10/18/2026
Ive always been curious about the zone ratings for roses, and when I was working on becoming a Consulting Rosarian & Máster Gardener, I found out that roses produce AFP’s (anti-freeze proteins). AFP’s primarily help by altering how ice crystals grow inside the plant tissue, NOT by lowering the liquid freeze temperature. So because different roses produce different amounts of AFP’s, hence the different levels of freeze they can ‘weather’.
I also thought if I were a scientist, I would find out how to help roses generate their own AFP’s, and not just from getting better results from breeding with a cold-hardy variety.
Then, this past year, I started reading about the use of ‘potassium drenching’ 4-6 weeks before the first frost as a way to further enhance the winter hardiness of a rose. Potassium can augment the rose’s natural AFP’s by lowering the liquid freeze temperature. So, in essence, a double dose of winter protection- changing how water freezes thru AFP’s, AND lowering the liquid freeze temperature.
From what I have read, using Sulphate of potash (Potassium Sulphate) is better that using Muriate of potash, because muriate has a much higher level of salt content which can adversely effect rose tissue in autumn.
Anyway, maybe I’ve not been paying attention, but I really haven’t heard, seen or read much about potassium drenching in the fall to winterize roses, even though there is a lot of science that backs it up. For instance, there is a lot of info about how to protect the canes against dessication & applying mounds of compost to protect against freezing, but only recently have I seen some information about using Sulphate of potash.
So, because I have SO many plants that are new this year or under 3 years old, I started applying sulphate of potash a couple of weeks ago: 1.5tsp/gal per rose, 1x weekly, 4 weeks before the expected first frost. It takes me 1-1/2 hrs to mix the powder in water & apply to 60 roses by hand. I am hoping to see a significant improvement in survival, especially for the roses that are barely a year old.
Has anyone else used potassium drenching in the autumn to increase a roses ability to handle deeper freezes? I would love to know how it’s worked for you. TIA
