Determining a Plant's Hardiness

If a particular plant isn't hardy to your specific USDA growing zone, chances are it won't survive. But how do you know if a plant is hardy? Master gardener Paul James explains:

"It's fairly safe to assume that landscape plants commonly sold in area nurseries and home improvement centers are hardy to your zone. Exceptions, however, include annuals, tender perennials and tropical plants. Most plants at the nursery are clearly marked with the USDA growing zone information, usually found somewhere on the plant tag or label. But every now and then you'll come across a plant that has no such information, and in that case, you'll have to do a little research.

"All-purpose gardening reference books almost always have hardiness information, as do plant catalogs and web sites. But just how dependable is hardiness zone information and who determines how hardy a plant is in the first place?

"In most cases, in particularly common landscape plants, the information is based on decades of information and experience gathered from a variety of sources, including primarily growers and researchers. But believe it or not, the hardiness rating of some plants is little more than an educated guess. And I've been pleasantly surprised more than once by just how hardy a particular plant really is.

Figure A

Take for instance, the sago palm (figure A), which is supposed to be hardy only in Zones 9 and 10, meaning the minimum temperature it can tolerate is about 55 degrees F.

Figure B

This is new growth emerging from another sago I left out all winter in temperatures that dropped down to 8 degrees F, and I never even watered it (figure B).

I'm not necessarily saying you should cheat the USDA hardiness zone for most plants. However, every now and then, you may come across a plant that is a little bit hardier by at least one zone than even the experts recognize. And that can be especially gratifying because it means you can add one more plant to your landscape with little or no worry."