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Propagating Cases and Mist Propagation


Propagating Cases
The simplest way to maintain high humidity for greenhouse propagation is by means of a plastic-topped case or frame. These can be bought from any garden centre and come in a range of sizes and shapes. Some cover just one seed tray or pot; others cover several trays. And for potted cuttings, there are deep cases in the form of miniature greenhouses.

It may seem odd to put a miniature greenhouse within a green house, but unless you are prepared to hose down the floor and staging every few hours, or invest in a more sophisticated humidifying system, this is the best method, especially for small amounts of cuttings or seeds.

As a money-saving compromise, you can use ordinary plastic boxes designed for packing sandwiches or for presenting and storing food. Those with transparent lids are excellent for raising seedlings.

For individual pots of cuttings or seeds, the simplest covering is a transparent plastic bag. This must be large enough to fit over the top of the pot without touching the plant leaves. A few sticks or hoops of wire inserted in the compost will hold the bag upright.

Once rooting has occurred, high humidity is less essential, so remove the cover to let more air reach the leaves and prevent fungal diseases getting a hold. With the more elaborate propagating cases, this can be done in easy stages with the aid of one or more adjustable vents in the cover. If you are using a plastic bag as a cover, slice open one side of the bag at first, and remove the bag entirely only after a few days of acclimatization.

Heated cases have a heating element incorporated in the plastic base or in separate heating trays.

You will need a safely installed supply of mains electricity in the greenhouse if you choose this type.

Small electrically heated cases have a built-in pre-set thermostat which is set to a moderate temperature suitable for the propagation of most plants. Larger cases have an adjustable thermostat. Consider carefully your needs before you buy — the larger cases often give more value for money and provide much more scope for raising a selection of plants.

There is no need to operate the case at high heat. Most seeds will germinate at a maximum temperature of 18°C (64°F) and cuttings root readily at around 21°C (70°F). Excessive temperatures may even inhibit seed germination.

To prevent scorching of the plant material, all propagating cases should be shaded lightly during periods of direct sunlight. 

Mist propagation

Intermittent mist spraying is an expensive but more efficient alternative to keeping cuttings or seedlings in a closed case or in a bag. A thin film of moisture is maintained over the leaf surfaces using a water jet controlled automatically by an electronic sensor.

With this method, propagation can be carried out in full sunlight or with just very light shading, and the normal processes of plant growth and food production within the leaves can take place at maximum efficiency.

Heat from underneath the cuttings or seedlings — known as bottom heat — is usually needed to balance the cooling effect of moisture passing constantly through the compost and to encourage healing, callus formation and rooting of cuttings.

With mist propagation, rooting is more rapid and certain — especially with species that are difficult by other means. Also there is less risk of disease because spores of grey mould and other fungal organisms are washed away before they do any damage.

However, cuttings rooted by mist propagation need to be weaned gradually to drier air if they are to grow vigorously without a check.

Small mist units, with a transparent surrounding screen to keep the mist within bounds, are available, but cost more than a heated propagator.

Alternatively, you can build your own mist unit. In addition to basic materials for making the case, you will need water pipes, mist nozzles, heating cables, a thermostat and humidity sensor, and a control unit. Seek advice from a specialist supplier and from a qualified electrician before starting such a project.