Seed Starting and Transplant Calculator
Tell us your ZIP code and the plants you want to grow. We look up the last spring freeze dates for your ZIP and turn standard extension timing into date windows for starting seeds indoors, hardening seedlings off, transplanting, and sowing outside.
How the dates are worked out
Every window is built from two freeze dates for your ZIP. The typical date is the median last spring freeze at 32 °F: half of years see their last freeze before it and half after. The cautious date is later, with only a 1 in 10 chance of a freeze after it. Indoor start windows run from the typical date minus the longest start time to the cautious date minus the shortest start time, so starting anywhere in the window is reasonable. Planting out and sowing windows run from the typical date to the cautious date, shifted by each plant's timing. Tender plants belong at the cautious end of their window.
About the timing
The weeks for each plant follow published university extension guidance, including seed starting charts and planting calendars from the sources listed below. Timing varies a little by variety, so treat these windows as a planning guide and check your seed packet for variety specific notes. Plants started indoors need strong light from the day they sprout, and seedlings raised inside should always be hardened off, moved outside for a little longer each day, before they are planted out for good.
Plant timing sources: University of Minnesota Extension, Cornell Cooperative Extension, Utah State University Extension, Iowa State University Extension, NC State Extension. Freeze dates: NOAA 1991-2020 Climate Normals (NOAA NCEI). Zone data: PRISM Climate Group, Oregon State University, for the 2023 USDA Plant Hardiness Zone Map.