Vegetables: growing in your greenhouse

A greenhouse is suitable for growing vegetables in a number of ways:

• Starting off hardy vegetable plants earlier than outdoors
• Getting earlier harvests of tender plants such as French beans
• Growing tender crops such as aubergines, cucumbers, peppers and chillies and tomatoes through the summer months
• Trying more exotic, heat-loving plants such as okra, melons and sweet potatoes, 
• Making use of the autumn sun to raise late salad crops, French beans and even calabrese

A productive greenhouse can be in use for most of the year. Heated greenhouses allow for maximum, year-round use but are rarely cost effective.

Late winter to early spring

• Sow hardy plants such as Brussels sprouts, cabbage, celeriac, early leeks, lettuce, onions and peas for planting out when the warmer weather arrives. A heated propagator will help ensure germination
• In heated greenhouses, tomatoes, peppers and other tender plants can be sown early in a propagator

Mid-spring

• Sow fast-growing tender plants such as courgettes, squashes and pumpkins, cucumbers, French beans, melons and sweetcorn so they are ready for planting in their final positions under glass in late spring or outside in early summer. A heated propagator will help ensure germination
• Buy ready-grown pepper and tomato plants for introducing to unheated greenhouses
• Sow basil for growing on indoors or moving outside in summer

Late spring to early summer

• Plant summer greenhouse plants into their final positions indoors
• Harden off and plant out young plants of outdoor crops once the frost has passed

Mid-summer

• Harvest summer crops
• Remove spent cucumber, French bean or melon crops (these can be replaced by later sown plants)
• Sow calabrese, French beans and parsley outside for bringing in when summer crops have finished

Late summer

• If space allows, sow lettuces, spicy salad leaves and baby carrots indoors to make use of the autumn sun and provide late harvests
• Plant new potatoes for Christmas crops in heated greenhouses

Autumn

• Remove spent summer crops
• Plant out lettuces into beds or growbags to harvest over winter
• Plant calabrese, French beans and parsley seedling to mature indoors
• Sow broad beans and peas to over-winter and be planted out once clay soils are more manageable in early in spring
• Sow hardy lettuces cultivars and pea shoots to be harvested indoors in spring
• Move herbs in pots indoors to keep them cropping longer

1. Planning

• Measure out your greenhouse beds and floor space for growbags. Check you have room to space out all the summer greenhouse crops you plan to grow
• Benches should provide plenty of space for seedlings, many of which will be moved outside when the space is needed for summer greenhouse crops
• A catch crop of salad leaves can be sown in greenhouse borders before the space is needed for summer greenhouse crops

2. Sowing seed indoors

• Use clean pots and trays and fresh, peat-free seed or multipurpose compost
• Follow instructions on the seed packet
• Seeds will germinate on a sunny windowsill indoors or a heated propagator unit in the greenhouse (be aware that in cold weather, in unheated greenhouses, the propagator may struggle to maintain warm temperatures)

3. Growing on

• Once germinated, seedlings will need somewhere light and frost-free to grow on; an unheated greenhouse may not be sufficiently warm until April
• Consider fleecing and heating a partitioned section of your greenhouse to make a suitable environment for growing on tender plants
• Watch weather forecasts and be prepared to protect young plants with fleece on frosty nights or provide supplementary heating when needed

4. Planting

• Plant protected crops into their final positions as soon as they are sturdy and well-rooted
• Plant into greenhouse borders, containers or growing bags
• Growbags can be used for autumn salads – remove a long panel from the top to create a shallow bed
• Ensure climbing plants such as cucumbers and melons have sufficient support and tie cordon tomatoes into strings or canes

5. Summer maintenance

• Check watering daily or install irrigation; uneven watering can result in problems such as blossom end rot in tomatoes
• Ventilate greenhouses on warm days by opening doors and vents; automatic ventilation is ideal
• For warmth-loving okra and cucumbers, vents can be kept closed but humidity must be raised by damping down. Alternatively, partition off a section of the greenhouse with fleece or clear plastic
• Some shading will be necessary; it is best to add this gradually, as it will initially reduce growth
• Hang yellow sticky traps to provide early warning of pests. Biological controls can then be ordered promptly
• Tie new growth into supports regularly; pinch out side-shoots of cordon tomatoes

6. Winter maintenance

• Keep structures, especially glazing, clean
• Fit insulation – bubble wrap is sufficient for many structures
• In heated greenhouses, ensure a thermostat is working to maintain a minimum night-time temperature
• Invest in a minimum/maximum thermometer to monitor conditions

When growing in greenhouse borders, reduce the build-up of soil borne diseases by digging out the soil every three to five years and replacing it with bought-in top soil or good garden loam. Grafted plants may help where soil problems are suspected (aubergines and tomatoes are now available).

Large pots and growbags are an alternative where soil is suspect. Cover soil with plastic sheeting, ideally white, and place pots or bags on this. Potting media including growbags can be re-used at least once for different crops if no diseases are present.

Links

Damping off
Grey mould
Glasshouse red spider mite
Glasshouse whitefly
Glasshouse leafhoppers
Powdery mildew

Greenhouse choosing
Greenhouse: heating efficiency
Greenhouse: ventilation and shading
Raised beds
RHS video: vegetable seed sowing
Vegetables for exhibition