What to Grow in a Greenhouse in Late Summer and Fall

Last week we talked about why it's not too late to start a fall garden if you have a greenhouse or high tunnel. This week let's get into the specifics — because "cool-season crops" covers a lot of ground, and knowing which ones to prioritize right now, and which ones to plan for next, makes a big difference in how productive your fall actually turns out.

If you're in mid-to-late July, you're in a sweet spot. You still have enough warm days to get plants established before growth slows down, and you've got several weeks before day length drops off enough to affect new plantings. Here's how I'd think about it, crop by crop.

Brassicas like cabbage and collards thrive in the cooler temperatures that fall brings.

What to Grow in a Greenhouse Now: The Fast Growers

These are your best bets for getting something in the ground this week and actually harvesting it before conditions get too cold.

Lettuce is one of the easiest and most rewarding fall crops. It actually prefers the cooler temperatures that fall brings — summer heat is what makes lettuce bolt and turn bitter, so a late July or August planting often produces far better heads than a spring planting that ran into summer heat too quickly. Loose-leaf varieties mature faster than heads, so if you're planting late, start with those.

Radishes are ready in as little as 25 days — perfect for keeping something coming in while slower crops develop.

Radishes are about as close to instant gratification as vegetable gardening gets. Most varieties are ready in 25 to 30 days, which means you can succession plant these every few weeks well into fall and keep pulling harvests. They're also great as a companion planting between slower crops, filling in space while other things develop.

Spinach is one of my favorite fall and winter crops in a high tunnel. It's genuinely cold-hardy, handles frost beautifully, and a planting started now can carry you through repeated cuttings well into early spring. If I had to pick one crop to prioritize right now, spinach would be at the top of the list.

Arugula grows fast, tolerates cold well, and has a flavor that actually improves slightly in cooler weather. It's ready to cut in about 30 to 40 days and keeps coming back after cutting, making it a great succession crop for fall.

Asian greens — things like bok choy, tatsoi, and mizuna — are underrated fall performers. They handle the temperature swings of fall better than most crops, grow quickly, and fill out a CSA box or farmers market display beautifully.

The Medium-Term Crops: Get These In Soon

These take a little longer to mature but are absolutely worth planting now if you move quickly.

Kale is one of the most reliable fall and winter crops you can grow. It's cold-hardy enough to handle temperatures well below freezing in a protected structure, and the flavor actually sweetens after a frost. Start seeds now and you'll have harvests running from fall through winter.

Kale is one of the most reliable fall and winter crops you can grow — and the flavor gets even better after a frost.

Swiss chard is a workhorse. It bridges the gap between your summer crops winding down and your cool-season crops hitting their stride, and it keeps producing over a long harvest window. It handles both heat and cold better than almost anything else in the garden.

Broccoli and cauliflower need the most lead time of any of the fall crops, so if you want to harvest these before winter, starting transplants now is the right move. These are crops that benefit enormously from the temperature protection a greenhouse provides — the cool nights that would stress an outdoor planting just make brassicas sweeter and more productive inside a tunnel.

Carrots need to be in the ground by early August if you want them properly sized up by winter. They develop slowly, but a fall-harvested carrot that's been sweetened by cool temperatures is hard to beat. Get seeds in soon and be patient.

The Transition Plan: What's Coming Out and What's Going In

This is also the time to start thinking about your transition from summer crops to fall crops. As your tomatoes, peppers, and cucumbers start slowing down — or as you pull spent plants — that's valuable space that can go right into fall greens. Don't let it sit empty.

A simple rotation: as a summer plant comes out, direct seed spinach, lettuce, or arugula in that spot the same week. By the time you're fully done with summer harvesting, your fall crops will already be establishing.

What to Save for September

Not everything needs to go in right now. Garlic, for example, is one of the most rewarding things you can grow in a greenhouse or high tunnel, but it wants to go in the ground in September and October. Same with certain cover crops that double as winter green manures. Planning your September planting now means you'll have the space ready when the timing is right.

A Note on Variety Selection

One thing worth mentioning: not all varieties of these crops are created equal for fall planting. Look for varieties specifically noted as cold-tolerant or recommended for fall production. Seed packets and catalogs usually flag these, and it makes a real difference in how well your crops hold up as temperatures drop.

If you've been growing the same varieties for years and struggling with fall performance, switching to a cold-tolerant variety of the same crop is sometimes all it takes.

The Big Picture

Late summer and fall greenhouse growing is genuinely one of the most satisfying stretches of the year. The intense heat of summer is backing off, your crops are happy in the cooler air, and you're harvesting when most outdoor gardeners have already called it a season. That's the payoff for having a structure.

If you're not set up with a greenhouse or high tunnel yet and this is making you wish you were, give us a call. Our kits are manufactured to order, and ordering now gets you set up in time to take full advantage of next year's fall season — or even the tail end of this one, depending on your timeline.

Happy growing!