Plants that need help now, during periods of sustained low temperatures:
1) plants you found and bought at nurseries outside the area that originate –grow in the wild–in areas that don’t routinely experience freezing.
2) plants that originate in low elevation areas that you planted in your high (2000′ feet and above) garden.
3) plants that are frost-hardy, but you just planted them in fall 2012 (especially if you planted in November or December).
The most vulnerable plants are small, young plants that have not yet developed a deep root system. Older plants, even those that are marginally frost-hardy (like many from Baja, Channel Islands and Northern Coastal) can survive our winters once they are established.
Most of the plants in category #3 will be fine as long as you have mulched with at least four inches of organic material or two inches of rock or gravel. If you haven’t yet done this, at least mulch heavily around the base of your new plants, about the diameter of the original nursery container. You can pile on the mulch even if you practically bury the the plant, as long as you remember to uncover it when the weather warms up. Sometimes landscape flags help with this. Construction shims work too –try painting them pretty colors–and birds can perch on them to pick out insects & seeds in the mulch.
For categories # 1 and #2, you can try the above –just mulching heavily around the plant– or use a frost-protection fabric to also protect the above-ground portion. If you have put a frost-tender plant into really high-elevation ground that freezes every winter, the plant may not survive and you’ve learned a good lesson.
Many plants survive as long as their roots and crown do, so sometimes it is a matter of forgetting about the above-ground parts and concentrating on what will make the difference. Sometimes just making small branch and twig piles over plants can help, or cover with fabric (not plastic, which can make things worse). Old timers in veggie gardens would pile fresh manure around the base of plants and watch it steam through the cold, but for native plants, you’d want to remove the manure later and fresh manure often has weed and other pest issues.
If above-ground parts of your plant freeze, don’t prune it off yet! I know it may not look that great, and you may think the whole plant has died, but wait until frost danger has passed — end of March for Vally and low-elevation gardens- and then see if the plant revives. The dead material acts like insulation and is the plant’s natural way of protecting the crown and regulating soil temperature in its root zone. The time to prune is once you see new growth and frost-danger has ended. Think about it later, in spring. Winter is all about protection.
I don’t generally sell plants that can’t withstand our average winter, whatever your elevation is. But I know we often see lovely native plants when we are out travelling and “just want to give them a try.” Hopefully these tips will help get them through winter, and each year they do they become a little stronger.