Thornton, CO Vegetable Planting Calendar

What to plant now in Thornton

In October:

• Plant garlic cloves (Oct 1–14)

The list above is for the month this page was built; the box is refreshed in your browser for today's date.

Spring planting dates

“Plant out” means transplanting for crops started indoors and sowing or planting directly in the garden for the others. Each column applies CSU Extension's timing rule to a different last-frost anchor: aggressive = typical date (May 2, 50% risk), balanced = May 7 (30%), safe = May 14 (10%). For hardy crops the cell shows the CSU “as early as” window.

Before sowing, check the soil too. CSU recommends taking its temperature at 4 inches (6 inches for beans). Since NOAA publishes no soil-temperature normals, we give CSU's minimum soil temperature for each crop rather than a date.

Fall planting dates

Latest dates for a fall harvest, using the University of Nebraska–Lincoln Extension count-back formula: days to harvest (CSU) + 14-day fall factor + 14-day frost-tender factor for frost-sensitive crops, counted back from the first-frost date. For transplanted crops (CSU days are from transplant) the result is the latest transplant date; seeds go in CSU's transplant-age weeks earlier. We add no harvest period, so these dates give you a first harvest by the first frost, not a long harvest.

Month-by-month: what to plant in Thornton

What to plant in Thornton in February

• Start broccoli seeds indoors (Feb 19 – Mar 5)
• Start cabbage seeds indoors (Feb 19 – Mar 5)

What to plant in Thornton in March

• Start cauliflower seeds indoors (Mar 5–19)
• Start eggplant seeds indoors (Mar 19 – Apr 9)
• Start pepper seeds indoors (Mar 26 – Apr 9)
• Start broccoli seeds indoors (Feb 19 – Mar 5)
• Start cabbage seeds indoors (Feb 19 – Mar 5)

What to plant in Thornton in April

• Onions (dry, from sets): plant sets outdoors (Apr 4–30)
• Peas: direct sow outdoors (Apr 4–30)
• Lettuce (leaf): direct sow outdoors (Apr 4–30)
• Broccoli: transplant outdoors (Apr 4–30)
• Cabbage: transplant outdoors (Apr 4–30)
• Kohlrabi: direct sow outdoors (Apr 4–30)
• Green onions: direct sow outdoors (Apr 4–30)
• Radishes: direct sow outdoors (Apr 4–30)
• Spinach: direct sow outdoors (Apr 4–30)
• Turnips: direct sow outdoors (Apr 4–30)
• Potatoes: plant (seed pieces) outdoors (Apr 18 – May 14)
• Carrots: direct sow outdoors (Apr 18 – May 14)
• Cauliflower: transplant outdoors (Apr 18 – May 14)
• Beets: direct sow outdoors (Apr 18 – May 14)
• Swiss chard: direct sow outdoors (Apr 18 – May 14)
• Parsnips: direct sow outdoors (Apr 18 – May 14)
• Start tomato seeds indoors (Apr 22 – May 6)
• Start eggplant seeds indoors (Mar 19 – Apr 9)
• Start pepper seeds indoors (Mar 26 – Apr 9)

What to plant in Thornton in May

• Beans (snap): direct sow outdoors (May 2–14)
• Summer squash: direct sow outdoors (May 2–14)
• Sweet corn: direct sow outdoors (May 2–14)
• Cucumbers: direct sow outdoors (May 2–14)
• Peppers: transplant outdoors (May 16–28)
• Start cabbage seeds indoors for a fall crop (May 16–30)
• Eggplant: transplant outdoors (May 16–28)
• Cantaloupe: direct sow outdoors (May 16–28)
• Watermelon: direct sow outdoors (May 16–28)
• Winter squash: direct sow outdoors (May 16–28)
• Potatoes: plant (seed pieces) outdoors (Apr 18 – May 14)
• Carrots: direct sow outdoors (Apr 18 – May 14)
• Cauliflower: transplant outdoors (Apr 18 – May 14)
• Beets: direct sow outdoors (Apr 18 – May 14)
• Swiss chard: direct sow outdoors (Apr 18 – May 14)
• Parsnips: direct sow outdoors (Apr 18 – May 14)
• Start tomato seeds indoors (Apr 22 – May 6)

What to plant in Thornton in June

• Start broccoli seeds indoors for a fall crop (Jun 5–19)
• Tomatoes: transplant outdoors (Jun 10)

