October 23, 2026

How to start a tomato garden by your yard's last frost

Start a tomato garden by planting out after your own yard's last frost, which can fall weeks after the published date. On still nights the heavier cold air slides downhill and settles around beds in low ground. The forecast describes the wider area and misses that pool. Our station learned an offset of -7°F for our garden, which pushes the published April 28 last frost back to May 20.

Step 1. Find your published last frost date

Look up the published last spring frost for your area. It comes from long-term records at official weather sites, and it marks the date after which frost is less likely than not. Use it as your starting point. Why your last frost date is probably wrong explains how far a single yard can drift from it.

Step 2. Measure how much colder your yard runs

Your offset is the gap between your yard's overnight low and the forecast low. A yard in a hollow or under open sky loses heat faster on calm nights, so its low sits below the forecast. You can estimate the gap three ways.

• The area checker. The microclimate checker lines up the nightly lows from home weather stations near you against the modelled low for recent nights, cloudy ones included. It returns a range and a median for your area, which can differ from your exact yard.
• A min-max thermometer. Hang one at bed height in the open and compare each morning's low with the forecast low for that night. Average the gaps over a couple of weeks.
• A connected weather station. After about 14 days of overlap, Siltwell learns your offset by setting the station's outdoor readings against the forecast. Connecting a station is part of the paid plan described below this guide.

Step 3. Enter the offset in Siltwell

Open Settings, then Garden, then Edit garden, and fill in Microclimate offset. Enter a negative number if your yard runs colder than the forecast. A colder yard reaches freezing on more spring nights, so its last frost lands later. Siltwell shifts your garden's first and last frost dates by the offset, by at most 30 days. The same offset adjusts the displayed forecast lows and the lows your frost alerts check.

Step 4. Prepare the bed

Pick the sunniest bed you have and loosen the soil with compost worked through it. Raised soil clears the cold ground beneath it and sheds rain sooner, so a raised bed warms faster in spring. Setting up a raised tomato bed covers depth, fill, and where a soil probe goes.

Step 5. Harden off the seedlings

Seedlings grown indoors have soft leaves that scorch in direct sun and tear in wind. Set them outside in shade for a few hours a day, then lengthen the time and add sun over several days. Bring them in on any night your adjusted forecast drops near freezing.

Step 6. Plant out in a frost-free week

Once your adjusted date has passed, read the coming week's lows in Siltwell. They already carry your offset. Every evening at 5 p.m. local time, Siltwell checks the adjusted lows for tomorrow and the day after. It sends a frost alert when either one reaches your threshold, 32°F by default. Plant on a day with no alert ahead, set each seedling deep so the buried stem grows roots, and water it in.

We set our transplants into raised bed 2 on May 18, two days before our adjusted date. From August 17 to September 7 we picked 30 fruits from it, weighing 1.7 lb together.

Step 7. Water by the soil

New transplants need steady moisture while their roots move into the bed. Rain in our garden ran from 0.54 in during April to 10.89 in during July, so a fixed schedule would have missed at both ends. Siltwell's Today screen tells you to hold off once about 0.2 in (5 mm) of rain is due across today and tomorrow combined. It marks you covered when that much has fallen over the past 3 days. That guidance applies to the whole garden. Map a soil probe to the tomato bed and the paid plan adds a dry alert, which fires when the bed's last two readings both sit under 25% moisture.

Station and probes for a tomato garden

The frost offset needs an outdoor air temperature reading. A per-bed dry alert needs a soil probe in that bed.

• Tempest. It reads outdoor air temperature, so it can learn your frost offset. It has no soil sensor.
• Ecowitt gateway with WH51 probes. Each WH51 reads soil moisture as a percentage in one bed. A gateway paired only with WH51s has no outdoor temperature reading, so it gets soil alerts and no learned offset.
• Both in one garden. A garden can hold several connections, so a Tempest plus an Ecowitt gateway with WH51s covers the frost date and each bed.

What can go wrong

• A late cold night. Your adjusted date shows how your yard compares with the forecast, and it can't tell you whether this particular spring runs late or early. Our last recorded spring frost came on April 30, 20 days before our adjusted date.
• A thermometer in the wrong spot. A sensor against a sunny wall or on a roof stays warmer than the bed overnight, so the offset comes out too small. Mount it near bed height in open air.
• Cold soil. Soil warms more slowly than air in spring, and roots in cold, wet soil barely grow. Transplants sit still until the bed warms.
• Summer heat. Very hot days make tomato pollen sterile, so flowers drop without setting fruit. Our garden logged 15 days at or above 86°F this year, with a high of 97°F on August 5.

If a frost alert lands after plant-out, cover the bed before dark and pull the cover off the next morning. Protecting tomato plants from frost lists which covers hold heat and how to anchor them.