There are several simple ways of growing new greenhouse plants from existing ones, and some plants respond to one method better than another. However, softwood cut¬tings and division are the easiest methods. They also produce new plants identical to the parent plant.
In all cases, make sure that you propagate only from completely healthy plants. Never use plants with yellow or mottled foliage, deformities or striped or distorted flowers – they may be suffering from virus diseases, and these will be passed on to the cuttings too.
Also, always avoid taking cuttings from plants that you know to have been affected by pests of any kind, as they may fail to root.
Softwood cuttings – an easy method
One of the most popular and easy methods of propagating greenhouse plants is by softwood cuttings. This is used for such widely grown plants as fuchsias, regal pelargoniums and zonal pelargoniums (or geraniums). It can be done at any time except winter, but spring is best.
Choose a healthy shoot without flowers or flower buds. Cut off a few inches with a sharp knife, cleanly and immediately below a leaf joint.
Gently pull or cut away the lower leaves and any stipules, or ‘whiskers’.
Dipping the base of each cutting in hormone rooting powder can hasten rooting and make it more certain, but is by on means essential. Fill a 2 1/2or 3 in. pot with John Innes seed compost or a compost of equal parts peat and sand. The compost should be moist.
Press the cutting gently into the compost to just below the lowest leaf. If you are taking more than one cutting, several can be put round the side of a slightly larger pot.
Now cover the pot to keep the plant in a moist, warm atmosphere and shade it to prevent loss of moisture through its leaf surfaces. Special pots with a transparent dome are available for this purpose. Alternatively, ordinary pots or seed trays can be placed in a closed propagating case, or a polythene tent can be constructed over each pot or tray. Do this by inserting bent lengths of wire into the compost, the ends next to the rim of the container, and putting a plastic bag or sheet of polythene over them. Tuck the polythene in under the base. The pots can be put on greenhouse staging but away from strong light.
Cuttings from many easily grown pot plants can also be rooted in polythene bags. Place a little compost at the bottom of the bag, insert the cuttings in this, seal the top of the bag and hang it in a warm place – in a kitchen would be ideal.
With all methods, keep the compost moist by spraying occasionally with a fine mist of water.
Bottom heat is essential for the rooting of some warm greenhouse plants. This is best provided by soil-warming cables in a greenhouse propagator.
The formation of top growth is a sign that rooting has occurred. This should take about three weeks. The plants must then be potted on into 3 1/2 in. pots of John Innes No. 1 or 2 compost or a proprietary potting compost.
Most young plants from rooted sunlight even if they are sun lovers. Full light should be given to them only very gradually as their roots develop.
Keep the compost moist, but not wet – waterlogging may prevent formation of roots in many cases, although some plants, such as Impatiens sultanii (busy lizzie), will root simply by putting cuttings in a glass of clean water.
Leaf cuttings for popular pot plants
Many widely grown pot plants, such as foliage begonias, saintpaulias, streptocarpus, gloxinias and sanse- vierias, can be propagated from leaf cuttings in various ways.
One method, often successful with begonias, is to pick a leaf and remove the stalk, then carefully cut across the veins in several places.
Fill a seed tray with seed compost and lay the leaves flat on the surface of the compost.
Weigh them down with stones, or peg with bent wire to keep the veins in contact with the soil. Place a sheet of glass or polythene over the tray to retain moisture, and put the covered tray in a warm propagator at a temperature of 18-21°C (64-70°F). After a few weeks, roots should form where the slits were made. Each rooted part can then be separated and potted individually in 3 1/2 in. pots of potting compost. Put them on greenhouse staging, shaded from strong light for a few days.
Another method, used for begonias and peperomias, is to cut the leaves into small triangular sections with a piece of the stalk at the apex of each. The triangles are inserted apex down in the compost, covered and treated as before.
The long leaves of plants such as sansevierias and streptocarpus can be cut into sections about 2 in. long. Each section is inserted vertically in the compost.
The best method for saintpaulias, and other small-leaved plants is to cut off leaves with 1/2-1 in. of stem.
Insert the stalks individually in a in. pot, or three in a 3 1/2 in. pot, of seed compost so that the base of each leaf just touches the compost. Cover the containers and place them in warmth. Bottom heat of 16-18°C (61-64°F) will produce rooting in 10-14 days, and new growth should appear above the surface about one month after insertion.
Intermittent mist propagation is a more expensive but more efficient alternative to keeping cuttings 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 leaf element or detector unit. This switches itself on only when the cuttings begin to dry.
With this method, propagation can be carried out in full sunlight, and the normal processes involved in plant food formation – known as
A mist unit with surround
photosynthesis – can take place.
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.
With mist propagation rooting is more rapid and certain, and cuttings, that with other methods root with difficulty only or not at all, usually succeed. Also there is usually less disease (particularly grey mould), because spores are washed away before they can do any damage.
The only drawback to mist propagation is that cuttings rooted by this method are more tender than those raised under glass in the ordinary way and need to be hardened off with great care.
Small misting units, with a surround to keep the mist within bounds, can be bought for slightly more than a heated propagator.
Increasing perennial pot plants by division
Division is an easy way of propagating perennial greenhouse plants. It should be done in early spring, before new season’s growth begins.
Tap the plant, with its compost, out of its pot. If the compost is dry and difficult to dislodge, water it.
Examine the top of the plant to discover where stems or tufts of growth arise. With a very sharp knife, cut firmly down through the plant between the tufts of growth.
It does not matter if roots are not cut cleanly, but the less damage done to them the better. Old or dead roots, or those that arc damaged, can be cut away.
Pot each segment separately in potting compost. Water the plants moderately and keep them in shade for a few days. Most plants will flower the same year.
Many greenhouse bulbs, including hippeastrums, form offsets – small bulbs that sprout from the parent bulb. They can be pulled away during repotting and potted individually in 3-5 in. pots. Use the same compost and the same heat as for the adult plants. It is often necessary to grow these small bulbs for two years or more until they have reached flowering size.
1. In early spring cut down through the plant between the tufts of growth
2. After removing old or dead roots, pot each segment separately
Some other methods of increasing plants
Layering is an effective method of propagating some climbing and trailing plants. Bend down a branch so that its stem touches some seed compost placed in a 3-4 in. pot. Slit the stem at this point or peel away a tiny piece of the outer skill. Keep the wound in contact with the compost with a staple made from bent wire, or a weight such as a small stone. When roots have formed, which should usually take between three and six months, cut the stem carefully away from the parent plant.
Greenhouse shrubs that do not root easily from cuttings can be propagated by air layering. Examples are gardenia and stephanotis. Air layering is also used to shorten leggy plants such as the rubber plant, Ficus elastica.
Sometimes, seed can be saved as a means of propagation. It is no use saving seed from hybrid plants as it cannot be relied on to yield identical plants, but species plants will come true to type. Make sure seed is ripe before collecting, and sow as soon as possible afterwards. Popular plants which provide good results from seed are: celosia, eccremocarpus, freesia, Solanum capsicastrum and torenia.
Some greenhouse plants, such as Saxifragastolonifera, produce runners with plantlets. These can be pegged down on to compost between spring and autumn in the same way as strawberries.
Passiflora can be propagated by detaching the suckers it throws up, each with some roots attached, and planting them.