Last frost dates are important guides for planting, but aren't always accurate.
Gardeners always know their planting zone, and the local lore around the date it’s safe to plant outside. Around my parts, that’s Mothers Day. In more northern parts of the U.S., it’s Memorial Day. We use those dates to backtrack and decide when to start seeds inside. But these dates, while not arbitrary, are also not written in stone. The weather changes every year, and we’ve been seeing huge weather changes the last few years due to climate change. That's why I’m suggesting you watch the weather, not the calendar, for your summer planting.
Each spring is a gamble for gardeners: when you start seeds and when you risk planting them outside. Start seeds too early, and your plants become large, leggy and rootbound inside. Start seeds too late, and your plants won’t be large enough to produce crops over the short summer season. You’d think if your plants are big enough, you could just plunk them outside in your garden, but that’s where the biggest risk lies. These tender summer plants have temperature requirements, and spring is notorious for surprise frosts. Your plants won’t survive, or at best, will be stunted, if they go through a frost without some serious protection. To hedge against this risk, each planting zone in the U.S. has a last frost date, which is an average date that should be safe to plant outside. But that’s all it is—an average. Some years the frost date is well into weeks of warm weather, and some years, there’s a surprise May snowstorm.
The frost date was never meant to be more than a guide. The real indicators we should be paying attention to are soil temperatures, overnight temperatures and sunshine. The soil, at a depth of eight or so inches, does not rapidly shift temperature based on one day’s weather. Instead, that temperature represents the shift of seasons, and it slowly warms as the ambient temperature does. To sustain summer plantings, the soil needs to be at least 60 degrees Fahrenheit. While plants may survive in temperatures below that, they won’t thrive. You can gauge your temperature with a probe specifically for that, and while there are fancy ones, I prefer a simple, analog long stem thermometer for the task.
While daytime conditions are important, the temperature is always going to dip once the sun goes down. Your summer plants can’t tolerate frosty temps, so you want to wait until the overnight temperature remains steadily above 50 degrees Fahrenheit (although 55 is better). Of course, this will happen in tandem with the soil temperature, but not precisely, and even one night of frost or cold temperatures can be harmful for your plants, so it’s a delicate balance. Any weather reporting site like Wunderground or Weather.com will report on these overnight temperatures.
Lastly, you want to look for the sunshine—this is less objective and more observational. Spring is still the rainy season, and plants benefit from all that free water. But they will still need sunshine to thrive, and so we’re looking for a mix of quality days of sunshine mixed with rain before you plant. Long stretches of overcast days will not produce happy plants, and instead, create conditions for virus and fungus. As rain splashes on the soil, microbes in the soil splash up onto the plants, where those wet conditions are perfect for fungal and viral growth and spread. If you’re really skilled, you’ll choose the last stretch of overcast days to plant everything, so it gets a gentle stretch of time away from the sun to settle into the new digs, and then gets a drink of sunshine so it can start to grow.
There are ways to start the season early by offering your plants protection outside, even if you plant them before conditions are met. There are greenhouses, both permanent and temporary. If you don’t have a greenhouse, you can buy a temporary popup to put over your bed. You can consider coldframes, which are outdoor beds with removable covers. There are insulating products like waterwalls for tomatoes, and there is always Agribond, which you can use to build low tunnels over your beds.
Whenever gardeners talk about seeding and planting and harvesting online, they’re usually talking about a median growing zone for the U.S. There are zones with completely different timetables, like the southeast or really northern states. When considering the advice, it’s smart to remember the local lore around planting and seeding dates, but it’s more important to just look outside. Our planting seasons are becoming more volatile due to climate change, and so we want to get the maximum amount of growing season possible. Rely on the indicators.
