Seed Starting Calculator

In short Almost every sowing instruction is written relative to your last spring frost date: start tomatoes six weeks before it, plant them out two weeks after it, sow peas six weeks before it. So the whole calculation is one subtraction, done once per crop, and the value is entirely in getting the offsets right rather than in the arithmetic. Enter your frost date and this page turns the packet advice for twenty-one common crops into actual dates. THIS PAGE DOES NOT KNOW YOUR FROST DATE AND DELIBERATELY DOES NOT GUESS. Frost dates vary by a fortnight across a single county, and by more between a valley floor and the slope a hundred metres above it. A built-in lookup table would be confidently wrong for a lot of people, and stale for the rest. Your national met service or agricultural extension publishes them, and so do most seed suppliers; the figure usually quoted is the 50% date, which means frost after it is a coin flip rather than an impossibility. There is one trap worth knowing about, and it catches experienced gardeners. "Start onions ten weeks before the last frost" does not mean ten weeks in a pot. Onions go OUT four weeks before the frost date, because they are hardy, so they spend six weeks under lights, not ten. Tomatoes are the opposite: started six weeks before, planted out two weeks after, so eight weeks in a pot rather than six. The two numbers only agree for a crop planted out exactly on the frost date, and almost nothing is. Get this wrong and you either run out of bench space or set out plants that are badly pot-bound. Two limits. Days to maturity on a packet is measured in good conditions, and for anything transplanted it counts from planting out rather than from sowing — a cool summer adds a fortnight, so treat the harvest date here as the earliest plausible one. And if you enter your first autumn frost as well, this page will tell you the last sensible date to start something, which is the honest way to find out that a long-season crop simply does not fit where you live.