When Do You Plant Tomato Plants In A Greenhouse

If you have a greenhouse, you already have a massive advantage for growing tomatoes. The key to making the most of it is timing. Knowing exactly when do you plant tomato plants in a greenhouse is the first step to a huge, healthy harvest.

Getting this date right means your plants will be strong and ready to produce fruit for months. It protects them from outdoor frosts and gives you a much longer growing season. Let’s look at how to find that perfect planting day for your specific situation.

There is no single calendar date that works for everyone. The best time depends on two main factors: your local climate and the type of greenhouse you have. A heated greenhouse offers different options than an unheated one.

For most gardeners, the goal is to plant seedlings into the greenhouse soil (or their final pots) after the last spring frost date for your area. The greenhouse itself will be warmer than the outside air, allowing you to plant several weeks earlier than you could in the garden.

Timeline for an Unheated Greenhouse

An unheated greenhouse relies on the sun to warm it. It protects from wind and frost, but nighttime temperatures can still drop low.

Sow Seeds Indoors: Start your tomato seeds indoors 6-8 weeks before you plan to move them to the greenhouse. For many regions, this is late February to mid-March.
Harden Off Seedlings: About 7-10 days before transplanting, begin hardening off your plants. This means exposing them to the greenhouse environment for a few hours each day to get them use to the different conditions.
Transplant Date: Move your young plants into the greenhouse 2-4 weeks before your last expected outdoor frost date. The soil inside should be consistently above 50°F (10°C) for good root growth. A simple soil thermometer is your best friend here.

Timeline for a Heated Greenhouse

With a heat source, you have much more flexibility and can start significantly earlier.

Sow Seeds: You can sow seeds directly in the greenhouse bench or in pots inside the greenhouse in late winter, as long as temperatures are kept above 60°F (15°C).
Transplant: You can transplant seedlings into their final positions 6-8 weeks before the last frost, often in early to mid-February for a spring crop. This allows for extremely early fruit set.

Signs Your Greenhouse is Ready for Tomatoes

Don’t just go by the calendar. Check these conditions before you plant:

Nighttime Temperature: It should reliably stay above 45°F (7°C). Tomatoes will not grow well and can be damaged if its colder.
Soil Temperature: As mentioned, aim for 50-60°F (10-15°C) minimum. Cold soil shocks roots and stunts growth.
Daytime Ventilation: Make sure you can manage heat on sunny days. Temperatures soaring above 90°F (32°C) can stress young plants.

Step-by-Step Guide to Planting in Your Greenhouse

Once your timing and conditions are right, follow these steps for success.

1. Preparing the Greenhouse Soil

Tomatoes are heavy feeders. Don’t just use old soil from last year.

Mix in plenty of well-rotted compost or manure.
Add a balanced, slow-release organic fertilizer according to package instructions.
Ensure the soil is loose and well-draining. Raised beds or mounds can help with this.

2. The Transplanting Process

Water your seedlings thoroughly an hour before transplanting.
Dig a hole deep enough to bury the stem up to the first set of true leaves. Tomato stems develop roots all along them, creating a stronger plant.
Gently remove the plant from its pot, teasing the roots apart if they are pot-bound.
Place the plant in the hole and fill it with soil, firming gently around the base.
Water well immediately after planting to settle the soil around the roots.

3. Spacing for Good Health

Crowding plants is a common mistake. Good air flow prevents disease.

Space determinate (bush) varieties 18-24 inches apart.
Space indeterminate (vining) varieties 24-36 inches apart, or more if you can.

Essential Care After Planting

Your job isn’t done after planting. Consistent care leads to optimal growth.

Watering and Feeding

Greenhouse tomatoes need regular watering since they don’t get rain.

Water deeply at the base of the plant, avoiding the leaves, early in the day.
Let the soil surface dry slightly between waterings to prevent root rot.
Begin feeding with a high-potassium tomato fertilizer once the first fruits are set, following the product’s schedule.

Temperature and Humidity Control

This is the biggest daily task in a greenhouse.

Ventilate on all but the coldest days to reduce humidity and prevent fungal diseases like blight.
Use shade cloth in peak summer to prevent scorching.
Consider a small fan to improve air circulation, which strengthens plant stems and reduces pest issues.

Support and Pruning

All tomatoes need support, especially vining types.

Install tall stakes, trellis, or strings at planting time to avoid damaging roots later.
As plants grow, tie them loosely to the support every 6-8 inches.
For indeterminate varieties, regularly pinch out the side shoots (suckers) that grow in the leaf joints. This directs energy into fruit production.

Common Problems and Solutions in Greenhouse

Even in a controlled environment, issues can arise. Here’s how to handle them.

Poor Fruit Set: If flowers aren’t turning into fruit, it’s often due to high heat or lack of pollination. Ensure good ventilation and gently shake flowering branches mid-day to distribute pollen, or introduce a small fan.
Blossom End Rot: A dark, leathery patch on the fruit bottom is usually caused by irregular watering which leads to calcium uptake issues. Maintain consistent soil moisture.
Whiteflies & Aphids: These pests love greenhouses. Check leaf undersides regularly. Use yellow sticky traps and introduce beneficial insects like ladybugs for control.

FAQs: Greenhouse Tomato Planting

What is the earliest you can plant tomatoes in a greenhouse?

In a reliably heated greenhouse, you can plant as early as late January for some varieties. In an unheated one, wait until soil temperatures are warm enough, typically 2-4 weeks before your last frost.

Can you grow tomatoes in a greenhouse all year?

In mild climates with sufficient winter light and heat, you can grow tomatoes year-round. For most, it’s best to grow a summer crop and then a separate, late-started autumn crop for winter harvests.

Do greenhouse tomatoes need pollinating?

Yes. Since bees and wind are limited, you need to assist. Gently vibrate flower trusses with your hand or an electric toothbrush around midday when humidity is lower.

What are the best tomato varieties for greenhouse growing?

Indeterminate (vining) varieties like ‘Sungold’, ‘Beefsteak’, and ‘Brandywine’ are excellent as they produce over a long season. Disease-resistant hybrids like ‘Shirley’ are also a great, reliable choice.

Mastering the timing of when to plant is the foundation of greenhouse tomato success. By paying attention to your specific conditions and providing consistent care, you can enjoy a bountiful harvest that lasts for months on end. There’s nothing quite like picking a sun-warmed, ripe tomato from your own greenhouse in early summer.