Greenhouse Propagation

So that you can make the most of your greenhouse, it is a good idea to become familiar with the techniques used to propagate plants. Once you are proficient in using these you can increase your stock, decrease your expenditure and sometimes help to preserve plants already in your collection. Most forms of propagation are also fascinating and give pleasure in themselves.

Some plants are very easy to propagate, others very difficult. The easy ones tend to become far too numerous, so be wary of accepting all propagating material, such as cuttings, offered by friends. Never use plant material which is itself unhealthy or has been taken from a diseased or pest-infected plant. For propagation, only use the best in both general quality and health. Undesirable characteristics can be inherited as well as desirable ones, and certain diseases, such as virus infections, can also be passed on.


For the quick and efficient propagation of most greenhouse plants it is best to set aside a specific area. It will need to be warmed from underneath and have a transparent cover to retain heat and moisture while letting in light. You can make up such an area yourself by laying electric warming cables over a part of the staging, covering it with sand, then enclosing it with transparent plastic sheeting or a glass frame. The suppliers of warming cables give recommendations for their use in propagation. As a rough guide, a 120-150 watt cable will heat a space about 1.5m x 600mm (5ft x 2ft). It can be thermostatically controlled. Other forms of electric greenhouse heater, such as tubes, can be used, either controlled manually or thermostatically. A paraffin lamp is another possible heat source.

You may prefer to buy a ready-made propagator. These are available with electric or paraffin heaters. Electric ones are preferable, if you have the power supply, because they have a thermostat which makes them easy to control and set for different temperatures as required. For most greenhouse propagation, a range of temperature from little more than frost free, for hardy plants, to about 27CC (80 degrees F), for sub-tropicals, is necessary. Many plants need a high temperature for propagation, higher than they normally re- quire once established, so it’s important to check that a propagator can give high temperatures when necessary.

The propagators available vary greatly in size, temperature range, and design. For the greenhouse, choose a covered design that has a variable temperature control and is of a suitable size for your requirements and the amount of space you have available. It is often useful to have room enough to keep plants in congenial warmth after propagation, until they become well established. Large enclosed propagators can be used as a permanent home for a small sub-tropical plant collection.


A propagator should have a base of sand which is always kept moist. The moist sand distributes the warmth evenly and keeps the atmosphere in the propagator humid. Dry conditions must be avoided because propagating material, such as seeds and cuttings, can quickly become dehydrated and die before roots have a chance to form. A transparent cover is also desirable to retain moisture and warmth, particularly for plants needing higher temperatures. Light is another essential for much propagation, but you should avoid direct sunlight. Site the propagator or propagation area where there’s slight shade. If necessary, shade an area of the greenhouse glass with white Coolglass. Excessively high temperatures can also cause severe damage, and it is important to remember that the temperature in the propagator is affected by the temperature of its surroundings.

The propagator comes into its own about late winter to spring, when it is used for such purposes as striking cuttings, seed germination, and starting tubers etc. into growth. During the rest of the year there is usually sufficient natural warmth for the types of plants then propagated.


This is an area of staging set aside for potting, seed sowing, preparation of cuttings, mixing composts and similar operations. A portable bench is a good idea. This can be simply made using a sheet of aluminium or zinc, which is available from most builders’ merchants or DIY shops. Select the right size for the chosen section of your staging, then turn up the sides of die sheet to form a three-sided tray to fit the space exactly. Keep the surface clean and wipe down from time to time with disinfectant.


Seed is a cheap source of plants and, nowadays, is widely available from plants that grow all over the world.




Sometimes you can save seed from your own plants or from other growing sources. Seed from hybrids, however, particularly F1 hybrids, is usually unreliable. It may not yield plants true to type and should not be saved.

Seed should be ripe and reasonably fresh. Old seed generally germinates poorly. Sow as soon as possible after you get it or, if it has to be kept a while, store in a cool, dry place in the home. Do not leave packets in the damp and heat of a greenhouse. Most seed companies now supply seed in special sealed packs with controlled humidity. Seed will remain viable for a long time when kept in these packs but normal ageing begins once they have been opened.

To make tiny seed easier to handle, some varieties are pelleted with an inert material by the seedsmen. However, there is some difference of opinion about whether this is an advantage, since germination of this type is sometimes poor. To hasten germination, large seeds can be soaked in water overnight before sowing. Another common practice which is used particularly for tough or hard-coated seeds, is to slice off a tiny sliver of the outer coating with a razor blade, being very careful not to damage the interior.

