How to Grow Vegetables in a Greenhouse | A Beginner’s Routine

Growing vegetables in a greenhouse works when you manage light, temperature, water, and airflow while choosing the right crops.

Growing vegetables in a greenhouse comes down to one thing: giving plants steadier conditions than they’d get outside. The structure traps heat and blocks wind, but it also traps humidity and can bake crops on a sunny afternoon. Get the balance right and you’ll harvest tomatoes, cucumbers, and salad leaves weeks earlier and later than outdoor gardeners. Here’s the routine that works.

Plan Light, Temperature, and Airflow First

Before buying seeds, measure your greenhouse floor and bed space. You need room for staging, a central path at least 60 cm wide, and space for summer crops. Clean the structure before planting — remove old pots, debris, and algae, then wash the glazing, frames, and benches. Check that vents, doors, and seals work properly.

Hobby-greenhouse temperatures run from 50°F to 70°F in the day and 45°F to 55°F at night, with short-term extremes from 35°F to 90°F. Different crops prefer different spots in that range, so group plants with similar needs together.

Two things kill more greenhouse crops than pests: overheating and stagnant air. Open doors and vents on warm days; automatic vent openers help if you’re away during the day. Add shading gradually in hot weather — sudden heavy shade slows growth at first.

Choosing Crops and Starting Seeds

Good first-year greenhouse crops include tomatoes, cucumbers, peppers, aubergines, salad leaves, and herbs. Pick just a handful of crops and a few varieties — keeping too many types is the most common beginner mistake, and each crop wants its own conditions.

Start seeds in clean pots with fresh, peat-free seed compost. Germinate on a sunny windowsill or in a propagator. Here’s the catch with an unheated greenhouse: it often stays too cold for germination and early growth until April. If you start seeds in late winter, use a propagator or keep trays indoors. After germination, move seedlings somewhere light and frost-free, and protect tender plants with fleece on cold nights.

Once seedlings are sturdy and well-rooted, plant them into borders, containers, or growbags. Use containers at least 4 inches deep for shallow-rooted crops like lettuce and 10–12 inches for deeper-rooted crops.

Watering, Feeding, and Supporting Plants

Water early in the day so foliage dries before night — wet leaves overnight invite fungal disease. Seedlings need frequent watering because shallow roots dry fast; established plants need less. Drip irrigation or soaker hoses keep foliage dry and cut water waste. Inconsistent watering is the classic cause of blossom end rot in tomatoes.

Use balanced fertilizer generally, then switch fruiting crops like tomatoes and peppers to a high-potassium feed once flowers appear. Train tomatoes vertically on strings or canes and remove side shoots; support cucumbers on trellises or nets. In an enclosed greenhouse, pollination often needs help — gently shake plants or use a small brush to move pollen.

Crop Management and Common Mistakes

Follow spacing on seed packets — overcrowding reduces airflow and spreads disease. Use yellow sticky traps to catch early pest warnings and order biological controls promptly if you spot trouble.

For winter production, time crops to mature before day length drops below 10 hours. Insulate with row covers, polyethylene over hoops, or water barrels for thermal mass.

The most common greenhouse mistakes are all avoidable: overcrowding plants, watering inconsistently, skipping ventilation on warm days, starting tender seedlings too early in an unheated greenhouse, and skipping supports for vining crops. Succession-sow quick crops like salad greens to keep the greenhouse productive all season.

Once your greenhouse runs smoothly, you’ll want a setup that makes the work easier. Our tested roundup of the best greenhouses for vegetables compares sizes, frames, and glazing to match your space.