Yes, in nearly every part of North Dakota. Grafted roses such as hybrid teas, grandifloras and many floribundas need a mound of clean soil over the base of the plant once hard freezes arrive, usually from late October into early November. Hardy shrub roses on their own roots can often go without. The rest of the job comes down to protecting one spot on the plant: the graft union.
Do roses need winter protection in North Dakota?
The USDA Plant Hardiness Zone Map places the state between zone 3a and zone 4b. That system rests on a single number, the average annual extreme minimum temperature. Even the mild end of that range sits well below what a hybrid tea rose can take.
A grafted rose is two plants in one. The flowering variety is joined to a tougher rootstock at a knob near the soil line called the graft union. The flowering wood is the tender half. When a hard winter kills the canes down to the ground, the shoots that sprout the next summer come from the rootstock. You end up with a vigorous plant that bears small, plain flowers nothing like the one you bought.
Cold alone does not do all the damage. Wind strips moisture from exposed canes while the frozen soil cannot send any back up. Midwinter thaws, which the northern Plains deliver regularly, wake the sap and then a refreeze splits the bark. Snow is a fine insulator when it stays put, but open ground here loses its snow cover to wind, sometimes within a day.
So hardly any part of the state is warm enough for a grafted hybrid tea rose to skip protection. The picture is different for roses grown on their own roots, where a dieback to the snow line still leaves the true variety alive. Some shrub and species-type roses handle zone 3 with little or no cover, and the rose varieties that come through North Dakota winters are worth sorting out before you buy. For site choice, soil and general care, see growing roses in North Dakota. What follows covers only the winter side.
When to start, and what triggers it
A run of hard freezes sets the schedule. The calendar does not. A rose has to shut down on its own before you cover it. As days shorten and nights chill, the plant stops pushing new growth, the leaves yellow, and the canes firm up. Pile soil on a plant that is still growing and the covered canes stay soft and damp in a warm, humid pocket. That is exactly where rot and canker start.
At the Fargo airport reference station, the first fall frost typically arrives between September 25 and October 5. That frost is only a warning. Roses often keep their leaves after it and sometimes put out another flush. Wait for the nights to stay below 32 F, for leaves to blacken or drop, and for the top of the soil to stiffen. At the reference station that usually lands in late October to early November. Gardens outside the city, and those on higher ground, can see the cold arrive sooner, so watch the plant and the ground rather than the date.
You can nudge the rose toward dormancy earlier in the season. Stop fertilizing by midsummer, since nitrogen pushes the soft growth that winter kills first. In late summer, quit deadheading and let the plant form hips, which tells it to slow down. Keep watering through autumn dry spells, because a rose heading into frozen ground with dry roots suffers more from winter drying.
The window for mounding is short. Once the soil freezes solid, you can no longer dig, and bagged material can turn into a frozen brick in the yard. Buy it in September, stack it near the beds, and keep a tarp over it so it stays workable. Gardeners who wait for the perfect day often end up chipping at frozen ground in December.
Spring has its own clock. The reference station’s last spring frost falls between May 4 and May 15, and rural gardens usually see it later. The mound comes off in stages well before then.
How to mound, and how deep
The graft union is the target. Canes are secondary, and they often die back to the mound line anyway. Your goal is to bury the graft union under loose, insulating material so that the part of the plant that gives you the flowers survives to leaf out in spring.
Start with cleanup. Rake up fallen leaves and remove any with black spots, since the fungus overwinters on debris and gets sealed in with the plant if you leave it. Tie long canes loosely together, then shorten them so they do not whip in the wind and rock the crown. Save the real pruning for spring. The right time to prune roses in North Dakota comes after the cover is off and you can see which wood is alive.
Then build a cone of soil 10 to 12 inches deep over the base of the plant, wide enough that the sides do not slump. Mound with bagged topsoil or a bagged soil and compost blend. Soil dug from around the plant is the wrong source for three reasons. Digging cuts the shallow feeder roots right where the plant needs them. It leaves a trench that collects meltwater, which then freezes around the crown. And garden soil near a rose often carries the spores of its leaf diseases. Bagged soil is clean and loose, and it traps a lot of air, which is what insulates.
Once the mound has frozen on the surface, you can add a layer of shredded leaves or straw on top, held in place with a ring of wire mesh. That extra layer slows the freeze-thaw swings. Do not pack it down. Insulation works because of the air inside it.
What does not work in this climate
Several popular shortcuts can do more harm than leaving the rose bare. All three trap moisture and heat around live tissue, and that combination is what kills canes in a northern winter.
Does plastic wrap protect roses?
Plastic sheeting wrapped around canes holds in condensation and blocks air movement. On a sunny January afternoon, the covered canes warm up and wake, then freeze hard again at dusk. Wet bark under plastic is also an open invitation to canker and rot. Burlap or a loose screen of evergreen boughs lets air through and cuts the wind, which is the job a wrap should be doing.
Can you leave a rose cone on all winter?
A foam rose cone placed over the plant and left sealed acts like a small greenhouse. Sun heats the inside well above the outside air, and the humid interior encourages mold on the canes. A cone also gives mice and voles a sheltered spot to nest and chew bark. If you use one, cut vent holes at the top, fill it with leaves or soil, and open it on mild days rather than leaving it shut until spring. Even then, a soil mound does the job better.
Is it okay to mulch roses early?
Piling mulch or leaves over the crown while the ground is still warm creates a soft, damp blanket that keeps the plant growing. Rodents move into it long before the cold arrives. The crown can stay wet and begin to rot well before the first deep freeze. Mulch belongs on the plant after the ground has started to freeze, when the goal has changed from keeping the soil warm to keeping it frozen and steady.
Checking this against your own county
The frost dates here come from the Fargo reference station (NOAA NCEI Climate Normals for 1991-2020). Elevation changes things quickly. Higher ground and open rural sites run colder and see their last spring frost later and their first fall frost earlier than an airport in a valley. Call your county extension office or check with NDSU Extension for the local hard-freeze timing and for the mounding window in your part of the state. Ask which of your roses are grafted, and when the soil in your area typically freezes. Those two answers set your schedule better than any date in a guide.
Common questions
When should I take the mound off in spring?
Remove it in stages through April as the soil thaws and the worst of the hard freezes pass. Pull off the straw or leaves first, then scrape the soil back a few inches at a time over a week or two. Keep the material close, because a late cold snap can send you back out to re-cover. Since the last spring frost at the Fargo reference station falls between May 4 and May 15, and later in many rural gardens, frost cloth is handy for the early weeks. Once the canes are exposed, check them before pruning.
Do climbing roses need the same protection?
Climbers carry long canes that stand fully exposed to wind. Untie them from the support in late fall, lay them gently along the ground, and cover them with soil, straw or evergreen boughs. Bend canes gradually and a little at a time, since stiff wood snaps if you force it. Mound the base just as you would for a bush rose, burying the graft union under soil.
Should I cut my roses back before winter?
Only enough to make mounding possible. Shortening canes keeps them from whipping in the wind. Heavier pruning in fall can stimulate new growth and leaves open cuts to freeze. Wait until spring, when you can see which canes survived, and cut back to healthy, green-centered wood.
Do roses on their own roots need to be covered?
Many hardy shrub roses on their own roots come back from the base even after a rough winter, so a thin layer of mulch after the ground freezes is often enough. Tender types on their own roots still benefit from a soil mound, because it saves you the dieback and the late bloom. If you are unsure whether a plant is grafted, look for a swollen knob near the soil line, or check the tag or the nursery’s listing.