Not Really Cold Hardy Plants
Sep-14-2005 at 8:40pm
This is a rant post. I've been researching Star Jasmine, Trachelospermun jasminoides, that was mentioned in a previous post asking about it's cold hardiness. Some box stores have been selling this and other plants as permanent plants when, at best, they are plants that need special care. The reason this gets to me is that they are setting people up for failure and when the plant dies they will think it's something they did instead of just the weather. I don't want people to get discouraged with gardening, I want them to see what fun it is.
For a reference point most of Western Washington is zone 7, Oregon zone 8 according to the USDA hardiness map. If you live farther away from the water you may be a zone colder, or an "a" or "b" difference. According to The Sunset Western Gardening book zones we are zone 5 or 6. I consider them the most reliable source of information around here. They list Star Jasmine as hardy to zone 8. Their zone 8 has a 20 year low of 29-13 degrees F. That is not us in the Northwest, so Star Jasmine needs extra protection like being next to a warm wall to live. Back to the USDA zone map, "Hortus the Third" lists it as being hardy to zone 9, USDA map, also not in this area. An older book, "Landscaping with Vines" says it is hardy to 24 F. We usually get colder then that. Now, according to the Monrovia Nursery website and tags they say it is hardy to zone 8, USDA map. I don't know where they get their information but I plan to find out more about this, because it doesn't fit in with my personal knowledge and with these known references.
One other plant is Euryops pectinatus, Monrovia lists it as USDA zone 8, but in "Sunset", they list it in their zone 8, a really big difference. I know of some other plants I have some real doubts about their hardiness, what do you think about this? I'm trying to find out more about this as time allows.
Posted: Sep-14-2005 at 9:34pm
Plant hardiness is hard to define. That is why Sunset zones, although not perfect, tell us better if it will survive in our northwest gardens or not.
Using the 'dis-mini-mal' USDA zones is not adequate for us for several reasons.
Winter Wet Excess moisture does more to kill a hardy plant than we realize. Root rot probably does more plants in that are hardy in zone 8, but needs dry winter conditions.
Not Enough Heat Many plants that are hardy in southern states that are zone 8 can not survive because of our cool summers. The reason is, the plant produces more sugars in heat, which in turn protects the plant from the cold.
Tony Avent of Plant Delights Nursery in North Carolina explains:" Another phenomenon, seen in England and in the cool areas of the West Coast of the US is the difference in winter hardiness due to a lack of summer heat. In many plants native to warmer climates, summer heat causes increased sugar production, which allows the plants to survive more stress in the winter. In areas without summer heat, a particular plant may only be hardy to 20 degrees F, while in an area with hot summers, the same plant may easily be hardy to 0 degrees F."
This is why a plant on a south facing wall can thrive as it is making more sugars because it receives more heat. I believe star jasmine maybe one of these plants that needs more sugar production to stay hardy enough in zone 8.
Good rant material, Fern!
Posted: Sep-17-2005 at 11:59am
I am glad you all put this info on here. I knew that the sunset zone were supposed to be more accurate but it confuses the average person...well the tag says hardy to this zone but sunset says we are this zone...not the same zone at all. That is hard to expalin to people that just dont get it in the first place. But your explanation of the sugars and wetness of the soil helps me out very much becasue I did not really understand the nuances of our zone differences here in wa. I mean we don't get as cold but still why don't some things produce here anyways? Well now I get it!