When To Plant Tomatoes In Greenhouse

Getting your tomato planting timing right in a greenhouse is the single biggest factor for a successful harvest. If you plant too early, seedlings can struggle with cold and low light. Plant to late, and you miss out on the longest possible growing season. This guide will help you nail the optimal timing for greenhouse planting, so you can enjoy ripe, homegrown tomatoes weeks or even months before your neighbors.

The perfect date depends on where you live, your greenhouse setup, and the varieties you choose. We’ll break it all down into simple steps. You’ll learn how to work with your local climate, prepare your greenhouse, and get those seedlings off to a roaring start.

This is your core rule of thumb. The optimal time to plant tomatoes in a greenhouse is typically 4-6 weeks before your region’s last expected spring frost date. For most temperate climates, this falls in a window from late March to mid-May. The greenhouse protects them from that final frost, allowing you to get a significant head start on the outdoor growing season.

Here’s a quick reference based on general USDA Hardiness Zones:

Zones 3-4: Aim for early to mid-May.
Zones 5-6: Target mid to late April.
Zones 7-8: You can often plant in late March or early April.
Zones 9-10: Planting can begin as early as late February or March, and you may even manage a fall crop.

Key Factors That Influence Your Planting Date

Your local frost date is just the starting point. These three elements will fine-tune your schedule.

1. Your Greenhouse Type and Heating

An unheated greenhouse relies solely on the sun. You must wait until the inside temperature consistently stays above 50°F (10°C) at night. A heated greenhouse gives you much more flexibility, allowing for planting 6-8 weeks before the last frost. Even a simple heat source like a horticultural heater can extend your season dramatically.

2. Nighttime Temperatures Are Crucial

Tomato growth stalls below 50°F (10°C), and plants can suffer chill damage. Consistently warm nights are more important than warm days in early spring. Use a minimum-maximum thermometer to track your greenhouse lows. If your’s dipping to cold, hold off on planting.

3. Tomato Variety Matters

Some varieties are more cold-tolerant than others. Early-season varieties (like ‘Early Girl’ or ‘Glacier’) are bred to handle cooler conditions and can be planted a little sooner. Tender, long-season heirlooms need warmer soil and air, so give them an extra week or two of warmth before transplanting.

Step-by-Step: Preparing for Transplant Day

Don’t just wait for a date on the calendar. Get your plants and greenhouse ready with this checklist.

Start Seeds Indoors: Begin seeds 6-8 weeks before your planned greenhouse transplant date. This means starting them in late winter, often in February or March.
Harden Off Seedlings: About 7-10 days before moving them to the greenhouse permanently, start hardening off. This involves placing them in the greenhouse for a few hours each day, gradually increasing their exposure to the stronger light and cooler nights compared to your house.
Prepare the Greenhouse Soil: Ensure beds or pots are filled with a rich, well-draining mix. Add plenty of compost. Consider warming the soil for a week prior by covering it with black plastic or using cloches.
Ensure Adequate Light: If you’re planting in early spring and experiencing cloudy weather, supplemental grow lights can prevent seedlings from becoming leggy and weak.

What to Do If You Plant Too Early

Sometimes enthusiasm gets the better of us. If a cold snap hits after you’ve planted, don’t panic. Here are some rescue tactics:

Add Extra Protection: Cover young plants with row cover, bubble wrap, or individual cloches at night.
Use Thermal Mass: Place jugs of water in the greenhouse. They absorb heat during the day and release it slowly at night.
Provide Bottom Heat: If plants are still in pots, place them on a seedling heat mat to warm the root zone.
Avoid watering in the evening, as damp soil and cold nights are a bad combination for disease.

Signs Your Greenhouse is Ready for Tomatoes

Nature gives you signals. Look for these indicators that conditions are right:

Daytime greenhouse temperatures reliably reach 70-80°F (21-27°C).
Nighttime temperatures inside the greenhouse do not fall below 55°F (13°C) for several consecutive nights.
The soil temperature at planting depth (4-6 inches) is at least 60°F (15°C). A simple soil thermometer is a great investment.
Your seedlings are stocky, 6-10 inches tall, and have their first flower cluster (called the first “truss”) just starting to form.

Maximizing Your Season with Succession Planting

For a non-stop harvest, don’t put all your tomatoes in at once. Try succession planting. Put in your first, most cold-tolerant varieties at the earliest safe date. Then, 2-3 weeks later, plant your main crop of mid-season varieties. Finally, you can plant a fast-maturing variety in mid-summer for a late fall harvest. This staggers your bounty and spreads out the work.

Another trick is to plant a fast-growing crop like lettuce or spinach around your tomato seedlings early on. You’ll harvest the greens just as the tomatoes need the space to spread out.

Common Timing Mistakes to Avoid

Even experienced gardeners can make these errors. Here’s what to watch out for:

Rushing the Season: Tempting as it is, a few extra weeks of growth isn’t worth the risk of a cold shock that sets plants back for month.
Ignoring Soil Temperature: Cold soil stops root growth dead. Warm air means nothing if the roots are sitting in chilly dirt.
Forgetting to Ventilate: On unexpectedly warm spring days, greenhouses can overheat rapidly, stressing young plants. Always provide adequate airflow.
Neglecting to Monitor Forecasts: Keep a close eye on the long-range weather forecast right before you plant. A predicted week of cold, cloudy weather is a sign to wait.

FAQ: Your Greenhouse Tomato Timing Questions

When is the best time to plant tomatoes in an unheated greenhouse?

The best time is usually 2-4 weeks before your last frost date, but only when nighttime temps inside stay above 50°F (10°C). Always verify with a thermometer.

Can I plant greenhouse tomatoes in February?

In most climates, no. Unless you are in a very warm zone (9+) or have a professionally heated greenhouse, February is too early. Light levels are to low for good growth.

How early can you put tomato plants in a greenhouse?

With heating and grow lights, you can start as early as late winter. But for typical hobby greenhouses, early to mid-spring is the standard. It’s better to have smaller, healthy plants than large, stressed ones.

What is the latest date to plant tomatoes in a greenhouse?

For a main summer crop, planting later than June may not give the plant enough time to mature before fall light and temperatures drop. For a fall crop, choose a quick-maturing variety and plant by mid-summer.

Do greenhouse tomatoes produce all year?

In commercial setups with perfect climate control, yes. In a home greenhouse, production slows or stops in winter due to low light and cold. The goal is to extend the season at both ends, not grow year-round.

By focusing on your local conditions, preparing your space, and listening to the cues your plants and greenhouse provide, you’ll find the perfect planting window. Getting your timing right means stronger plants, fewer problems, and the ultimate reward: that first sun-warmed tomato of the season, picked long before the outdoor vines have even set fruit. With a little planning, your greenhouse can become a tomato paradise.