I have new double knockout rose bushes. Please recommend a cover for our long winter where temperatures go as low as minus 20 F. Thanks, Armande
Answer: For Zone 4 here are some general suggestions. First you want to make sure you have cleaned away any leaves or old mulch that might be left, eliminating over-wintering of any fungal spores. In Zone 4 it is suggested that after mid-October you spray the plants with a fungicide/dormant spray. After the first hard frost trim back the canes to a few feet. You can also tie the canes together to keep the wind from blowing and breaking them. Then using mulch or wood chips mixed with soil, compost mound up about 10 inches over the graft or crown of the plant. You can continue adding mulch to a depth of 15-18 inches. Once the ground is completely frozen and mice are not a threat, you can continue to protect them by creating a fence around them and fill with straw, hay or strawy manure to a depth of 3 feet. Start the reverse process around April, removing the last layer, and add water.
Rose cones can also be used but they are more expensive if you have a number of plants. For these you would want to wait until there have been several hard frosts and the roses are completely dormant. You will need to cut the canes back and tie together so they will fit completely under the cones. Mound your soil or mulch over the base of each rose. Punch 4-6 holes around the top to allow ventilation. If the heat builds up inside, the rose might break dormancy too early. Before putting the cone over the rose, add some dry mulching material, such as leaves or straw. Make sure to weigh down the cones with a brick or heavy rock and mound some mulch or soil over the base of the cones. Remove the cones after the threat of hard frost is over in the spring.
Knock-out roses are pretty hardy so either method should be fine; however, speaking from experience I find trying to wrangle roses into rose cones a bit of a pain, quite literally, so I prefer the fencing and mounding method for winter protection.
Best of luck with them.