozen, then add the layer |
Plastic is a particular risk. It looks like a raincoat, but a rose does not need to stay dry in winter. It needs to stay dormant and consistently cold. A plastic sleeve over the canes sits in the sun on a clear February day and overheats, then drops far below freezing by night. The repeated swing does more harm than steady cold.
Sealed rose cones fail in the same way. The foam shell works only if you can open it. A cone sealed in October and forgotten until May cooks the plant on spring afternoons. If you use cones at all, fill them with soil and loose bough cover rather than leaving them hollow, and open the top as soon as the days turn mild.
Early mulching has a quieter cost. Wood chips or leaves piled around a warm crown hold in heat, damp, and rodents. The animals chew bark just above the soil line, and a ring of damage there can kill a cane in one winter. Wait for the surface to freeze, then pile on the cover.
Checking this against your own community
Yellowknife is one city, and the Northwest Territories covers a very large area. Elevation and the distance from open water change frost dates by weeks. A garden on a hill may get its first frost earlier than one in a sheltered hollow, though cold air also pools in low ground, so the answer depends on your lot. Use the Yellowknife dates as a starting range and then check your own site.
The Government of the Northwest Territories agriculture services are a good local starting point, and you can look up your community’s zone on Natural Resources Canada’s Canada’s Plant Hardiness Site. Compare your own first hard freeze with the dates above, and adjust your mounding week to match your own fall.
Common questions
Can I leave roses unprotected if they are rugosas or other hardy shrub types?
Some very hardy own-root shrub roses can survive on snow cover alone, but a winter with thin snow can still kill the canes back to the ground. Mounding costs little and takes little time, so it is worth doing for any rose you would be sorry to lose.
What if my rose is on its own roots and not grafted?
Own-root roses have no graft union to protect, and if the top freezes they regrow true from the roots. They are the safer choice for the north. A light mound still helps by holding the crown at a steadier temperature.
How much soil do I need per bush?
A mound 10 to 12 inches tall over a typical bush takes roughly two to three bags of topsoil. Buy a little extra for the bushes that surprise you, and keep it dry in a shed until the day you use it.
When do I take the mound off in spring?
Remove it in stages as the ground thaws, starting with the boughs. Because May 25 is only the midpoint of Yellowknife’s last-frost window, hold some soil back until the worst of the freezes has passed. A late frost on exposed new buds can set the plant back a full season.