A couple of weeks ago I started seeding directly into the field and despite the fact that we’ve gotten pretty nice consistent rain and I have an irrigation system in place, I’m getting very poor germination. This season I’ve been pushing to start a little bit earlier than usual and the difference in soil temperature seems to be a problem. Most gardeners in the US have to deal with the issue of the ground being too cold to plant seeds early, but here in South Florida, planting too early means the sun is still warming our soils to temperatures high enough to affect germination of most seeds.
The ideal soil temperature for most annual vegetables is generally between 60 and 80 degrees F. During the past couple of weeks I’m almost certain the soil temperature in my fields was in the 90′s on sunny days. Today I finally decided to take a more active role in monitoring soil temperature and micro managing soil moisture. I’ve ordered a soil thermometer and printed a few germination charts I found on the internet. One in particular, found on a rather obscure website called Tom Clothier’s Garden Walk and Talk, seems very helpful because it gives the germination percentage of different seeds at various temperatures. With this chart I can find the percentage of seeds that should be germinating at the actual temperature of my soil. The scary thing is that besides heat loving crops like cucumbers, tomatoes and peppers, most other vegetable seeds, especially the brassicas, have 0-35 percent germination at 95 degrees! I’ll be able to use this chart to choose which crops are worth pushing early and which are simply a waste of time and seeds.