GreenlyWhat to plant in September in the Mountain West
Worked out for 15 cities from their own weather and frost dates, by zone.
What to sow and plant in September in the Mountain West, by zone: frost-free, highs above 82 °F (Denver, Salt Lake City and 3 more): garlic and flower bulbs; frost-free, highs of 72–82 °F (Colorado Springs, Reno and 6 more): garlic, broad bean, peas, onion, spinach and 11 more; frost-free, highs of 61–72 °F (Helena and Missoula): garlic, broad bean, peas, onion, spinach and 13 more.
In the Mountain West, September is fall.
Frost-free, highs above 82 °F: Denver, Salt Lake City, Grand Junction, Pueblo and 1 more
In September, on average, these cities have highs of 83 to 92 °F and lows of 52 to 65 °F, 0.4 to 1.3 in of rain on 2 to 5 days; the day lasts 12 h 27 min to 12 h 31 min (on the 15th).
No frost in September in 10 years of data: frost-tender crops can go in too, if it is warm enough for them.
| How | What | Where |
|---|---|---|
| plant cloves | Garlic | Denver, Salt Lake City, Grand Junction and 1 more |
| sow direct | Zucchini, green beans, basil, bok choy, borage | St. George |
| plant bulbs | Crocus, daffodil, hyacinth, tulip | Denver, Salt Lake City, Grand Junction and 1 more |
Compared with August: new — garlic, crocus, daffodil, hyacinth and tulip.
Not starting in September: tomato (no month outdoors by these rules), bell pepper (no month outdoors by these rules), lettuce (October, sown direct), peas (October, sown direct) and spinach (October, sown direct) — dates for Grand Junction.
Frost-free, highs of 72–82 °F: Colorado Springs, Reno, Billings, Cheyenne and 4 more
In September, on average, these cities have highs of 76 to 82 °F and lows of 51 to 61 °F, 0.4 to 1.5 in of rain on 2 to 5 days; the day lasts 12 h 29 min to 12 h 36 min (on the 15th).
No frost in September in 10 years of data: frost-tender crops can go in too, if it is warm enough for them.
| How | What | Where |
|---|---|---|
| plant cloves | Garlic | Colorado Springs, Reno, Billings and 4 more |
| sow direct | Broad bean, peas, cabbage, beet, parsley, cilantro, arugula, chamomile, dill, mustard greens | Billings, Cheyenne, Carson City and 1 more |
| plant sets | Onion | Billings, Cheyenne, Carson City and 1 more |
| start indoors, plant out in October | Spinach, lettuce, kohlrabi | Billings, Cheyenne, Carson City and 1 more |
| start indoors, plant out in October | Swiss Chard | Carson City |
| sow direct | Chickpea | Carson City |
| plant sets | Shallot | Carson City |
| plant bulbs | Crocus, daffodil, hyacinth, tulip | Colorado Springs, Reno, Billings and 4 more |
Compared with August: new — garlic, broad bean, peas, onion, spinach, lettuce, cabbage, Swiss chard and 14 more; no longer sown — zucchini, green beans, watermelon, pumpkin, sweet corn, basil, bok choy, borage and 10 more.
Not starting in September: tomato (March, started indoors), bell pepper (March, started indoors), zucchini (April, started indoors) and green beans (June, sown direct) — dates for Billings.
Ready to harvest in September in Billings, if sown in the right month: zucchini (July), green beans (July), watermelon (June), sweet corn (June), basil (July), bok choy (July), borage (July) and corn (June).
Frost-free, highs of 61–72 °F: Helena and Missoula
In September, on average, these cities have highs of 70 to 71 °F and lows of 45 to 46 °F, 1.6 to 1.7 in of rain on 6 to 7 days; the day lasts 12 h 36 min to 12 h 37 min (on the 15th).
No frost in September in 10 years of data: frost-tender crops can go in too, if it is warm enough for them.
| How | What | Where |
|---|---|---|
| plant cloves | Garlic | all |
| sow direct | Broad bean, peas, lettuce, cabbage, broccoli, Swiss chard, beet, parsley, cilantro, arugula, chamomile, dill, kohlrabi, mustard greens | all |
| plant sets | Onion | all |
| start indoors, plant out in October | Spinach | all |
| plant bulbs | Crocus, daffodil, hyacinth, tulip | all |
Compared with August: new — garlic, spinach, broccoli, crocus, daffodil, hyacinth and tulip; no longer sown — zucchini, green beans, basil, chickpea, shallot, bok choy and borage.
Not starting in September: tomato (April, started indoors), bell pepper (April, started indoors), zucchini (May, started indoors) and green beans (July, sown direct) — dates for Helena.
Ready to harvest in September in Helena, if sown in the right month: zucchini (July), green beans (July), sweet corn (June), basil (July), arugula (August), bok choy (July), borage (July) and corn (June).
How we work it out
For each city we use its average weather in September and the months after (highs, lows and rain for 2016–2025, ERA5 reanalysis via Open-Meteo) and its average frost dates. A crop starts in September if the month's mean temperature reaches the minimum soil temperature for germination on its card (we use the mean air temperature as a stand-in for the soil's) and if, over its first months (up to three), the average highs stay within its group's range, with no frost for crops that cannot take it. Crops started indoors count from the month they are planted out.
Temperature groups from Colorado State University Extension: cool-season crops prefer 60–80 °F and lose quality in heat; hardy ones grow in days as cool as 40 °F, semi-hardy ones from 40–50 °F (we use 50 °F); warm-season crops need days above 55 °F, and the most tender (tomato, pepper, eggplant, melon, watermelon, squash) above 60 °F, up to 95 °F. Crops the guide does not name go into the stricter group by their frost hardiness. Fava (broad) beans, which the guide does not name, count as hardy: hardy varieties are sown in fall and overwinter (RHS). Garlic and bulbs: a month before the first frost, and later only while the ground is not frozen (as on each city's page).
This is a general rule from averages: your garden can be a week or two earlier or later, and a greenhouse or a cold frame brings the season forward. Greenly works out the dates for your location.
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