Before any insect shows up, the weather can take a pepper crop apart. At the Salt Lake City reference station, 52.1 days a year reach 90 F or hotter. Utah State University Extension says pepper fruit set fails above that line, and also on nights below 55 F. Pests and disease do their own damage, but this page tracks the weather side, because that is where you can count the dates. For the full crop plan, start with Growing Peppers in Utah.
The short answer
Start with the hot end. The NOAA NCEI Climate Normals put the mean at 52.1 days a year at or above 90 F at the Salt Lake City reference station. That is more than seven weeks’ worth of days, scattered through the warm season rather than stacked in a row. It measures summer heat load, which is different from the average summer temperature. A summer can feel pleasant and still rack up a lot of 90-degree afternoons. Utah State University Extension (Peppers in the Garden) says pepper fruit set fails when days climb above 90 F. A flower that opens in that heat is a flower at risk of dropping instead of turning into a pepper.
The annual total matters less than where those days land against your plants’ flowering. A bed that flowers in a mild stretch can do fine in a hot year, and a bed that flowers right into a hot spell can stall in a mild one. That is why a count of 52.1 reads as a warning about exposure, not a verdict on whether peppers can grow here.
Now the other end, which gardeners forget. The same Extension guidance says fruit set also fails on nights below 55 F. This site does not carry summer night minima, so there is no night count to offer next to the 52.1. I won’t guess one from a map or from how cold the winter gets, because a bitter January says nothing about a July night. What works is cheap: a min/max thermometer set at plant height, read each morning, with the lows written down for a couple of weeks once the plants are in the ground. Your own garden’s numbers beat any regional average, especially if you sit in a low spot where cold air pools.
One more caution about the 52.1. It belongs to the Salt Lake valley, which is a mild spot by Utah standards. Most of the state sits higher and colder, and southern Utah runs warmer. Your hot-day count could land well away from the reference figure in either direction, so treat 52.1 as an anchor, not your local number. Pepper is squeezed between two thresholds, hot days on one side and cool nights on the other, and the usable window is narrower than the growing season looks on a calendar. If you are still choosing what to plant, Pepper Varieties That Ripen in a Utah Season covers the pick side of that squeeze.
When to start seed here
Seed timing works backward from one date: the day you can safely put transplants outside. Utah State University Extension says transplants need 8 to 10 weeks indoors, and that you should transplant when the soil reaches 60 F, after frost. Here is the arithmetic, step by step, so you can redo it for your county.
- Find your last spring frost. For the Salt Lake City station, NOAA NCEI puts the last 32 F freeze between April 12 and April 29, using the 30 to 70 percent probability band (station USW00024127). That band is a middle range, not a guarantee, and it describes a mild valley. Higher, colder parts of Utah will run later, and southern Utah earlier.
- Wait for the soil. Frost-free is not the same as warm enough. Push a soil thermometer a few inches down in the morning, and treat the first day it reads 60 F or more, after your last frost, as your transplant date. If soil reaches 60 F later than the frost window, the soil date wins.
- Subtract 8 to 10 weeks. Take a made-up example, not a Utah forecast: soil hits 60 F on May 20. Ten weeks is 70 days, which lands on March 11. Eight weeks is 56 days, which lands on March 25. So seed goes in between March 11 and March 25. Plug in your own date and the same two subtractions give your window.
- Add the harvest count. Clemson Cooperative Extension says peppers should be ready for harvest about 70 to 85 days after transplanting. From the sample May 20 transplant, that points to July 29 at the early end and August 13 at the late end. Clemson is in South Carolina, so treat those days as a rough guide for Utah, not a promise.
- Read any packet carefully. A figure like 35 to 45 days refers to the time from flowering, not from sowing or transplanting. Counting it from the seed date would put harvest weeks ahead of where it really falls.
Starting earlier than the window does not buy a head start. Overgrown seedlings sit in cramped cells waiting on the weather, and the date they can go out has not moved.
What goes wrong here
Four failures account for most of the grief, and each has a fix that costs little. If flowers are forming but nothing is setting, Why Peppers Fail to Set Fruit in Utah goes deeper on that one.
- Blossom drop in a hot spell. Days above 90 F are the hot-end line for fruit set, and Salt Lake’s reference count is 52.1 of them a year. Flowers fall off instead of setting. Afternoon shade cloth over the bed, steady watering, and a mulch layer to keep roots cooler help the plant ride out a stretch. Pick off flowers if you must, and expect new ones when the heat breaks.
- Blossom drop on a cool night. Nights below 55 F cause the same failure from the other side. This site does not carry summer night minima, so check your own with a thermometer. When a cold night is forecast, a row cover or a water-filled cloche over the plants takes the edge off, and removing it by midmorning keeps the bed from overheating.
- Sunscald. It shows up as pale, papery, sunken patches on the side of the fruit facing the sun, often after leaves were stripped or lost to disease. Leave the canopy intact, avoid heavy pruning, and use light shade cloth in the strongest afternoon sun. Fruit hidden under leaves is protected fruit.
- Blossom-end rot. A dark, leathery patch on the bottom of the fruit points to uneven moisture, which limits the calcium reaching the pepper. Water on a steady schedule, mulch to hold moisture, and get a soil test before adding calcium products blindly. A heavy dose rarely fixes a watering problem.
Common questions
Can I keep harvested peppers in the refrigerator?
You can, but it is colder than peppers like. Clemson Cooperative Extension gives the optimal storage conditions as 45 to 50 °F with 80 to 90 percent relative humidity, for two to three weeks (Clemson HGIC). A household fridge runs colder than that range, and below about 45 °F peppers pit and soften from chilling injury. Two to three weeks is a ceiling under ideal conditions, not a promise. A cool pantry or basement shelf that stays in that range often beats the crisper drawer.
Do hot days or cool nights hurt more?
Neither one outranks the other in the Extension guidance. Fruit set fails at both ends: nights below 55 F and days above 90 F. The hot end has a number here, 52.1 days a year at the Salt Lake City reference station. The cool end does not, because summer night minima are not carried on this site. Watch your own nights.
When can transplants go outside in Salt Lake?
After the last frost and once the soil reaches 60 F. NOAA NCEI’s 32 F freeze probabilities put the last spring frost between April 12 and April 29 at the Salt Lake City station (30 to 70 percent band). The soil often reaches 60 F later than that, and it is the later date that counts. Colder, higher areas of Utah come later still.
Is 8 to 10 weeks indoors a firm rule?
It is the range Utah State University Extension gives, so work inside it. A seedling younger than 8 weeks may be too small to cope with a cool night or a hot afternoon, while one older than 10 will be cramped in its container. Because the count runs back from your transplant date and not from the frost date, a cold spring that pushes soil warmth later moves your seed date later too.