Longmont, CO Planting Dates — Moon Phase Garden

The four dates that matter

Date	When	What it means
Last spring frost (50%)	May 20	Half of all years see a freeze after this date. It is an average, not a guarantee.
Last spring frost (10%)	Jun 3	Only one year in ten freezes after this. Use it for anything you cannot afford to replant.
First fall frost (50%)	Sep 26	Half of all years freeze before this date. Fall crops are counted back from here.
First fall frost (10%)	Sep 12	An early-freeze year. If a crop must finish, aim to harvest before this.

USDA zone 5b 129 frost-free days Modelled estimate

Spring & summer planting in Longmont

Dates are calculated from this location's own last-frost date, not from a generic zone chart. “Best moon days” are the days inside each window that fall in the crop's traditional phase — the same rule the app uses.

Crop	Start seed indoors	Plant outdoors	Best moon days	Days to harvest
Strawberry (crowns) transplant	—	Mar 21 – Apr 29	Apr 1 Full Moon	perennial — first real harvest next season
Garlic cloves/sets Better planted in fall — see below	—	Apr 3 – May 4	Apr 3–8 Waning Gibbous	240–300
Asparagus (crowns) transplant	—	Apr 8 – May 13	Apr 23 First Quarter	perennial — first real harvest 2–3 years
Chives either	Mar 25	Apr 8 – May 13	Apr 18–22 Waxing Crescent	75–90
Onion (from sets/transplants) cloves/sets	—	Apr 8 – May 13	Apr 8, May 2–6 Waning Gibbous	90–120
Peas (English/snap) direct sow	—	Apr 8 – May 13	Apr 23 First Quarter	55–75
Spinach direct sow	—	Apr 8 – May 13	Apr 18–22 Waxing Crescent	35–50
Cabbage transplant	Mar 18	Apr 15 – May 13	Apr 18–22 Waxing Crescent	65–90
Collards either	Apr 8	Apr 15 – Jun 3	Apr 18–22, May 17 Waxing Crescent	55–85
Kale either	Apr 8	Apr 15 – May 20	Apr 18–22, May 17 Waxing Crescent	50–65
Lettuce (leaf) either	Apr 15	Apr 15 – Jun 10	Apr 18–22, May 17 Waxing Crescent	40–55
Potato cloves/sets	—	Apr 15 – May 20	May 1 Full Moon	70–120
Radish direct sow	—	Apr 15 – Jun 10	May 2–7 Waning Gibbous	22–35
Turnip direct sow	—	Apr 15 – May 27	May 2–7 Waning Gibbous	40–60
Broccoli transplant	Mar 18	Apr 20 – May 20	Apr 20–22, May 17–19 Waxing Crescent	55–80
Brussels Sprouts transplant	Mar 18	Apr 20 – May 20	Apr 20–22, May 17–19 Waxing Crescent	90–110
Leek transplant	Mar 11	Apr 20 – May 20	May 2–7 Waning Gibbous	100–120
Lettuce (head) transplant	Apr 1	Apr 20 – May 20	Apr 20–22, May 17–19 Waxing Crescent	60–85
Parsley either	Mar 18	Apr 20 – May 27	Apr 20–22, May 17–19 Waxing Crescent	70–90
Parsnip direct sow	—	Apr 20 – May 27	May 1 Full Moon	100–150
Shallot cloves/sets Better planted in fall — see below	—	Apr 20 – May 20	May 2–7 Waning Gibbous	90–120
Beet direct sow	—	Apr 22 – Jun 10	May 1, May 31 Full Moon	50–65
Kohlrabi either	Apr 8	Apr 22 – May 27	Apr 22, May 17–21 Waxing Crescent	45–60
Carrot direct sow	—	Apr 29 – Jun 10	May 1, May 31 Full Moon	65–85
Cauliflower transplant	Mar 18	Apr 29 – May 20	May 17–20 Waxing Crescent	55–80
Cilantro direct sow	—	Apr 29 – Jun 10	May 17–22 Waxing Crescent	45–70
Swiss Chard direct sow	—	Apr 29 – Jun 19	May 17–22 Waxing Crescent	50–60
Dill direct sow	—	May 6 – Jun 19	May 17–22 Waxing Crescent	40–60
Cucumber either	Apr 29	May 20 – Jun 19	May 23 First Quarter	50–70
Green Beans (bush) direct sow	—	May 20 – Jul 4	May 23, Jun 21 First Quarter	50–65
Oregano transplant	Mar 25	May 20 – Jun 10	May 23 First Quarter	cut as needed
Sage transplant	Mar 25	May 20 – Jun 10	May 23 First Quarter	cut as needed
Sweet Corn direct sow	—	May 20 – Jul 4	May 23, Jun 21 First Quarter	60–100
Thyme transplant	Mar 25	May 20 – Jun 10	May 23 First Quarter	cut as needed
Zucchini / Summer Squash either	Apr 29	May 20 – Jun 19	May 23 First Quarter	45–60
Basil either	Apr 8	May 27 – Jun 19	Jun 15–19 Waxing Crescent	30–60
Dry Beans direct sow	—	May 27 – Jun 24	Jun 21 First Quarter	85–100
Pole Beans direct sow	—	May 27 – Jun 29	Jun 21 First Quarter	60–75
Pumpkin direct sow	Apr 29	May 27 – Jun 24	Jun 21 First Quarter	90–120
Rosemary transplant	May 20	May 27 – Jun 19	— First Quarter	cut as needed
Tomatillo transplant	Apr 8	May 27 – Jun 19	— First Quarter	60–75
Tomato transplant	Apr 8	May 27 – Jun 19	— First Quarter	60–85
Winter Squash direct sow	Apr 29	May 27 – Jun 24	Jun 21 First Quarter	85–110
Cantaloupe / Muskmelon either	Apr 29	Jun 3 – Jun 24	Jun 21 First Quarter	75–90
Eggplant transplant	Mar 25	Jun 3 – Jun 24	Jun 21 First Quarter	60–90
Okra direct sow	Apr 22	Jun 3 – Jun 29	Jun 21 First Quarter	50–70
Pepper transplant	Mar 25	Jun 3 – Jun 24	Jun 21 First Quarter	60–90
Sweet Potato slips	—	Jun 3 – Jun 29	Jun 3–7 Waning Gibbous	90–150
Watermelon either	Apr 29	Jun 3 – Jun 24	Jun 21 First Quarter	70–90

