Highlands Ranch, CO Planting Dates

The four dates that matter

Date	When	What it means
Last spring frost (50%)	May 5	Half of all years see a freeze after this date. It is an average, not a guarantee.
Last spring frost (10%)	May 18	Only one year in ten freezes after this. Use it for anything you cannot afford to replant.
First fall frost (50%)	Oct 8	Half of all years freeze before this date. Fall crops are counted back from here.
First fall frost (10%)	Sep 25	An early-freeze year. If a crop must finish, aim to harvest before this.

USDA zone 6a 156 frost-free days Modelled estimate

Spring & summer planting in Highlands Ranch

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 6 – Apr 14	Apr 1 Full Moon	perennial — first real harvest next season
Garlic cloves/sets Better planted in fall — see below	—	Mar 19 – Apr 19	Apr 2–7 Waning Gibbous	240–300
Asparagus (crowns) transplant	—	Mar 24 – Apr 28	Mar 25, Apr 23 First Quarter	perennial — first real harvest 2–3 years
Chives either	Mar 10	Mar 24 – Apr 28	Mar 24, Apr 18–22 Waxing Crescent	75–90
Onion (from sets/transplants) cloves/sets	—	Mar 24 – Apr 28	Apr 2–7 Waning Gibbous	90–120
Peas (English/snap) direct sow	—	Mar 24 – Apr 28	Mar 25, Apr 23 First Quarter	55–75
Spinach direct sow	—	Mar 24 – Apr 28	Mar 24, Apr 18–22 Waxing Crescent	35–50
Cabbage transplant	Mar 3	Mar 31 – Apr 28	Apr 18–22 Waxing Crescent	65–90
Collards either	Mar 24	Mar 31 – May 19	Apr 18–22, May 17 Waxing Crescent	55–85
Kale either	Mar 24	Mar 31 – May 5	Apr 18–22 Waxing Crescent	50–65
Lettuce (leaf) either	Mar 31	Mar 31 – May 26	Apr 18–22, May 17 Waxing Crescent	40–55
Potato cloves/sets	—	Mar 31 – May 5	Apr 1, May 1 Full Moon	70–120
Radish direct sow	—	Mar 31 – May 26	Apr 2–7 Waning Gibbous	22–35
Turnip direct sow	—	Mar 31 – May 12	Apr 2–7 Waning Gibbous	40–60
Broccoli transplant	Mar 3	Apr 5 – May 5	Apr 18–22 Waxing Crescent	55–80
Brussels Sprouts transplant	Mar 3	Apr 5 – May 5	Apr 18–22 Waxing Crescent	90–110
Leek transplant	Feb 24	Apr 5 – May 5	Apr 5–8, May 2–3 Waning Gibbous	100–120
Lettuce (head) transplant	Mar 17	Apr 5 – May 5	Apr 18–22 Waxing Crescent	60–85
Parsley either	Mar 3	Apr 5 – May 12	Apr 18–22 Waxing Crescent	70–90
Parsnip direct sow	—	Apr 5 – May 12	May 1 Full Moon	100–150
Shallot cloves/sets Better planted in fall — see below	—	Apr 5 – May 5	Apr 5–8, May 2–3 Waning Gibbous	90–120
Beet direct sow	—	Apr 7 – May 26	May 1 Full Moon	50–65
Kohlrabi either	Mar 24	Apr 7 – May 12	Apr 18–22 Waxing Crescent	45–60
Carrot direct sow	—	Apr 14 – May 26	May 1 Full Moon	65–85
Cauliflower transplant	Mar 3	Apr 14 – May 5	Apr 18–22 Waxing Crescent	55–80
Cilantro direct sow	—	Apr 14 – May 26	Apr 18–22, May 17 Waxing Crescent	45–70
Swiss Chard direct sow	—	Apr 14 – Jun 4	Apr 18–22, May 17 Waxing Crescent	50–60
Dill direct sow	—	Apr 21 – Jun 4	Apr 21–22, May 17–20 Waxing Crescent	40–60
Cucumber either	Apr 14	May 5 – Jun 4	May 23 First Quarter	50–70
Green Beans (bush) direct sow	—	May 5 – Jun 19	May 23 First Quarter	50–65
Oregano transplant	Mar 10	May 5 – May 26	May 23 First Quarter	cut as needed
Sage transplant	Mar 10	May 5 – May 26	May 23 First Quarter	cut as needed
Sweet Corn direct sow	—	May 5 – Jun 19	May 23 First Quarter	60–100
Thyme transplant	Mar 10	May 5 – May 26	May 23 First Quarter	cut as needed
Zucchini / Summer Squash either	Apr 14	May 5 – Jun 4	May 23 First Quarter	45–60
Basil either	Mar 24	May 12 – Jun 4	May 17–22 Waxing Crescent	30–60
Dry Beans direct sow	—	May 12 – Jun 9	May 23 First Quarter	85–100
Pole Beans direct sow	—	May 12 – Jun 14	May 23 First Quarter	60–75
Pumpkin direct sow	Apr 14	May 12 – Jun 9	May 23 First Quarter	90–120
Rosemary transplant	May 5	May 12 – Jun 4	May 23 First Quarter	cut as needed
Tomatillo transplant	Mar 24	May 12 – Jun 4	May 23 First Quarter	60–75
Tomato transplant	Mar 24	May 12 – Jun 4	May 23 First Quarter	60–85
Winter Squash direct sow	Apr 14	May 12 – Jun 9	May 23 First Quarter	85–110
Cantaloupe / Muskmelon either	Apr 14	May 19 – Jun 9	May 23 First Quarter	75–90
Eggplant transplant	Mar 10	May 19 – Jun 9	May 23 First Quarter	60–90
Okra direct sow	Apr 7	May 19 – Jun 14	May 23 First Quarter	50–70
Pepper transplant	Mar 10	May 19 – Jun 9	May 23 First Quarter	60–90
Sweet Potato slips	—	May 19 – Jun 14	Jun 1–6 Waning Gibbous	90–150
Watermelon either	Apr 14	May 19 – Jun 9	May 23 First Quarter	70–90

