Greenhouse Planting Schedule vs. Garden Planting Schedule: What's Different and Why It Matters

One of the questions we get from new greenhouse owners, especially folks who are coming from years of outdoor gardening, is some version of: "When do I plant?"

And the answer is — it depends, but probably a lot earlier than you think.

If you've spent years timing everything around your last frost date, a greenhouse changes the rules. You're not waiting on the weather anymore, which is an especially good thing since it’s so unpredictable, like that surprise early cold snap that might wipe out your crop! You're deciding when things happen, and that shift in mindset is one of the best parts of having a greenhouse or high tunnel.

Here's how a greenhouse planting schedule actually works, and how it's different from what you've been doing outside.

Starting seeds in flats inside your greenhouse lets you get a jump on the season weeks before outdoor gardeners can.

How a Greenhouse Planting Schedule Frees You From Frost Dates

Outdoor gardening is basically a negotiation with your local weather. Everything you plant gets timed around two dates — your average last spring frost and your average first fall frost. Depending on where you are in the country, that outdoor window might be 120 days or 200 days — but either way, it's fixed, and you're working around it rather than controlling it. And I’m not sure “fixed” is the right word actually, since it seems like temperatures aren’t quite as predictable as they once were (at least in my region).

A greenhouse or high tunnel changes that equation significantly. You still want to be aware of outside temperatures, especially if your structure isn't heated, but you're no longer held hostage by them. A hard frost outside doesn't automatically mean your growing season is over or hasn't started yet.

For most of our customers, a high tunnel or greenhouse effectively adds 4 to 8 weeks on each end of the season — sometimes more, depending on what you're growing, where you are located, and whether you're supplementing heat.

Starting Seeds Earlier Than You'd Think

Here's where it gets exciting for people who love crops like tomatoes and peppers. In an outdoor garden, you'd typically start tomato seeds indoors 6 to 8 weeks before your last frost date, then transplant outside once nighttime temps are reliably above 50°F.

In a greenhouse, you can start those same seeds earlier — sometimes weeks earlier than your outdoor last frost date would ever allow — because your transplants aren't going outside at all. They're staying in the structure, where you control the environment. We successfully grew tomatoes in our high tunnels a full month before they would have ever been safe to put a plant in the ground outside, and several of our customers have reported the same.

For cool-season crops like lettuce, spinach, kale, and radishes, a greenhouse lets you start planting well before outdoor gardeners would dream of it too, even in colder climates where the ground is still frozen.

Thinking in Successions, Not Just Seasons

Outdoor gardeners often plant once and harvest once. A greenhouse opens the door to succession planting, which just means staggering your planting dates, so you've always got something coming in rather than everything ripening at the same time.

Because you have more control over the environment, you can plant a new row of lettuce or radishes every few weeks and keep a steady supply coming rather than having a huge glut all at once. This takes a little planning up front, but once you get the rhythm of it, it's not difficult, and the more consistent crop yield is so worth it! I have customers who continue to grow cold weather crop in their high tunnels in winter without even adding heat!  The protection of the structure along with some frost cloth on the worst nights allows for them to continue harvesting all winter long.

Fall Planting Looks Different Too

Outdoor gardeners race the fall frost. Greenhouse growers have a few more weeks — and a bit more breathing room — to get fall crops in the ground. Cool-season crops that you'd be reluctant to start outdoors in August (because they wouldn't have enough time before frost) are perfectly reasonable greenhouse plantings in late summer.

This is also when a lot of growers start prepping for a winter harvest. If you want to keep pulling greens through December and January, the seeds need to go in during September and early October — before day length shortens too much for significant new growth. Once days get short, plants slow way down, but they'll hold, and you can harvest slowly from mature plants all winter even without supplemental lighting.

Cool-season crops planted in late summer can carry you well into winter inside a high tunnel.

What Doesn't Change

Even though a greenhouse frees you from frost dates, a few things still remain important to pay attention to:

Day length

Plants don't know what temperature it is, but they respond strongly to day length. Short days in winter will slow growth significantly, no matter how warm your structure is. You can add grow lights to your structure to increase the time, but if you don’t want to do this, just plan your fall plantings early enough that crops are close to maturity before mid-November, when day length drops off.

Soil temperature

Seeds germinate based on soil temperature, not air temperature. A warm greenhouse with cold soil will give you poor germination. If you're starting early in a cold structure, warming the soil first matters more than warming the air.

Variety selection

Some varieties are specifically bred for greenhouse production and handle the different light and humidity conditions better than field varieties. This is especially true for tomatoes and cucumbers. Descriptions on the packages or in the store will usually let you know if they are geared more towards one or the other.

The Short Version

If you're a seasoned outdoor gardener stepping into a greenhouse for the first time, the main thing to internalize is this: stop looking at the calendar the same way you used to. Trust me, I know it takes some getting used to, but although your frost dates are still useful as reference points, they no longer must control your growing season. With your greenhouse or high tunnel, you’re in control!

Start seeds earlier than feels comfortable. Plant cool-season crops before outdoor gardeners would dream of it. Push your fall season further than you ever could outside. That's what the structure is for.

If you're not sure where to start or want to talk through what a planting schedule might look like for your specific setup — whether that's a Four Season Kit, an All Inclusive Package, or one of our high tunnel options — give us a call. We're happy to walk through it with you.

Happy growing!