Hello there. I live in North Oshawa which is supposedly Zone 5a-5b. The bed has fairly good loamy soil which is about 40% compost and manure and 60% percent loam. The bed is in full sun and is along the foundation of the house on a south facing exposure, and the bed drains quite well. There are some large rocks which may or may not contribute to the microclimate as in the winter they are covered by the snow. I recently purchased some Kniphofia (1 Flamenco, and 1 Royal Castle), and I was wondering what are some surefire ways to make sure they survive the winter as they are a zone 5-6 plant and I’m located in zone 5?
Thanks, Sufian
Kniphofia uvaria (common names red-hot poker or torch lily) is indeed on the borderline, and so the short answer to your question is that there is no surefire way to make sure that they survive the winter. However, it seems that you have a microclimate in that sheltered part of your garden, and so it is well worth giving this plant a chance. Here is an interesting read on using microclimates to try to grow plants that theoretically should not be able to survive. Note that this author indicates that large rocks can provide radiated heat: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19289 Snow also provides protection, and some authors recommend a layer of mulch as a protection through the winter and the sometimes extreme freeze and thaw cycles we experience in the spring.
The question of hardiness zones is a complicated one, because the USDA zones are one digit different from the Canadian. If you are referring to an American publication, you will need to add 1 to the hardiness zone indicated. Here, two Toronto garden writers provide a clear explanation of the differences: https://www.torontogardens.com/2009/05/why-toronto-is-zone-6-and-zone-5.html/
The Government of Canada’s Plant Hardiness chart, revised for climate changes to 2010, indicates that Oshawa is a zone 6a: https://www.planthardiness.gc.ca/?m=22&lang=en&prov=Ontario&val=O
Very best of luck in overwintering these two Kniphofia cultivars in your sheltered space.