Vegetables: growing in your greenhouse

A greenhouse is a great asset to any vegetable plot, enabling gardeners to make the most of the sun. Even the smallest, unheated structure will allow gardeners to extend the seasons and produce good crops of a wide range of vegetables.

Growing vegetables in your greenhouse

Quick facts

Suitable for A wide range of vegetables
Timing Spring to autumn
Difficulty Moderate

Suitable for...

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

When to use a greenhouse to grow vegetables

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

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

Late summer

Autumn

How to use a greenhouse to grow vegetables

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

Problems

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.

Diseases to watch out for include damping off, grey mould and powdery mildew. Commons pests under glass include glasshouse red spider mite, glasshouse whitefly and glasshouse leafhoppers.

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