Most people treat late summer as the garden winding down — tomatoes slowing, weeds winning, and a quiet plan to give up around September. That's a missed season. Late summer is secretly the year's second planting window, and the crops you put in now often taste better than the spring ones, because they mature into cool nights instead of summer heat. The trick is knowing what to sow, and when to stop.
First, find the one date that matters
Fall gardening runs on a single number: your average first frost date. Everything counts backward from it. Take a crop's "days to maturity" from the seed packet, add two weeks for the harvest you want to keep picking, add another two weeks (a "fall factor," because growth slows as the days shorten), and add two more if frost kills the crop outright. Count that many days back from your frost date — that's your last sensible sowing day for that crop. Penn State Extension's cutoffs for Pennsylvania give you a feel for it: kale by August 15; leaf lettuce, spinach and early broccoli by August 30; radishes by September 10. Miss the window and the plant simply runs out of daylight before it finishes.
The good news: most fall crops are cool-season plants that actually shrug off a light frost — kale and spinach carry on right through one, and it's a stretch of steady cold, not a single frosty night, that improves their flavor — so the deadline is more forgiving than it sounds.
Waxing moon: sow the greens
As the moon fills toward full, follow the same cue as spring — start what grows above the ground. This is the window for a fast fall salad garden: spinach, leaf lettuce, arugula, Asian greens, Swiss chard, and another round of kale. They germinate happily in warm late-summer soil and hit their stride as the weather cools.
Full moon: gather and take stock
The full moon is the traditional peak for harvesting and gathering. Bring in the last of the summer tomatoes and peppers, cut and dry herbs, and — this is the season for it — save seeds from your best-performing plants for next year. Walk the beds and note what thrived and what didn't while it's fresh.
Waning moon: roots, garlic, and soil
After the full moon the rhythm turns downward — toward roots, soil, and cleanup, which is most of what fall prep actually is. Sow your root crops now: radishes, turnips, and beets. Clear out the spent, tired summer plants before they harbor pests over winter, and turn the compost.
Two waning-moon jobs pay off big months from now. First, garlic — a bulb, so a "grows-down" crop — goes in during the fall for a fat summer harvest; wait until a few weeks before your ground freezes so it roots without sprouting. Second, if a bed is done for the year, sow a cover crop like winter rye or crimson clover instead of leaving bare soil. It blankets the bed against erosion, smothers weeds, and — in the case of clover — pulls nitrogen down into the ground for free, so spring's soil is richer than fall's.
Working out your own dates: the planting calendar gives your town's frost dates and fall windows, and what you can still plant this fall shows how to count back from them.