Plant tomatoes in Pennsylvania after the last spring frost, which falls between mid-May and early June depending on your location in the state

Pennsylvania's frost dates divide the state into zones. The southern counties around Philadelphia and Pittsburgh have their last spring frost around mid-May. Central Pennsylvania (around State College) sees frost until late May. Northern counties near the New York border may not be frost-free until early June. Tomatoes planted before the soil warms will sit in cold ground and rot before they germinate. Waiting until after your local frost date gives seedlings the warm soil they need to establish roots and grow.

You can start seeds indoors 6 to 8 weeks before your frost date if you want to transplant larger seedlings outdoors. That means starting seeds indoors in late March or early April for most of Pennsylvania. Seedlings started this way will be ready to move outside right around your frost date, when the soil has warmed enough to support them.

Key Takeaways

  • Southern Pennsylvania's last frost is mid-May; central and northern regions see frost until late May or early June.
  • Soil temperature matters more than air temperature—wait until soil reaches 60°F before planting, ideally 65–70°F for faster growth.
  • Start seeds indoors 6 to 8 weeks before your local frost date if you want to transplant seedlings rather than direct-sow seeds.
  • Pennsylvania's growing season is long enough for both early-season and late-season tomato varieties, and fall planting is possible in southern counties.

Finding Your Exact Frost Date in Pennsylvania

Your county extension office has your precise frost date. Penn State's College of Agricultural Sciences publishes frost date maps by county, and your local extension office can tell you the average date for your area. Search "Penn State Extension" plus your county name to find the office phone number or website. They can also tell you the frost date for your specific neighborhood if you live in a microclimate—a valley, hilltop, or area near water that warms or cools differently than surrounding land.

If you cannot reach your extension office, the USDA Hardiness Zone map (planthardiness.ars.usda.gov) shows your zone by zip code. Pennsylvania spans zones 5a through 7a. Once you know your zone, you can search "last frost date zone [your number]" to find the typical date range. This is less precise than your county extension office, but it gives you a working date to plan from.

Soil Temperature: The Real Trigger for Planting

Air temperature and soil temperature are not the same. A warm spring day can fool you into planting too early if the soil is still cold. Tomatoes need soil at least 60°F to germinate and grow without rotting. Ideally, wait for 65–70°F soil for faster, healthier growth. You can measure soil temperature with a straightforward soil thermometer from any garden center—push it 4 inches into the ground in the morning and read it after 5 minutes.

In most of Pennsylvania, soil reaches 60°F around the time of your last frost date, but not always. A cold, wet spring can delay soil warming by a week or two. If you plant seedlings into 50°F soil, they will sit dormant and may rot before they grow. If you wait for 65°F soil, you may plant a week or two after your frost date, but your plants will take off when ready and catch up to earlier plantings by mid-summer.

Starting Seeds Indoors vs. Direct Sowing

Most Pennsylvania gardeners start tomato seeds indoors because the growing season is just long enough to reach full maturity before frost. Starting seeds indoors 6 to 8 weeks before your frost date gives you transplants ready to go into the ground right when soil warms. This method works well for determinate varieties (bush types that ripen all at once) and indeterminate varieties (vining types that produce all season).

Direct sowing seeds outdoors is possible but slower. Seeds planted directly in the garden after your frost date will germinate in warm soil and grow into productive plants, but they will start producing fruit later than transplants. If you live in southern Pennsylvania with a frost date around May 15, direct-sown seeds will still mature before the first fall frost (usually around October 10). If you live in northern Pennsylvania with a frost date near June 1, direct sowing cuts your season short—transplants are the safer choice.

Fall Planting for a Second Harvest

Southern Pennsylvania (zones 6b and 7a) can support a fall tomato crop. Plant transplants in late July or early August so they mature before the first fall frost, which typically arrives in early to mid-October. Fall tomatoes grow in cooling weather, so they mature more slowly than spring plantings, but they often taste sweeter because nighttime temperatures drop and sugars concentrate in the fruit.

Fall planting requires transplants, not seeds—there is not enough time for seeds to germinate and grow to maturity. Buy transplants from a garden center in late July or start your own seeds indoors in late May or early June. Central and northern Pennsylvania rarely have time for a fall crop because the first frost arrives too early, but it is worth trying in a sheltered spot if you want to experiment.

Hardening Off Seedlings Before Transplanting

Seedlings started indoors are tender and will wilt or die if moved straight from a warm house to outdoor sun and wind. Harden off seedlings by gradually exposing them to outdoor conditions over 7 to 10 days before planting. Start by placing them in a shaded, sheltered spot outdoors for 2 to 3 hours. Each day, increase their time outside and move them into brighter light. By day 7 or 8, they should spend the full day outside. On the final day or two, leave them out overnight if temperatures stay above 50°F.

Hardening off takes time but prevents transplant shock, which can stunt growth or kill young plants. If you skip this step and plant tender seedlings directly into full sun, they will look pale and droopy for weeks while they recover. A week of gradual adjustment means your transplants will grow vigorously from the moment they go in the ground.

Protecting Early and Late Plantings from Frost

If you plant transplants right at your frost date and a late frost arrives, row covers or frost cloth will protect them. Drape the cloth over plants and anchor it to the ground with soil, rocks, or garden staples. Remove the cover during the day if temperatures rise above 75°F so plants do not overheat. A late frost in May is uncommon in southern Pennsylvania but happens regularly in central and northern regions, so keep frost cloth on hand if you plant early.

For fall plantings, watch the forecast in September and October. When temperatures drop toward 32°F, cover plants overnight. Remove the cover in the morning. This straightforward step can extend your harvest by 2 to 4 weeks and let late-season fruit ripen fully before hard frost kills the plant.

Frequently Asked Questions

Can I plant tomatoes in April in Pennsylvania?

Not safely. Soil is still too cold in April across most of Pennsylvania, and seedlings will rot before they grow. Wait until late May or early June when soil reaches 60°F or warmer. Starting seeds indoors in March or April is fine—just do not move them outside until after your frost date.

What if I plant too early and frost kills my seedlings?

You will need to replant. If you have frost cloth, cover plants when frost is forecast and you may save them. If plants freeze and die, pull them out and plant new transplants once soil warms again. This delays your harvest by a few weeks but is better than losing the whole crop to rot from cold soil.

Do I need to wait for the last frost date if I use row covers?

Row covers protect plants from frost but not from cold soil. Soil still needs to reach 60°F for roots to grow. You can plant under row covers a week or two before your frost date if soil is warm enough, but checking soil temperature is more important than the calendar date.

Can I plant tomatoes in June in Pennsylvania?

Yes, but later plantings will produce less fruit before the first fall frost. A June planting in southern Pennsylvania will still mature and produce, but you will harvest fewer tomatoes than a May planting. In northern Pennsylvania, a June planting may not reach full maturity before frost arrives in early October.

What is the difference between determinate and indeterminate tomatoes for Pennsylvania planting?

Determinate varieties grow as bushes and ripen most fruit at once, making them good for canning. Indeterminate varieties grow as vines and produce fruit all season until frost. Both grow well in Pennsylvania. Indeterminate varieties need more time to mature, so they benefit from early planting in northern regions where the season is shorter.