Do I have wait for Mother's Day?
“When should I plant my tomatoes?” A common answer in central Iowa is “any time after Mother’s Day”. Depending on the year, Mother’s Day in the U.S. can be as early as May 8th or as late as May 14th. While that date lines up well with when our chance of another 32 F frost nears zero (see the risk of frost chart below for Ankeny, Iowa), does that rule of thumb apply everywhere every year?
The best planting time for tomatoes depends on your weather conditions, but here are a few guidelines that can help you decide when to plant regardless of where you live.
The first thing to understand about planting tomatoes (and peppers, eggplants, melons, and squash) is that they are warm weather plants. Tomatoes will not grow in temperatures below 50 F. The first sign that it is warm enough to plant is the night time temperature stays consistently above 50 F. The ten-day weather forecast for my town in central Iowa indicates only 2 of 10 days in the forecast will reach that minimum requirement which means I will wait another week or more to consider planting warm season crops.
Why should I care about the soil temperature?
The second thing to consider is the temperature of the soil about 4" deep. Ideally, tomatoes should be planted when the soil temperature is at least 60 F in the early morning. You can use a soil thermometer, a kitchen thermometer, or just stick your finger in the soil for a minute and see if it feels cold (if it feels uncomfortably cold it is probably below 60 F). So far this year, the reported soil temperatures have only reached about 50 F. Raised beds or a garden with a good southern exposure could be warmer than the reported temperatures.
Why should I care about the weather forecast?
The third thing to consider is the weather forecast. Even if there have been a few warm days and nights and the soil temperature warms to 60 F, a forecast for significant cold weather indicates it may be better to wait to plant until warmer temperatures are in the forecast. This next week looks windy, wet, and cool. I'll be planting lettuce, potatoes, onions, carrots, beets, radishes and other cool season crops, but leaving the tomatoes and peppers in the greenhouses. They wouldn't like the cooler nights.
What about this year?
So where does my garden in central Iowa stand on these 3 considerations? Our night time low temperatures this past week have been in the 40's or warmer most nights with one frost a couple nights ago. The soil temperature in my west-facing raised garden beds this morning has warmed, but the night time lows in the 10-day forecast has a mix of 40's and 50's. All of that suggests I should wait another week or so, which puts us pretty close to….Mother’s Day.
What about Peppers, Eggplants, Squash and Melons?
If you are like us, tomatoes are not the only type of plant you like to grow. We also grow peppers, eggplant, squash, and melons. Add 10 degrees to all of the tomato temperatures above when deciding when to plant peppers, eggplant, squash, and melons. Cool season plants like peas, potatoes, onions, celery, kale, carrots, broccoli, and cabbage can be planted now if not already done.
If you haven’t purchased flower seeds, tomato, pepper, eggplant, greens, potatoes, onions, strawberries, herbs, or squash plants yet, check out our plant sale page at https://www.iowabackyardfarmer.org/springplantsale.html