When To Plant Seeds In Greenhouse

Getting the timing right for when to plant seeds in greenhouse is the single biggest factor for your success. It’s the difference between strong, healthy seedlings and weak, leggy plants that struggle. This guide will help you nail the optimal schedule for sowing, no matter where you live.

Your greenhouse is a powerful tool. It extends your season dramatically. But it’s not magic. You still need to work with nature’s rhythms, just on a more forgiving timeline. Let’s break down how to find your perfect planting dates.

This isn’t a single date. It’s a moving target based on two key things: your local last spring frost date and the needs of each specific plant. Your last frost date is your anchor. You can find it with a quick online search for your zip code or by asking at a local nursery.

Once you have that date, the greenhouse gives you a head start. Here’s the basic rule:

Frost-Tender Plants (like tomatoes, peppers, cucumbers): Start these 6-8 weeks before your last frost date. The greenhouse protects them from late cold snaps.
Hardy Plants (like kale, broccoli, lettuce): These can be started even earlier, 8-10 weeks before the last frost. Some can even be grown all winter in a cool greenhouse.
Very Tender & Summer Crops (like melons, basil): Start these 4-6 weeks before the last frost. They grow fast and don’t like being pot-bound.

Understanding Your Greenhouse Microclimate

Not all greenhouses are the same. A heated greenhouse is very different from an unheated one. Most home gardeners use an unheated or “cold” greenhouse, which relies on the sun. This is what we’ll focus on.

An unheated greenhouse can be about 5-10 degrees Fahrenheit warmer than the outside air. It protects from wind and frost, but it can still get very cold at night. You need to monitor temperatures closely, sometimes with a simple max-min thermometer.

Essential Tools for Perfect Timing

A Soil Thermometer: Seed germination depends on soil temp, not air temp. This is crucial.
A Calendar: Mark your last frost date and count backwards for each crop.
Seed Packets: They contain goldmine of timing info for that variety. Always read them.
Insulation: Bubble wrap or horticultural fleece for unexpectedly cold nights.

A Month-by-Month Sowing Guide (Temperate Climates)

This is a general guide. Adjust for your specific zone.

Late Winter (January – February)

Focus on hardy greens and slow-starters. The days are short, so growth is slow. Use a heated propagator for best results if your greenhouse is very cold.

Sow: Onions, leeks, celery, early lettuce, spinach, kale.
Tip: Provide as much light as possible. Clean greenhouse windows to maximize sunlight.

Early Spring (March)

This is the busiest sowing time. Light levels improve, and temperatures begin to rise more reliably.

Sow: Tomatoes, peppers, aubergines (eggplant), broccoli, cabbage, cauliflower, more lettuce, herbs.
Tip: Be prepared to pot on seedlings quickly as they grow. Don’t let them get crowded.

Mid-Spring (April)

You can start direct sowing some crops into greenhouse borders if the soil is warm enough. Check with your soil thermometer.

Sow: Courgettes (zucchini), cucumbers, melons, squash, beans, sweetcorn.
Tip: Ventilate on sunny days to prevent overheating and dampening off disease.

Step-by-Step: How to Sow Seeds in Your Greenhouse

Clean & Prepare: Start with clean trays and fresh, sterile seed-starting mix. This prevents disease.
Moisten the Mix: Dampen your mix before filling trays. It should feel like a wrung-out sponge.
Sow & Label: Sow seeds at the depth recommended on the packet. Label every tray immediately—you will forget what you planted.
Water Gently: Use a fine mist or a watering can with a rose to avoid disturbing the seeds.
Provide Warmth: Place trays in a warm part of the greenhouse or on a heat mat until they germinate.
Maximize Light: As soon as seeds sprout, move them to the brightest spot. Rotate trays regularly for even growth.
Pot On: When seedlings get their first set of true leaves, transplant them into individual pots.
Harden Off: About 7-10 days before planting outside, begin hardening off plants by exposing them to outdoor conditions for longer periods each day.

Common Timing Mistakes to Avoid

Even experienced gardeners make these errors. Here’s how to sidestep them.

Sowing Too Early: The biggest mistake. Overly mature seedlings become root-bound and stressed before they can go outside. They won’t perform as well.
Ignoring Soil Temperature: Seeds like tomatoes need warm soil (65-70°F) to germinate. Sowing in cold soil leads to rot or no germination at all.
Poor Ventilation: Still, damp air encourages fungal diseases like damping off. Good air flow is non-negotiable.
Insufficient Light: This causes leggy, weak seedlings. If natural light is low, consider a simple grow light for your seedling area.

Extending the Season: Fall and Winter Sowing

Your greenhouse isn’t just for spring. In late summer, you can sow crops for fall and winter harvest.

In August and September, sow hardy greens like spinach, mizuna, winter lettuce, and corn salad. These will grow slowly through the cooler months, providing fresh greens long after the summer garden is done. A little bit of fleece protection on the coldest nights is often all they need.

FAQ: Your Greenhouse Sowing Questions Answered

How early can I start seeds in an unheated greenhouse?
It depends on the plant’s cold tolerance. Hardy plants like kale can be started 10+ weeks before last frost. For tender plants, wait until you can consistently maintain the required soil temperature, usually 4-8 weeks before last frost.

What is the best month to plant seeds in a greenhouse?
For most gardeners, March is the peak month for starting the widest variety of common vegetable seeds. It offers the improving light and temperature conditions most seeds need.

Can you sow seeds directly in a greenhouse bed?
Yes, for crops that don’t transplant well or that you want to grow to maturity inside. This includes things like carrots, radishes, and salad greens. Just ensure the bed soil is warm, fertile, and well-draining.

How do I keep my greenhouse warm at night for seedlings?
Use thermal mass like water barrels that absorb heat during the day and release it at night. Insulate with bubble wrap. For critical seedlings, a small horticultural heat mat under the trays is very effective and energy-efficient.

What seeds should I start first in greenhouse?
Start with your slow-growing, long-season crops first. These are typically onions, leeks, celery, and certain flowers like geraniums. They need the extra time to develop before transplanting.

Getting your timing right is a skill that improves each year. Keep notes on what you planted when and how those seedlings performed. This personal record will be your most valuable tool for nailing the optimal timing in your unique greenhouse, season after season.