BLOG: Crazy Weather—When Can I Plant?

I almost missed the frost advisory for Thursday night. After our unseasonably warm winter and March, it seems odd to worry about cold nights. Over the past several warm weekends I’ve bought seedlings and a few shrubs—I'm ready to plant! The frost though warns me not to be too eager—it can still get cold enough to kill fragile plants (and then I’ve lost the money I’ve spent on them).

With the recent wide swings in temperatures, how do we know when is the right time to put those plants in the ground? We can use our instinct and experience as well as scientific data.

One source for data is the USDA Plant Hardiness Zone Map, generally considered the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. Knowing your zone, you can find the average last frost date for your area. For most of Harford County, that falls between April 11-20.

The USDA map was recently updated. The previous map reflected 1990 data. Interestingly, the new 2012 map shows that portions of Harford County are now rated as a warmer zone than in 1990. Checking zip codes, the central area around Bel Air used to be a zone 6b (lowest winter temperatures of -5 to 0F). Now those same zip codes are zone 7a (lowest winter temperatures of 0 to 5F)

For our readers in southern PA, the towns closest to the Maryland border are also warmer. Some zip codes that were zone 6a (lowest winter temperatures of -10 to -5F) are now rated 6b (-5 to 0F).

So, though it may be getting warmer, check the data and make your decision when it’s best for you to plant. As for me, I’ll keep protecting my seedlings and wait til mid-April to really get my garden going.

Do you have plants you're waiting to put in the ground?