What to plant in Thornton in July

• Last day to set out cabbage transplants for a fall harvest (by Jul 4)
• Last day to sow beans (snap) for a fall harvest (by Jul 15)
• Last day to sow carrots for a fall harvest (by Jul 19)
• Last day to sow cucumbers for a fall harvest (by Jul 20)
• Last day to sow peas for a fall harvest (by Jul 24)
• Last day to set out broccoli transplants for a fall harvest (by Jul 24)
• Last day to sow summer squash for a fall harvest (by Jul 25)
• Last day to sow lettuce (leaf) for a fall harvest (by Jul 29)
• Last day to sow green onions for a fall harvest (by Jul 29)
• Last day to sow beets for a fall harvest (by Jul 29)
• Last day to sow swiss chard for a fall harvest (by Jul 29)

What to plant in Thornton in August

• Last day to sow kohlrabi for a fall harvest (by Aug 8)
• Last day to sow turnips for a fall harvest (by Aug 8)
• Last day to sow spinach for a fall harvest (by Aug 18)
• Last day to sow radishes for a fall harvest (by Aug 28)

What to plant in Thornton in October

• Plant garlic cloves (Oct 1–14)

Crop-by-crop notes

When to plant tomatoes in Thornton

Very tender crop. CSU Extension's rules are to set tomatoes out at least 2 weeks after the average last frost and only once nights are above 52°F. In Thornton, the frost rule alone would allow about May 16, but the average nightly low at Northglenn isn't above 52°F until Jun 10, so the night rule sets the date at every risk level: about Jun 10. Start seeds indoors Apr 22 – May 6, 5–7 weeks before transplanting on Jun 10. Fruit set suffers when nights drop below 55°F (CSU). The normal low at Northglenn reaches 55°F on Jun 22. CSU's typical time from transplant to harvest for tomato plants is 65 days. Set out on Jun 10, they would start producing around Aug 14, 58 days before the typical first frost (Oct 11).

When to plant peppers in Thornton

Very tender crop. Following CSU Extension's rule (2+ weeks after the average last frost), set peppers out around May 16 with the typical date, May 21 for a balanced choice, or May 28 with the safer 10% date. Sow seeds indoors Mar 26 – Apr 9, 6–8 weeks before transplanting on May 21. CSU notes that nights under 55°F hurt fruit set. The normal low at Northglenn climbs to 55°F on Jun 22. Set out on May 21, pepper plants would start producing around Jul 30, based on CSU's typical 70 days from transplant to harvest. That leaves 73 days before the typical first frost (Oct 11).

When to plant potatoes in Thornton

Semi-hardy crop. CSU says potatoes can be planted as early as 0–2 weeks before the average last frost, in soil of at least 45°F. In Thornton, that's roughly Apr 18 – May 2 going by the typical date, or Apr 30 – May 14 going by the safer date.

When to plant onions (dry, from sets) in Thornton

Hardy crop. Per CSU, onions (dry, from sets) can be planted as early as 2–4 weeks before the average last frost, as long as the soil is workable and at least 35°F. For Thornton, that works out to roughly Apr 4–18 (typical date) or Apr 16–30 (safer date).

When to plant peas in Thornton

Hardy crop. CSU says peas can go in as early as 2–4 weeks before the average last frost: roughly Apr 4–18 in Thornton using the typical date, or Apr 16–30 using the safer one. Wait until the soil can be worked and is at least 40°F. For fall, sow by Jul 24 (Jul 9 to be safe), which is 79 days before the first-frost date.

When to plant lettuce (leaf) in Thornton

Hardy crop. CSU says lettuce (leaf) can go in as early as 2–4 weeks before the average last frost, once the soil can be worked and is at least 35°F. In Thornton, that's roughly Apr 4–18 going by the typical date, or Apr 16–30 going by the safer date. For fall, sow by Jul 29 (Jul 14 to be safe), which is 74 days before the first-frost date.

When to plant carrots in Thornton

Semi-hardy crop. CSU allows planting carrots as early as 0–2 weeks before the average last frost, in soil at 40°F or warmer. For Thornton, that's roughly Apr 18 – May 2 by the typical date or Apr 30 – May 14 by the safer date. For fall, count back 84 days from the first-frost date. That means sowing by Jul 19, or by Jul 4 to be safe.

