Spring is almost here, and with spring comes a number of gardening decisions. One of the biggest of these is “when should I move my plants/seedlings outside?”, and it can be a complex question indeed. Most gardeners know about the “first/last risk of frost dates” offered by many gardening publications, but what do those dates mean and when is it safe to start planting outside? Never fear, we’re here to help take some of the mystery out of it so let’s dig in and learn a bit about understanding frost dates.
What Does “Risk of Frost” Mean?
Risk of frost dates measure the average date on which a given area of the world will have its first or last frost of the season. With spring coming up soon, we’ll focus on the “last risk of frost” date for this article, but the same principles apply for the “first risk of frost” dates in late autumn. The idea is that the frost date is an average, based on climate projections and historical data. They’re not an exact prediction, which is why many almanacs and garden guides will give a range of potential dates with percentage odds of the first or last frost occurring rather than a single definitive date.
Further complicating things, most meteorological services and garden guides/almanacs break frosts down into three main categories, depending on how severe the frost is and how it’s likely to affect your plants:
· Light frosts or light freezes generally involve temperatures at or just below freezing—roughly 29F to 32F/-2C to 0C. These frosts will damage tender plants and new growth, generally killing new seedlings just peeking out of the ground. They may kill larger tender or cold-sensitive plants.
· Moderate frosts or moderate freezes occur when temperatures are between 25F to 28F/-4C to -2C. These temperatures are more serious, damaging most vegetation in temperate climates and killing most seedlings outright. They are likely to kill mature plants which aren’t suited for colder temperatures.
· Severe frosts or severe freezes happen when temperatures are below 24F/-4.5C. Most garden plants will suffer heavy damage or death at these temperatures. Even more robust plants adapted to the temperature may lose foliage or suffer other damage.
Obviously it can get much colder than this but at that point very little is growing in our gardens and starting seeds is effectively impossible. It’s important to understand the temperature “safe zones” for the various plants in your garden and to move plants outside, start seeds, or transplant when the weather is clement.
Frost Dates and Planting Dates
So now that we know a bit more about what frost dates are, how do we make use of them. There are two things to understand here: your local frost dates and planting dates. We recommending consulting a few different online sources to find your last risk of frost date; we’ve linked to some of our favorites here so please use those resources. We also caution you to remember that frost dates are just estimates and it’s a good idea to keep an eye on your local weather forecast just to be safe. We also recommend having a means to cover and protect your plants should an unexpected cold snap occur.
On to the next question: how do we use this information. The key thing is to plant your seeds and transplant your seedlings at the appropriate time. Seed packets will generally have information about when to plant and transplant, and we’ve got a handy guide here. It’s a big topic, but if you follow the guidelines here and start your seeds at the right time, exercise caution when moving them outdoors, and transplant when conditions are right you should be OK.
It’s an exciting time, with spring coming and garden season upon us. For both newcomers and veteran gardeners, it’s also a great time to review some of the fundamentals and plan for a successful garden this year. We hope this brief explanation of frost dates and planting times helps—and as always if you have any questions let us know in the comments or send us an email. We’re here to help!
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