Days to maturity when the season is short
Packet days are counted differently for seeds and transplants, and cool nights add time. How to tell whether a crop can finish.
“Sixty days” on a packet is not a promise that you will eat that crop sixty days after you tear the envelope. In a short, cool season the number is a clue, and it is easy to misread.
The Colorado Vegetable Guide says the phrase “days to maturity” can mean different things depending on the company. Some counts start at seeding. For crops that are usually started indoors, the count usually starts when the seedling is transplanted.
The guide’s plain conclusion is the one to keep: the number is not a precise measurement. Mountain communities are told to pick quicker cultivars than they would at lower elevation, and to leave a buffer, because cool temperatures slow growth.
Gilpin County’s extension summary, written for a community garden at 9,300 feet, is willing to be numeric. Add about 20 days to the packet figure.
That adjustment is for a high, cool site, not for a sheltered bed in Pueblo. At 5,300 feet on the Front Range the penalty is smaller, and it is not zero. June nights in the 40s and low 50s still stretch the calendar on tomatoes, peppers, and corn.
If you need a single rule before you have your own records, use the packet number below about 6,500 feet, add one to two weeks from there to about 8,000 feet, and use Gilpin’s 20 days above that until your notebook says otherwise. Label it as a buffer, not as a new scientific constant.
Count backward from a real date
You need two dates before the packet means anything. The first is the median last spring 32°F date for a station you trust. The second is the 1-in-10 late date, the right-hand column in the NOAA occurrence tables, explained in reading a spring frost.
Hardy crops can be timed from the median, and often from two to four weeks before it, which is the window in CSU’s planting guide, provided the soil can be worked. Very tender crops should be timed from the late date unless a cover is already built.
Then decide whether the packet is counting from seed or from transplant.
- Direct-sown crops such as pea, radish, spinach, carrot, beet, and bush bean: count from the day you intend to sow.
- Crops you transplant such as tomato, pepper, eggplant, and many brassicas: count from the day you intend to set them out. The weeks those plants spend under lights are not part of the packet number. They are part of your indoor schedule, below.
A fall limit closes the sum. If you do not have a published first-frost date, do not invent one. Use a conservative reading from a source that has one, such as Gilpin’s September 15 at 9,300 feet, the National Weather Service medians for Cheyenne and Laramie on the Wyoming and Montana calendar, or USU’s published first dates where the Utah page quotes them.
On the Front Range, the Colorado Climate Center’s tool is the check. A crop that matures on paper three days before a median first frost will miss in the early half of years.
Worked examples
These are illustrations of the arithmetic, not variety rankings.
Provider-class bush bean, packet about 50 days from seed, Fort Collins. The NOAA median last 32°F date at the Fort Collins station is May 4, and the 1-in-10 late date is May 15.
Beans are on CSU’s tender list, sown around the average last frost when days are above about 55°F. Sow on the median only if the week looks settled. Otherwise wait for the late date.
Fifty packet days from May 15 is early July. Even with a one-week cool-night buffer, that crop finishes in a Front Range summer. This is a bean you can grow.
A 100-day winter squash from seed, Gunnison. Gunnison’s median last 32°F date in the NOAA table is June 27, and the late date is July 16. A squash sown July 16 and given 100 packet days is late October, which is past any honest first frost for a 7,700-foot valley.
Adding a buffer makes it worse. That cultivar does not fit. A winter squash in the 80-day class, sown under a cover as early as you can protect it, is the conversation. Or you skip winter squash and grow summer squash, which is on the tender list and is eaten before a long count even starts.
A 65-day tomato, counted from transplant, Alamosa. Alamosa’s median is June 9 and the late date is June 23. Set the plant out June 23. Sixty-five packet days lands in late August.
Add Gilpin’s style of buffer, even a partial one, and you are in September, when high-valley nights are already falling. The Colorado Vegetable Guide’s mountain note says tomatoes are very hard without a short-season cultivar and a cover.
The arithmetic says the same thing. A 65-day tomato in Alamosa is a covered experiment. It is not the backbone of the garden. Greens and potatoes are.
Broccoli started indoors, Boulder. Boulder’s median last 32°F date is May 4. CSU’s mountain sheet says longer brassicas can be sown indoors 6 to 8 weeks before the last frost and planted out about two weeks before that frost.