When to plant beans (snap) in Thornton

Tender crop. CSU says to sow beans (snap) around the average last frost, about May 2 in Thornton. If you want only a 1-in-10 chance of a later 32°F night, wait until May 14. Seeds need soil of at least 55°F. For fall, sow by Jul 15 (Jun 30 to be safe), which is 88 days before the first-frost date.

When to plant summer squash in Thornton

Tender crop. CSU says to sow summer squash around the average last frost, about May 2 in Thornton. If you want only a 1-in-10 chance of a later 32°F night, wait until May 14. Seeds need soil of at least 60°F. For fall, sow by Jul 25 (Jul 10 to be safe), which is 78 days before the first-frost date.

When to plant broccoli in Thornton

Hardy crop. Per CSU, broccoli can be planted as early as 2–4 weeks before the average last frost, as long as the soil is workable and at least 40°F. For Thornton, that works out to roughly Apr 4–18 (typical date) or Apr 16–30 (safer date). CSU's typical time from transplant to harvest for broccoli plants is 65 days. Set out on Apr 9, they would start producing around Jun 13, 120 days before the typical first frost (Oct 11). For a fall crop, set broccoli transplants out by Jul 24. That's UNL Extension's count-back from the typical first frost (Oct 11), using CSU's 65 days from transplant to harvest. If you grow your own transplants, start seeds Jun 5–19 (CSU: 5–7 weeks to transplant size).

When to plant cabbage in Thornton

Hardy crop. CSU says cabbage can go in as early as 2–4 weeks before the average last frost, once the soil can be worked and is at least 40°F. In Thornton, that's roughly Apr 4–18 going by the typical date, or Apr 16–30 going by the safer date. Set out on Apr 9, cabbage plants would start producing around Jul 3, based on CSU's typical 85 days from transplant to harvest. That leaves 100 days before the typical first frost (Oct 11). For a fall harvest of cabbage, get transplants in the ground by Jul 4 (UNL Extension count-back from the typical first frost, Oct 11, with CSU's 85 days from transplant to harvest). To raise your own, sow seeds May 16–30, 5–7 weeks ahead (CSU).

When to plant cauliflower in Thornton

Semi-hardy crop. Per CSU, cauliflower can go in as early as 0–2 weeks before the average last frost: roughly Apr 18 – May 2 in Thornton (typical date) or Apr 30 – May 14 (safer date). The soil should be at least 40°F.

When to plant kohlrabi in Thornton

Hardy crop. CSU says kohlrabi can go in as early as 2–4 weeks before the average last frost: roughly Apr 4–18 in Thornton using the typical date, or Apr 16–30 using the safer one. Wait until the soil can be worked and is at least 40°F. For a fall crop, sow by Aug 8, or by Jul 24 to be safe. That's 64 days back from the first-frost date.

When to plant green onions in Thornton

Hardy crop. CSU says green onions can go in as early as 2–4 weeks before the average last frost, once the soil can be worked and is at least 35°F. In Thornton, that's roughly Apr 4–18 going by the typical date, or Apr 16–30 going by the safer date. For fall, sow by Jul 29 (Jul 14 to be safe), which is 74 days before the first-frost date.

When to plant radishes in Thornton

Hardy crop. CSU says radishes can go in as early as 2–4 weeks before the average last frost, once the soil can be worked and is at least 40°F. In Thornton, that's roughly Apr 4–18 going by the typical date, or Apr 16–30 going by the safer date. For fall, sow by Aug 28 (Aug 13 to be safe), which is 44 days before the first-frost date.

When to plant spinach in Thornton

Hardy crop. Per CSU, spinach can be planted as early as 2–4 weeks before the average last frost, as long as the soil is workable and at least 40°F. For Thornton, that works out to roughly Apr 4–18 (typical date) or Apr 16–30 (safer date). For a fall crop, sow by Aug 18, or by Aug 3 to be safe. That's 54 days back from the first-frost date.

When to plant turnips in Thornton

Hardy crop. Per CSU, turnips can be planted as early as 2–4 weeks before the average last frost, as long as the soil is workable and at least 40°F. For Thornton, that works out to roughly Apr 4–18 (typical date) or Apr 16–30 (safer date). For fall, sow by Aug 8 (Jul 24 to be safe), which is 64 days before the first-frost date.

When to plant beets in Thornton

Semi-hardy crop. CSU allows planting beets as early as 0–2 weeks before the average last frost, in soil at 40°F or warmer. For Thornton, that's roughly Apr 18 – May 2 by the typical date or Apr 30 – May 14 by the safer date. For fall, sow by Jul 29 (Jul 14 to be safe), which is 74 days before the first-frost date.

