Frost-Free Growing Season Extension

The Benefit of Covered Space

Greenhouses, High Tunnels, and Climate Batteries

A significant benefit to a greenhouse is the ability to extend your frost or freeze-free season. A greenhouse allows you to start plants earlier than you otherwise would, and it extends the growing season past the normal first frost or freeze. However, it does so at the cost of some sort of heating, usually from fossil fuels like propane or natural gas.

High tunnels or hoop houses typically refer to an unheated structure, and they still offer some slight protection from the temperature extremes of outside weather and don’t carry the expense of a fossil fuel heater. However, this lack of heating usually only buys you a couple weeks at the book ends of the growing season unless other means of protection are used, like row covers. Growers in a high tunnel can be caught off-guard by a late spring freeze after taking a chance on an early planting of tomatoes.

Climate Battery Greenhouse - Best of both worlds?

Our use of the climate battery greenhouses on our home farm tries to provide the benefits of both worlds: the lower cost production of the high tunnel due to a reduced or eliminated need for a fossil fuel heater with the ability to regulate temperature found in a greenhouse. But how does it perform in the real world? How long of a growing season is possible?

Our Normal Growing Season: ~170+ frost-free days

Our home farm is in South Central Pennsylvania, Zone 6b/7a. Our frost-free growing season typically runs early May through about mid October. PlantMaps.com agrees: they put our last frost around the end of April and our first frost around the end of October. Conservatively, that gives us a frost-free period of 170+ days for outdoor growing. That certainly doesn’t tell the whole story (first hard freeze, average temperatures across the growing season, growing degree days, soil temperatures, etc), but it does give us a starting point.

Our Climate Battery Greenhouse Growing Season - ~270 days

We’ve been keeping records on our climate battery greenhouse performance since 2017 to 1) gather data for ourselves (and others) and 2) determine how to best run our two greenhouses. Below are some approximations of when we encountered our last and first freezes in the greenhouses since we often didn’t record the precise date of the cold snaps.

2018 - Last week of March - First week of December - Approximately 250 days
2019 - Second week of March - First week of December - Approximately 270 days
2020 - Third week of February - Second week of December - Approximately 300 days

As an average, the climate battery greenhouse growing season is around 273 days long, a full 103 days above and beyond our normal growing season. Importantly, much of that time comes at the beginning of the season when the sun is strong. Of additional significance is that over our years of managing these houses we’ve gotten a better feel for how to operate them, perhaps accounting for at least some of gradual increase in the number of frost-free days.

Interestingly enough at the time of writing, I had just checked out in the greenhouse and our ginger plants are still green, meaning the minor cold that the greenhouse has experience (despite 7°F outside temperatures in early December) haven’t been enough to kill it back to the ground.

Rendering a Better Picture

Length of growing season is just one piece of the picture, though an important one.

Our tomatoes, for instance, really benefit from the increase in soil temperature early in the season due to climate battery operation, but then slow down significantly in the fall as the short days and weather that turns more cloudy. Without supplemental lighting, they slow down or just stop growing. Therefore, any additions to the growing season matters most in the early season when the sun is strong.

With other crops like greens, we’re able to more readily overwinter them without the use of row covers while a bump in soil and air temperatures keep them growing in the reduced sunlight environment of the winter season. The greenhouse is such a pleasant place to be and grow in the winter!

What are you growing or hoping to grow in your greenhouses? Could an extended growing season help your farm? If you’re considering a climate battery greenhouse, we’d love to help you out!