Climate guide

Cool-Season Greens in a Hot Climate: When the Window Opens

6 min read

Every fall, somewhere in Phoenix or Houston or the Central Valley, somebody sows a bed of lettuce because the calendar says September and the internet says fall is for greens. Two weeks later there's nothing. The seed was fine but the soil wasn't. It was 90F, and lettuce seed mostly won't germinate in soil that warm.

The window is real, and in a hot climate it's the best planting of the whole year. It just doesn't arrive on a date.

The short answer: In a hot climate the fall greens window opens when the heat lets go, and that arrives on its own schedule. Wait until soil at 2 inches holds below about 80F, or start seed indoors and set out transplants. In the low desert that means late September into October. Along the Gulf Coast it can be late October or November.

Why lettuce won't come up in September

Lettuce seed has a built-in stop. Above roughly 80F it goes dormant instead of germinating, and the hotter it gets the more completely it shuts down. Spinach does much the same thing. It's a property of the seed, and no amount of watering can circumvent it.

The number that matters is soil temperature, and this is where an otherwise careful gardener can get caught. Soil temperature at planting depth lags behind the air temperature in both directions. In spring the ground is colder than the air feels. In fall, the soil stays warmer, for weeks. A run of pleasant 85F afternoons in Phoenix can be sitting above soil that's still over 90F two inches down. If you've ever sown greens on the first comfortable weekend and gotten nothing, that's usually what happened. (Here's the same problem in summer, and the workarounds.)

Why a planting chart can be weeks off where you live

Most fall planting guides for hot climates give you a month, and that month is an average across an enormous amount of ground. The low desert, the Gulf Coast, the Texas hill country and the California coast all get filed under "hot," and they come out of summer on very different schedules. Two gardens in the same state can be weeks apart.

It moves from year to year, too. A long September heat wave pushes everything back, and an early cool front can pull it forward, which is why the date that worked last fall doesn't always work this one.

And the weather report is measuring the air. The first cool evening tells you the air has turned. The soil is still catching up, and the soil is what your seed reads.

How to tell the window has opened in your own bed

Buy a soil thermometer. They cost about as much as a packet of seed and it will give you an exact time, permanently.

Push it 2 inches down and read it in the late afternoon, when the soil is at its warmest for the day. A dawn reading will be lower, and can mislead you.

• Under about 80F, lettuce, spinach and arugula will come up.
• The tougher ones go in earlier. Chard, collards, kale, mustard, radishes and turnips will germinate in warmer soil than lettuce will tolerate.

If you're within a few degrees, you can hurry the bed along rather than wait for the weather. Water it deeply for several evenings running, then shade it, and the top couple of inches will come down in temperature faster than the air does.

How long the window stays open, and which greens quit first

In a mild-winter climate the cool season doesn't end in winter. It ends in spring, when the heat comes back. And the plants' own biology decides when that is.

Lettuce, spinach and cilantro go first. As the days lengthen and warm, they bolt: a flower stalk shoots up and the leaves turn bitter. Spinach responds to the longer days themselves, so even a cool spring won't hold it for long.

Broccoli, peas, beets and radishes hang on a little longer, tolerating a little more warmth.

Kale, collards, chard and cabbage are the tough ones. They'll keep producing into the warm-up well after the salad greens are done.

Carrots and other roots tend not to bolt in their first season, but they get tougher and less sweet as the soil heats up. It can be easy to miss, because nothing looks wrong from above.

The practical version: sow the short-lived plants at the front of your window. Lettuce and spinach run out of season first, so they want to be in the ground early, and kale and chard can follow behind them. (Succession sowing is what keeps the salad going through it.)

What to do if you can't wait

• Start seed indoors, or in a shaded spot, and transplant. Germination is the heat-sensitive step. A transplant skips it entirely and buys you two or three weeks.
• Pre-irrigate and shade the bed. Several deep waterings plus shade cloth will take a few degrees off the top 2 inches.
• Sow the heat-tolerant lettuce types. Jericho, Nevada and Muir are the names that come up most, and they'll germinate and hold a little further into warmth than the usual varieties.
• Sow in the right order. Chard, collards, kale and mustard will go in before lettuce will. Get them started and come back for the greens.
• Plant a little deeper than you would in spring. Seed at the bottom of a shallow trench is sitting in cooler soil, and the trench holds water where you want it.

Where Verna fits, and where it doesn't

Verna combines climate data for your exact location with what each plant needs, and puts a window on your calendar per plant. So "is it time for leaf lettuce" gets answered for leaf lettuce, in your garden, rather than for greens in general somewhere in the Sun Belt. And it keeps track of where each plant is in its life as the season turns. (The bigger picture of a hot-climate year is here.)

What it doesn't do: it isn't a forecast, so it won't tell you a late heat wave is coming next Thursday, and it doesn't know that your one good bed sits against a west wall that holds the afternoon heat. The thermometer in the ground is still worth owning, and your own read on your own garden still wins. (If you garden somewhere with a real frost, the September guide reads the month from both sides.)

See when the window opens in your garden

Verna maps sowing, transplanting, and harvest timing to your exact spot.