When to plant swiss chard in Thornton

Semi-hardy crop. Per CSU, swiss chard can go in as early as 0–2 weeks before the average last frost: roughly Apr 18 – May 2 in Thornton (typical date) or Apr 30 – May 14 (safer date). The soil should be at least 40°F. For fall, sow by Jul 29 (Jul 14 to be safe), which is 74 days before the first-frost date.

When to plant parsnips in Thornton

Semi-hardy crop. CSU allows planting parsnips as early as 0–2 weeks before the average last frost, in soil at 35°F or warmer. For Thornton, that's roughly Apr 18 – May 2 by the typical date or Apr 30 – May 14 by the safer date.

When to plant sweet corn in Thornton

Tender crop. CSU's timing for sweet corn is around the average last frost, which is about May 2 in Thornton. For only a 1-in-10 chance of a later 32°F night, hold off until May 14. Seeds want soil at 50°F or warmer.

When to plant cucumbers in Thornton

Tender crop. Sow cucumbers around the average last frost, per CSU: about May 2 in Thornton. Waiting until May 14 cuts the chance of a later 32°F night to 1 in 10. Seeds need soil of at least 60°F. For fall, sow by Jul 20 (Jul 5 to be safe), which is 83 days before the first-frost date.

When to plant eggplant in Thornton

Very tender crop. CSU Extension's rule is 2+ weeks after the average last frost, which puts eggplant outside around May 16 (typical date), May 21 (balanced) or May 28 (safer 10% date). Start seeds indoors Mar 19 – Apr 9, 6–9 weeks ahead of transplanting on May 21. Using CSU's typical 60 days from transplant to harvest, eggplant plants set out on May 21 would start producing around Jul 20. That's 83 days before the typical first frost (Oct 11).

When to plant cantaloupe in Thornton

Very tender crop. CSU Extension says to plant cantaloupe 2+ weeks after the average last frost: about May 16 (typical date), May 21 (balanced) or May 28 (safer 10% date). Wait for soil of at least 60°F.

When to plant watermelon in Thornton

Very tender crop. CSU Extension says to plant watermelon 2+ weeks after the average last frost: about May 16 (typical date), May 21 (balanced) or May 28 (safer 10% date). Wait for soil of at least 60°F.

When to plant winter squash in Thornton

Very tender crop. CSU Extension's rule is to plant winter squash at least 2 weeks after the average last frost. That's about May 16 using the typical date, May 21 as a balanced choice, or May 28 if you go by the safer 10% date. Seeds need soil of at least 60°F.

When to plant garlic in Thornton

CSU Extension's Colorado guidance is to plant garlic in early October, before October 15 (Oct 1–14). That's a fixed statewide window, not one computed from frost data. At Northglenn, the typical first hard freeze (28°F) is Oct 21, 7 days after the window ends.

How these dates are calculated

CSU states its spring rules relative to “the average last spring frost.” We use the 50% date as that average and also show the 30% and 10% anchors. See the methodology for details and limitations.

FAQ

When should I start seeds indoors in Thornton?

Start tomatoes Apr 22 – May 6 and peppers Mar 26 – Apr 9. Both dates count back from a balanced (30%-risk) transplant date using CSU's transplant ages.

When should I plant tomatoes outside in Thornton?

About Jun 10, at every risk level. CSU wants tomatoes out 2+ weeks after the average last frost (that rule alone gives about May 16 here) and nights above 52°F. At Northglenn, the average nightly low first rises above 52°F on Jun 10, later than the frost-rule date, so the night rule sets the date.

What can I plant in Thornton in the fall?

Counting back from the typical first frost (Oct 11) with UNL Extension's method: peas by Jul 24; lettuce (leaf) by Jul 29; carrots by Jul 19; beans (snap) by Jul 15; summer squash by Jul 25; broccoli by Jul 24 (transplants); cabbage by Jul 4 (transplants); kohlrabi by Aug 8; green onions by Jul 29; radishes by Aug 28; spinach by Aug 18; turnips by Aug 8; beets by Jul 29; swiss chard by Jul 29; cucumbers by Jul 20. Garlic goes in Oct 1–14.

When should I plant garlic in Thornton?

In early October, before October 15 (Oct 1–14), per CSU Extension's Colorado guidance.

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