Start pepper seed indoors 8 to 10 weeks before the day you plan to transplant. That day is the later of two dates: after your last spring frost, and when the soil reaches 60 F. In many Utah gardens the frost passes in late April and the soil warms in May, so seed goes in the tray sometime in March. Valley, high-elevation and southern Utah gardens each count back from a different date.
The short answer
Salt Lake City’s reference station reaches 90 F or higher on 52.1 days in a typical year, according to NOAA’s NCEI climate normals. That is well over seven weeks of hot days scattered through the summer. For peppers, it matters because Utah State University Extension says fruit set fails at both ends of the thermometer: nights below 55 F or days above 90 F. A plant can be healthy, green and covered in flowers and still hold no fruit when the temperature swings past either line.
So the seed date is not only about dodging frost. Peppers started on a sensible schedule and set out once the soil is warm have the longest stretch of growth before the hot weeks arrive. That gives them a bigger frame to carry fruit when the weather finally cooperates.
How hot is too hot for pepper blossoms in Utah?
The line is 90 F. A day above it can make blossoms drop instead of setting fruit. With 52.1 days a year at or above that mark at the Salt Lake station, a gardener on the valley floor should expect several hot spells every summer. Gardens at higher elevation will see fewer, and southern Utah’s warmer gardens will likely see more, the count is a guide to the pattern rather than a number for your own yard. When a spell is forecast, afternoon shade cloth over the bed takes the edge off. Blossoms that open after the heat breaks can still set fruit.
What about cool nights?
The other end is the one gardeners forget. Nights under 55 F also stop fruit from setting. Summer night lows differ from one yard to the next. The practical answer is to watch your own nights. A cheap min/max thermometer set at plant height shows the low every morning. If it reads under 55 F while the plants are in flower, cover them at dusk with a row cover or a spare sheet and pull it off in the morning.
For a closer look at both failures, see Why Peppers Fail to Set Fruit in Utah. The wider picture for the crop lives in Growing Peppers in Utah, and the season-length question is covered in Pepper Varieties That Ripen in a Utah Season.
When to start seed here
The method is a countdown from transplant day. Five steps get you a date for any Utah county, and you only need a calendar and a soil thermometer.
- Find your last spring frost. At the Salt Lake station, the 32 F freeze probability sits in the 30 to 70 percent band from April 12 to April 29, per NOAA NCEI normals for station USW00024127 (NOAA data). Salt Lake sits in a mild valley. Most of Utah is higher and colder, so frost lingers later. Southern Utah is warmer, so frost ends earlier. Your county extension office can tell you where your town falls.
- Find the date the soil reaches 60 F. Push a soil thermometer a few inches into the bed in the morning. Soil often warms later than the frost date passes, so this check matters.
- Choose the later of the two dates. That is your transplant day. Peppers set out into cold soil sit and wait, so there is no reward for rushing.
- Subtract 8 to 10 weeks. Eight weeks is 56 days and ten weeks is 70 days. USU Extension says transplants need that long indoors.
- Sow in that window. If a cold snap delays the soil, the plants wait a little longer in their pots. If the soil warms early, you still keep your plan, because you can’t shorten the indoor time.
What does the count look like for a real date?
Say your soil hits 60 F on May 10. Eight weeks back is March 15. Ten weeks back is March 1. Seed goes in the tray between those dates.
The edges of the Salt Lake frost window show the earliest case. A transplant day of April 29 points to a sowing window from February 18 to March 4. A transplant day of April 12 points to February 1 through February 15. Those are the earliest possible dates, and they only work if the soil is already warm, which is worth checking before you plant a single seed that early.
Gardeners at higher elevations push every date later. Gardeners in the warmer south can move everything earlier. The count itself never changes. Only the starting point does.
What goes wrong here
Utah’s mix of hot afternoons, strong sun and swings in night temperature produces a short list of pepper problems. Most are preventable once you know what each one looks like.
- Blossom drop in a hot spell or a cool night. Flowers fall off with no fruit behind them when days climb above 90 F or nights fall under 55 F. The plant is not sick. Shade cloth over the bed during a heat wave takes the edge off the afternoon. A row cover on cold evenings holds a little warmth overnight. Keep watering steady, and hold back heavy nitrogen feeding, which pushes leaves at the expense of flowers. New blossoms that open after the weather settles can still set fruit.
- Sunscald where the sun is strong. Fruit shows pale, papery, sunken patches on the side facing the sun, and the spot can later turn gray and moldy. Utah’s clear, dry sun is hard on exposed fruit. A full leaf canopy is the plant’s own shade, so don’t strip leaves to “let the fruit ripen.” A strip of shade cloth in the hottest part of the afternoon also helps, and so does keeping plants staked upright so fruit stays under foliage.
- Blossom-end rot. A dark, leathery, sunken patch forms on the bottom of the fruit, away from the stem. The cause is calcium failing to reach the fruit, and uneven watering is the usual trigger. Many Utah soils already hold plenty of calcium, so adding more rarely fixes it. Water on a regular schedule, mulch to hold moisture, and run a soil test through your county extension office before you add anything.
How do I tell which problem I have?
Look at where the damage sits. Flowers on the ground and no fruit forming points to temperature. A bleached patch on the sunny side of a fruit points to sunscald. A dark patch on the bottom points to blossom-end rot. Record the night low and the afternoon high for the days before the problem showed up, and the cause is usually clear.
Some gardeners also time this by the Moon: when to plant peppers by the moon, as almanac tradition rather than fact.
Common questions
Can I sow pepper seed straight into the garden in Utah?
Peppers are grown from transplants. USU Extension says they need 8 to 10 weeks indoors before they go out, and a seed sown in the ground after the soil warms leaves too little season to flower, set and ripen fruit.
Is April 12 to April 29 my last frost date?
That band is for the Salt Lake station, which sits in a mild valley. Most of Utah is higher and colder, so your last frost is likely later. Southern Utah is warmer, so yours may be earlier. Your county extension office can give you the local range.
How long until harvest after transplanting?
Clemson Extension says peppers should be ready for harvest in about 70 to 85 days after transplanting (Clemson Cooperative Extension). Compare that with the length of your own season when you pick a variety.
How should I store harvested peppers?
Not in the refrigerator, which is too cold. Clemson Extension puts the best storage at 45 to 50 F and 80 to 90 percent relative humidity, for two to three weeks at most under those ideal conditions. Below about 45 F, peppers pit and soften from chilling injury. A cool pantry or basement shelf is a better bet than the crisper drawer.