Follow the Weather, Not the Calendar, to Determine Planting Time
Gardeners always know their planting zone and local lore for when it is safe to plant outdoors. In my area today is Mother’s Day. In more northern parts of the US it is Memorial Day. We use these dates to go back and decide when to start the seeds inside. But these dates, while not arbitrary, are not set in stone. The weather changes every year and in the last few years we have seen huge changes in the weather due to climate change. This is why I suggest you watch the weather rather than the calendar when planting in the summer.
Last frost dates are estimates.
Every spring is a gamble for gardeners: when you sow your seeds and when you venture to plant them outdoors. If you start sowing seeds too early, your plants will become large, leggy, and with roots in them. Start your seeds too late and your plants won’t be large enough to produce a crop during the short summer season. You might think that if your plants are large enough you can just plant them in the garden, but this is where the biggest risk lies. These tender summer plants are picky about temperature, and spring is famous for unexpected frosts. Your plants will not survive, or at best will experience stunted growth, if they survive the frost without serious protection. To hedge against this risk, each planting zone in the United States has a last frost date , which is the average date when plants can be safely planted outdoors. But that’s all there is – an average. Some years the frost coincides with weeks of warm weather, and some years there is an unexpected snowstorm in May.
Soil temperature is the most important indicator
The frost date was never more than a guideline. The real indicators we need to look at are soil temperature, night temperature and sunlight. Soil about eight inches deep does not change temperature quickly with the weather in one day. Instead, this temperature reflects the changing seasons, and it warms slowly, just like the ambient temperature. To support summer planting, the soil temperature must be at least 60 degrees Fahrenheit. Although plants can survive at temperatures below this, they will not thrive. You can measure the temperature with a special probe, and while there are fancy models out there, I prefer a simple long-stem analog thermometer for this task.
Night temperatures should be above 50 degrees.
While daytime conditions are important, temperatures will always drop after sunset. Your summer plants won’t tolerate freezing temperatures, so you’ll need to wait until nighttime temperatures are consistently above 50 degrees Fahrenheit (though 55 is better). Of course, this will happen in tandem with soil temperature, but not exactly, and even one night of frost or cold temperatures can be harmful to your plants, so it’s a delicate balance. Any weather website such as Wunderground or Weather.com will report overnight temperatures.
Look for the sunshine
Finally, you need to look for sunlight – 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 sunlight to grow, so we look for a combination of quality sunshine and rain before planting. Long cloudy days will not produce happy plants, but will instead create conditions for viruses and fungi to spread. When rain falls on the soil, microbes from the soil are transferred to the plants, where the wet conditions are ideal for the growth and spread of fungi and viruses. If you’re really experienced, you’ll choose the last period of cloudy days to plant everything , so it gets a short period of time out of the sun to settle into its new digs, and then gets a breath of sunshine so it can start growing.
Extend the season
There are ways to start the season early by offering protection to your outdoor plants, even if you plant them before the conditions are met. There are greenhouses, both permanent and temporary. If you don’t have a greenhouse, you can buy a temporary greenhouse and place it above your bed. You might want to consider coldframes—outdoor beds with removable covers. There are insulating materials such as tomato water walls and there is always Agribond which can be used to build low tunnels over beds.
Whenever gardeners talk about sowing, planting and harvesting online, they are usually talking about the average growing area in the United States. There are areas with completely different schedules, such as the southeastern or even northern states. With this advice in mind, it’s wise to remember local lore about planting and sowing dates, but it’s more important to just look from the outside. Our growing seasons are becoming increasingly unstable due to climate change and so we want to get the longest growing season possible. Rely on indicators.