Plastic Greenhouses, Growing Tunnels, & Cloches

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Plastic greenhouses, growing tunnels, and cloches can give a gardener a head start to the growing season by providing a protective environment for young plants, and a warmer place to germinate seed.

Mini or Plastic Greenhouses

Mini or plastic greenhouses provide a warm and sheltered environment for young plants from late winter onwards. The glass or plastic cover captures the heat of the sun and retains it inside, whilst blocking any chilly winds getting to the plants. The shelves provide an efficient use of space to germinate and grow on a large number of plants.

A mini greenhouse offers less growing space than a standard size greenhouse, but they do have advantages in their own right:

• Most properties with a front or back garden can find space for a mini greenhouse, all they require is a sunny and sheltered position (ideally backing against a wall)
• They are easy to erect
• They are easy to take down
• They do not require a specially constructed base
• For plastic greenhouses, replacement covers are relatively cheap

Mini greenhouses come at different price points, with plastic greenhouses being most affordable compared to the more durable wooden framed glass growhouses.

From personal experience, plastic greenhouses can suffer from:

• Wind damage, especially when the wind gets underneath the cover, or the door is left open during storming weather
• The zip doors are a potential weak spot (a gardener needs to be careful to not break the zip when opening and closing)
• The cover needs to be UV stabilised (otherwise sunlight will cause the cover to fall apart)
• Some 'flat roof' designs suffer from water pooling on top of the plastic (the weight of the water can damage the plastic and supports)
• The strength of the racking and frame can be of variable quality

The usable life of a mini-greenhouse can be extended by:

• Purchasing greenhouses made of more durable materials (treated wood or galvanised steel)
• Positioning the greenhouse in a sheltered spot from the wind (and securing to a wall)
• If using a budget plastic greenhouse, taking it down and packing it away when not in use

Growing Tunnels

Growing tunnels provide protection for young plants in their final growing position, saving the time and effort of transplanting seedlings grown in pots. Plants benefit by avoiding any disturbance to their roots.

Seed is sown along a row and covered by a growing tunnel, which provides warmth and protection to aid germinate, and continues to protect the seedlings as they grow. Not all vegetables can be grown this way, as the plants need to be reasonably hardy to survive late winter and spring frosts. Early sowings of peas and broad beans benefit from this method.

There are several types of covering for a growing tunnel, each with their own advantages:

Plastic or polythene covers

These offer the warmest environment for seedlings and are ideal for spring germination in cold soil. The plastic traps the heat inside the tunnel, warming the soil and providing an ideal environment for germination. The narrowness of the growing tunnel means that there is no need to worry about watering the seedlings, as the soil will naturally stay moist from rain on the surrounding ground. The biggest risk will come with the onset of warmer weather, when the hot sun may potentially scorch the young plants.

Fleece covers

Fleece growing tunnels are not as warm as plastic growing tunnels, but they have their own advantages. They will retain enough heat during the night to protect young seedlings from frost damage, whilst on hot days the air flow will take the edge off scorching sunshine (compared to a plastic cover). The air flow helps keep the plants healthy and, if properly secured around the edges, will form an effective barrier against insect attack.

Net growing tunnels

These growing tunnels are more for protection than for extending the growing season. Netting comes in many grades and varieties depending upon its desired use, but on a growing tunnel its most frequent use is to protect against caterpillar or bird attack during late spring and summer growth. Netting will allow you to water the plants during the growing season, and allow the plants to receive their full quota of sunshine.

In addition to the type of cover used, a gardener needs to consider the height of the growing tunnel, especially if the intention is to keep the crop covered throughout the full lifecycle (rather than just when plants are small in spring). Brassicas like kale, cabbage, and cauliflower, may be grown inside a growing tunnel covered by butterfly netting (or insect mesh), to stop butterflies laying their eggs on the leaves, that later turn into caterpillars.

Plastic Cloches

These are ideal for individual plants that need to be spaced far apart when fully grown, with the cloche protecting them in the early stages of growth. Crops like squash, courgette, melon, and even tomatoes may benefit from growing under a cloche in spring.

Plastic cloches can be very useful:

• They provide protection against young plants being eaten by slugs and snails
• They reduce the risk of heat sensitive plants being killed by a late frost
• They stop wind burn (wind blowing cold air over the leaves)

Courgettes and squash are particularly susceptible to a late frost or an attack from slugs and snails. I find that a cloche can provide an extra two to four weeks protection in April and May. In June, a frosty morning is much less likely, and when the plants have larger leaves, they are less desirable to slugs and snails.

It is important to make sure that the bottom edge of a cloche is covered with soil all around the base. Without this, strong winds can blow them off the plot!

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