Fall planting in Highlands Ranch

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
Brussels Sprouts	May 26 – Jun 12	—	90–110	hardy
Potato	May 31 – Jul 2	May 31, Jun 29	70–120	semi-hardy
Pole Beans	Jun 15 – Jun 28	Jun 21	60–75	tender
Parsley	Jun 18 – Jul 2	Jun 18–20	70–90	semi-hardy
Okra	Jun 20 – Jul 20	Jun 21	50–70	very tender
Cabbage	Jun 25 – Jul 7	—	65–90	hardy
Carrot	Jun 25 – Jul 7	Jun 29	65–85	semi-hardy
Collards	Jun 25 – Jul 17	Jul 15–17	55–85	hardy
Cucumber	Jun 25 – Jul 8	—	50–70	tender
Broccoli	Jun 30 – Jul 17	Jul 15–17	55–80	hardy
Cauliflower	Jun 30 – Jul 17	Jul 15–17	55–80	semi-hardy
Green Beans (bush)	Jun 30 – Jul 8	—	50–65	tender
Lettuce (head)	Jun 30 – Jul 12	—	60–85	semi-hardy
Peas (English/snap)	Jun 30 – Jul 17	—	55–75	hardy
Beet	Jul 5 – Jul 22	—	50–65	semi-hardy
Kale	Jul 5 – Jul 22	Jul 15–20	50–65	hardy
Swiss Chard	Jul 5 – Jul 22	Jul 15–20	50–60	semi-hardy
Zucchini / Summer Squash	Jul 5 – Jul 13	—	45–60	tender
Basil	Jul 10 – Jul 28	Jul 15–20	30–60	very tender
Kohlrabi	Jul 10 – Jul 27	Jul 15–20	45–60	hardy
Cilantro	Jul 15 – Jul 27	Jul 15–20	45–70	semi-hardy
Lettuce (leaf)	Jul 15 – Aug 1	Jul 15–20	40–55	semi-hardy
Turnip	Jul 15 – Aug 1	Jul 30–Aug 1	40–60	hardy
Spinach	Jul 20 – Aug 6	Jul 20	35–50	hardy
Dill	Jul 25 – Aug 1	—	40–60	semi-hardy
Radish	Jul 25 – Aug 19	Jul 30–Aug 4	22–35	hardy
Garlic	Sep 8 – Oct 29	Sep 27–Oct 2	240–300	hardy
Shallot	Sep 22 – Oct 23	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 May 18 date instead and keep a row cover handy either way.

Where these numbers come from

There is no long-record weather station inside Highlands Ranch 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

Centennial, CO zone 6a Lakewood, CO zone 5b Aurora, CO zone 6a Arvada, CO zone 5b Westminster, CO zone 5b Thornton, CO zone 6a

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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