As autumn approaches, a woody plant begins preparing for the grueling task of surviving the winter in two main ways. The first is putting forth a strong burst of root growth, which enables the plant to absorb moisture and nutrients, which in turn strengthen it. The second is by reabsorbing the nutrients and chlorophyll in the leaves and leaf-stems (petioles) and storing them in the main trunk(s) and/or branches for the winter. Though, by and large, plants can fend for themselves, certain cultivated plants greatly benefit form human assistance. Roses are the perfect example of that! There are a few steps that you should take to help ensure that they will survive the winter with minimal damage. First and most importantly, keep in mind that a healthy rose plant is more likely to winter over successfully than a plant that has been stressed from insect or disease damage, or poor cultural conditions. Preventative maintenance is always a key factor!
1) Stop deadheading and doing any "ornamental" pruning by mid-September. Only prune-out any old, diseased and/or dying wood from the plant in the autumn. Leave any "ornamental" pruning for springtime.
2) Carefully clean up ALL leaf litter around the base of the plant. This will help minimize diseases, which over winter on dead plant material.
3) In mid-October, OR after a hard frost (which may be as late as November), pull the mulch away from the base of the plant about 3' in diameter.
4) Lightly scatter Potash fertilizer on the soil around the base of the plant and water it in well. Potash is very high in potassium, which is responsible for heat, cold and drought tolerance, disease resistance, strong cell-wall growth, and general vigor in plants. Smaller plants should receive ¼ to 1/3of a cup; older plants can get ½ to 2/3 of a cup.
5) If you have a problem with rodents, such as field mice and voles, you might consider dusting the base of the plant with bulb dust before step #5. Bulb dust not only helps prevent insect and disease damage, but also contains a chemical, which repels rodents from chewing the bark off your rose plants during the winter.
6) Heap garden soil in a mound 10 to 12 inches deep around the base of each rose, which will help protect the graft, a "knob-like" area at the base of the plant. Contrary to popular belief, mounding and mulching methods are not intended to keep the plant warm. They are meant to keep the plant consistently cold, or even frozen! A plant that remains frozen throughout the entire winter has much better survivability than one that is constantly freezing and thawing!
7) Replace the mulch on top of the soil mound.
8) Consider spraying the aboveground stems with an anti-desiccant, such as Wilt-Pruf. This will prevent the stems from dehydrating during the coldest part of the winter. It also helps to add Chlorothalonil fungicide in with the Wilt-Pruf spray. Aside from the obvious fungicidal benefits, it also leaves a white film on the bare stems, which will help prevent them from becoming sunburned on very bright, very cold winter days!
With the arrival of early April, you can remove the soil mound, fertilize and re-mulch your plants for the coming season, as well as doing any necessary pruning. Happy growing!