Sowing around mid-March and setting out around April 20 matches that pattern for a Boulder median. The packet days then run from April 20, not from mid-March. A 60-day broccoli is heading in late June.
A fall sowing, which the Colorado Vegetable Guide illustrates for cabbage with a midsummer direct sowing in its baseline climate, is a second crop on the Front Range and often a fantasy above 8,500 feet. One good heading is the mountain goal.
Indoor weeks are a separate clock
The packet does not include germination and early growth under lights. Typical lead times used by extension educators, and consistent with the CSU mountain sheet’s 6-to-8-week note for broccoli, cauliflower, leeks, and Brussels sprouts, look like this.
| Crop | Start indoors before the intended set-out date | Set out |
|---|---|---|
| Broccoli, cabbage, cauliflower, kale | 4 to 6 weeks; up to 6 to 8 for leeks and Brussels sprouts | Hardy crops about 2 weeks before the median last frost, if hardened |
| Tomato | 6 to 8 weeks | After the late frost date, or earlier only with a cover |
| Pepper, eggplant | 8 to 10 weeks | After the late date, in the warmest bed |
| Onion from seed | About 10 weeks, per the Colorado Vegetable Guide | The guide’s baseline is mid-April to mid-May; shift later at elevation |
| Cucumber, squash | 2 to 3 weeks if you must, or direct sow | Around or after the last frost; they resent cramped roots |
Lights matter more than a sunny windowsill. A leggy transplant loses a week sitting still after you plant it, and that week was part of your margin. Harden for wind and sun. High-elevation light burns tissue that only ever saw glass.
Onions are the crop where the indoor clock and the day length both matter.
The Colorado Vegetable Guide says seed takes about 110 days in the garden, so much of Colorado is better off with sets or transplants, and it recommends starting seed about ten weeks before the last spring frost.
Sets and plants in that guide go out from about mid-April to mid-May in the guide’s baseline season. Shift that later as your station’s median moves later. And buy long-day cultivars in Colorado, Wyoming, Montana, and Idaho. Onion bulbs form in response to day length. A short-day cultivar, the type grown for winter shipping from the South, bulbs too soon at these latitudes and stays small.
Cool-season crops get a discount
The Colorado Vegetable Guide makes an exception you should use. A cool-season crop whose packet days run a little longer than your frost-free stretch can still be worth growing, because it survives frost and may improve after a light one. Kale, spinach, and carrots are in this group. A warm-season crop does not get the discount.
If the packet days plus your buffer run past the first frost, the crop needs a cover or a different cultivar. “Warm-season crops with long days to maturity can only be grown to harvest under season extension cover” is the guide’s wording, and it is the right test for melons, long-season corn, and big winter squashes at elevation.
Corn has a second clock besides days. It is on CSU’s tender list, it wants warm soil, and it is wind-pollinated. The vegetable guide says to plant at least four rows of a single variety side by side, and it prefers a single variety in a home garden so pollen is not a muddle.
A 65-day early corn in a Front Range garden with a mid-May sowing can finish. The same packet in a valley whose median last frost is June 27 is a crop you will water until a September freeze ends the argument. Plant the early packet, or plant something else.
Checklist before the seed order
- Write your median and late spring dates, and a fall date only if you have a source.
- Mark each packet “from seed” or “from transplant.”
- Add a buffer of about a week on the Front Range in a cool June, and closer to Gilpin’s 20 days above 8,500 feet.
- Reject warm-season cultivars that miss by more than a few days. Do not reject a hardy green for the same miss.
- Count indoor weeks backward from the set-out date, not from New Year’s.
- Prefer one short-season cultivar you will cover over three long-season cultivars you will hope for.
- If two packets of the same crop differ by 20 days, buy the shorter one for the mountain garden and keep the longer one for a lower, hotter site if you have it.
Related reading: cool-season crops, warm-season crops, and what changes above 5,000 feet.
Sources
- CSU Extension, Colorado Vegetable Guide, sections on days to maturity, mountain communities, onions, corn, and season extension.
- Gilpin County Extension climate summary: add about 20 days at 9,300 feet.
- CSU Extension, Vegetable Planting Guide and Vegetable Gardening in the Mountains.
- NOAA NCEI Colorado freeze-occurrence table for the station dates used in the examples.