August 10, 2026

Winter Garden Seeds: What You Can Sow Now Depends on How Cold Your Yard Actually Runs

Search "winter garden seeds" and you get seed shops and tidy lists of cold-hardy crops — spinach, kale, garlic, broad beans. All useful. But every list quietly assumes the reader's yard behaves like the regional average, and it almost never does. The real question behind a winter sowing isn't what's hardy in my zone — it's how cold does my particular patch of ground actually get on a still night. That's what decides whether a sowing establishes or stalls.

Zone maps and seed packets give you a regional guess

A hardiness zone is drawn from decades of area-wide temperature data spread across a huge map cell. It's a reasonable starting point for "will this plant survive winter here in general." What it can't tell you is how your yard deviates from that average — and the deviations are large enough to change the answer.

A low corner where cold air settles on a clear, still night can run several degrees colder than the forecast low. A bed against a south-facing wall stays warmer and drier. A hedge or fence line traps frost on one side and shelters the other. Two beds ten metres apart can sit in different microclimates, and a winter seedling in the cold one may quietly fail while its neighbour shrugs off the same night.

So the honest version of "what seeds can I plant in winter" is: the regional list tells you the candidates, and your yard's real cold behaviour tells you which of them actually stand a chance where you'd put them.

Measure how cold your area really runs

Before you buy hardware or gamble a tray of seedlings, you can check the size of the gap. Siltwell's public microclimate checker compares the forecast low to what nearby home weather stations actually recorded over recent nights, and reports how much colder or warmer your area tends to run than the forecast — along with how many stations it found and a confidence level.

A couple of honest points about what that number is. It uses all recent nights, not a hand-picked set of clear ones, so it reflects your area's typical behaviour rather than only the coldest radiative nights. And it's an area indicator built from neighbouring stations — it's not a guaranteed reading for your exact bed or a promise about a single date. What it gives you is a realistic sense of whether your patch runs colder than the forecast suggests, which is exactly the correction a seed packet's regional assumption is missing.

If your area consistently runs a few degrees colder, that's your cue to sow the hardier end of the list, start earlier so plants mature before deep cold, or hold tender successions until the risk passes.

Timing the sowing: establish before the cold, not into it

Most winter and overwintering crops want to be established before real cold arrives — a plant with a root system and a few true leaves handles frost far better than a seed trying to germinate into cold soil. That's why the classic advice is to sow in late summer to early autumn.

• Hardy greens — spinach, mâche, mustard, winter lettuces — germinate slowly in cold soil and want a head start before growth stalls.
• Brassicas — kale, Brussels sprouts — need enough autumn warmth to bulk up before winter parks them.
• Alliums and broad beans — garlic and overwintering onions and beans are sown to root in now and sit through winter, sprinting in spring.

The catch is that "before real cold arrives" is a moving target that depends on when frost actually lands in your yard — which, in a cold pocket, can be noticeably earlier than the regional first-frost date implies. Getting that date wrong by a couple of weeks is the difference between a sowing that establishes and one that gets caught out. We dig into why the standard date so often misleads in this guide on frost dates.

Once you've got a station: frost alerts tuned to your site

If you go further and connect a weather station, Siltwell stops treating your yard like the regional average. On the Pro plan a connected station learns how your site differs from the forecast: it compares your station's readings to Open-Meteo and works out the bias — a colder site reads negative. That learned offset is only trusted after about 14 overlapping days of station-and-forecast data, so it's not instant; it earns your trust over a couple of weeks.

Once it's calibrated, frost alerts use your adjusted low rather than the raw forecast — so when an overnight low is expected to dip to your garden's threshold, you get a warning that says it's been adjusted to your station, in time to throw fleece over tender winter seedlings. On the free tier you can enter a manual temperature offset in Settings yourself, and frost alerts will use that instead; connecting the station is what lets Siltwell learn the offset for you. Connecting a station is a Pro feature ($22/year, with a 14-day free trial).

The honest limit

A microclimate reading and a calibrated station narrow the guesswork; they don't repeal the weather. A learned offset is an average, not a forecast of tomorrow's exact low, and a freak cold snap can still undercut it. The area indicator is built from neighbours, not your exact bed. What all of this buys you is fewer bad guesses — sowing the right crops, at roughly the right time, for a yard you actually understand — instead of trusting a zone map to speak for a corner it's never measured. Keep your fleece handy and your eyes open; let the data stop the obvious mistakes.