Fall planting in Longmont

Fall dates count backward from the first frost, and they add time back for the “fall factor” — days are shortening and cooling, so a crop grows slower than the seed packet promises. Sowing late is the single most common way a fall garden fails.

Crop	Sow or set out	Best moon days	Days to harvest	Hardiness
Parsley	Jun 6 – Jun 20	Jun 15–20	70–90	semi-hardy
Cabbage	Jun 13 – Jun 25	Jun 15–20	65–90	hardy
Carrot	Jun 13 – Jun 25	—	65–85	semi-hardy
Collards	Jun 13 – Jul 5	Jun 15–20	55–85	hardy
Broccoli	Jun 18 – Jul 5	Jun 18–20	55–80	hardy
Cauliflower	Jun 18 – Jul 5	Jun 18–20	55–80	semi-hardy
Lettuce (head)	Jun 18 – Jun 30	Jun 18–20	60–85	semi-hardy
Peas (English/snap)	Jun 18 – Jul 5	Jun 21	55–75	hardy
Beet	Jun 23 – Jul 10	Jun 29	50–65	semi-hardy
Kale	Jun 23 – Jul 10	—	50–65	hardy
Swiss Chard	Jun 23 – Jul 10	—	50–60	semi-hardy
Zucchini / Summer Squash	Jun 23 – Jul 1	—	45–60	tender
Basil	Jun 28 – Jul 16	Jul 15–16	30–60	very tender
Kohlrabi	Jun 28 – Jul 15	Jul 15	45–60	hardy
Cilantro	Jul 3 – Jul 15	Jul 15	45–70	semi-hardy
Lettuce (leaf)	Jul 3 – Jul 20	Jul 15–20	40–55	semi-hardy
Turnip	Jul 3 – Jul 20	Jul 3–6	40–60	hardy
Spinach	Jul 8 – Jul 25	Jul 15–20	35–50	hardy
Dill	Jul 13 – Jul 20	Jul 15–20	40–60	semi-hardy
Radish	Jul 13 – Aug 7	Jul 30–Aug 4	22–35	hardy
Garlic	Aug 27 – Oct 17	Aug 28–Sep 2	240–300	hardy
Shallot	Sep 10 – Oct 11	Sep 27–Oct 2	90–120	hardy

Bottom line: the 50% frost date is a coin flip, not a green light. If you would be sick about losing a flat of tomatoes, plant to the Jun 3 date instead and keep a row cover handy either way.

Where these numbers come from

There is no long-record weather station inside Longmont itself, so these frost dates are modelled, not observed. They come from a half-degree climate grid built by fitting the freeze dates of 6,808 NOAA stations (1991–2020 Normals) against terrain and the USDA 2023 hardiness raster, then reading the cell your city sits in. Checked against the stations themselves, that model lands within about a week on average — and its zone matches the station's exactly 67% of the time and within one half-zone 97% of the time. A cell covers roughly 35 by 28 miles, so this is a neighborhood answer, not a backyard one. If there is a station-backed location near you, prefer it.

How precise is any of this?

Extension recommendations for the same crop routinely disagree by two to four weeks, and occasionally more, so these offsets should be presented as windows and starting points rather than deadlines. Three things drive the spread. (1) Regional risk tolerance: for warm-season crops, Colorado State and Utah State put tomatoes, peppers and melons out about 14 days after the last frost, while University of Missouri's calendar works out to roughly 25-35 days after the last frost and University of Maryland's window runs from the frost date itself out to 45 days past it. That is a real 6-week spread for the single most popular garden crop, and it exists because a short-season mountain state is optimizing against a short season while a humid continental state is optimizing against cold, wet spring soil. (2) Soil temperature versus calendar date: most Extension services now say the honest trigger is soil temperature at 4 inches, not the frost date, and the frost-relative number is a convenience translation. A cold, wet spring can push the real date two or three weeks past the calendar recommendation. (3) Days-to-maturity is variety-specific and is measured differently for direct-seeded versus transplanted crops - seed catalogs count from transplant date for tomatoes, peppers, eggplant and cole crops, but from sowing for roots and greens - so a 'days to maturity' range of 60-85 for tomato is genuinely 60-85 depending on which packet you bought. Additionally, the average last frost is a 50% probability, meaning half of all years have a frost after that date; Extension services commonly advise waiting 1-2 weeks past it for anything frost-tender, or being prepared to cover. Treat every number here as plus or minus about 10-14 days.

On the station itself: a weather station is one point. Cities hold their heat, valleys pool cold air, and a garden on a north slope can run two weeks behind one a mile away. Treat this page as your starting point and let your own garden correct it — that is exactly what the journal in the app is for.

Nearby locations

Boulder, CO zone 5b Thornton, CO zone 6a Westminster, CO zone 5b Arvada, CO zone 5b Greeley, CO zone 5b Fort Collins, CO zone 5b

Want this on your phone, in your garden? Moon Phase Garden keeps your frost dates, your planting windows and tonight's moon phase in one place, works offline, and lets you override these dates with what your own garden actually does. Free and ad-free.

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