Seed must be sown in proper seed compost to ensure success. You can use John Innes loam-based seed compost or any of the numerous proprietary peat-based seed composts now available. Another essential is general cleanliness. Keep the seed compost in closed plastic bags or containers when not in use. Make sure that the rectangular plastic seed trays in which you put the seed compost are kept clean, as well as the plastic pots which are used for sowing some larger types of seed. Fill the tray or pot by lightly pressing the compost into it to about 1cm (1/2 inch) below the rim. Then level off the surface and firm it down with your hand. Before you begin sowing, label each container. It is a good idea to put the date as well as the name of the seeds. Be sure to use a waterproof pencil.

The compost must be made moist before sowing; it should be neither dry nor waterlogged. Sow thinly to make pricking out easy. Very fine seed should not be covered with compost after sowing. Otherwise the rule is to cover the seed with a depth of compost roughly equal to its own diameter. The best way to sow is to tap the seed from the packet with your forefinger while moving your hand over the surface. To aid even distribution of very fine seed when sowing, mix the seed with a little silver sand. Large seed can be sown with the fingers or a pair of tweezers. If there is a lot of seed in a packet, there is no need to sow it all at once. Often sowings can be staggered over a couple of weeks or so. This will provide batches of plants in different stages of development, giving you longer flowering and cropping periods. After sowing, water-in the seed using a mist of water from a fine sprayer. The same technique should be used for subsequent waterings, and also, for watering the tiny seedlings when they show through. It is not a good idea to water by immersing the sowing containers in water, as is so often suggested, because this leaches out all the soluble fertilizers added to the seed compost. Overwatering must in fact be avoided since too much water can suffocate the seed – air must be able to penetrate for it to germinate. On the other hand, complete drying out once the seed has started to germinate is equally fatal.


After sowing, cover the seed container with a sheet of clean white paper, then lay a piece of glass on top or slide it into a polythene bag to help retain moisture; the paper prevents drips of condensation from saturating the compost surface and water-logging the seed. Some greenhouse plant seed germinates best if exposed to a certain amount of light, which is all the more reason for not sowing too deeply. This is particularly worth doing with bromeliads, cacti and other succulents, calceolarias, rubber plants, gloxinias, lettuce, petunias, African violets and Cape primroses.

When the containers are put in a propagator, be careful to check that the temperature is optimum for the seed type. Obviously, one must not try to germinate different types of seed with widely different temperature requirements at the same time. If the temperature is excessively high, although germination will be speeded up, pale, weak, lanky seedlings will be produced.

Most of the popular greenhouse-sown seed germinates in one to three weeks. Some may take considerably longer, so do be patient. As soon as germination is seen to be taking place, remove all covering. Pricking out and transferring to potting compost must be done as soon as the seedlings can be safely handled without damage. The sooner this is done after germination, the less setback the seedling will suffer and the less likelihood there is of damaging the roots. The best tool for this job is a pair of long finely tipped tweezers, but be careful not to actually grasp the seedlings with them. Just lift using the ‘V shape made by the tips of the tweezers. You can adjust the distance between the tips, according to seedling root size, by varying your finger pressure. Another popular tool for pricking out is a thin strip of wood or plastic with a ‘V’ notch cut at the end. If you need to handle a seedling with the fingers, lift it by one of the first formed ‘seed leaves’ and not by the stem. Replant the seedlings using a blunt-pointed dibber to make holes in the potting compost just large enough to take the roots easily.

The seedlings can be transferred to pots or to plastic seedling trays, depending on their further treatment. Many pot plants can initially be grown-on for a time in trays, which avoids the use of lots of small pots. To prevent damping-off, water-in the seedlings with Cheshunt compound.

In the case of bedding plants, be particularly-careful not to overcrowd seed trays. About 24 plants per tray is average. Seed, such as lobelia that yields crowds of minute seedlings, should be ‘patched’ out. This means lifting them in tiny groups and not attempting to separate out individual seedlings. The groups can be subsequently treated as a single plant and will grow as a clump perfectly well.


Before bedding plants are put out in permanent positions in the garden, they must be gradually acclimatized to full light conditions and low temperatures. If this is not done, and they are suddenly transferred from the warmth of the greenhouse, they will immediately become sickly and take a long time to start growing. Start hardening-off about two weeks before planting-out time which is roughly May-June, depending on area. If you are not sure about the timing, find out what the local parks or keen neighbouring gardeners do. The hardening-off procedure begins with moving the plants to the coolest part of the greenhouse, then, later, to closed unheated frames outside. Shade the frames from full sun at first but reduce the degree of shade or the period of shading each day. At the same time, give more and more ventilation until the plants are fully exposed. In the early stages, however, it may be necessary to close the frames at night if you think that there is any risk of frost.



Cuttings taken from greenhouse plants are usually ‘softwood’, that is taken from soft immature growth. The best time to take cuttings is when the plants have just moved into their active growth period, as in spring. However, some cuttings can be taken in early autumn. Semi-hardwood and hardwood cuttings are cut from woody plants and shrubs. These can also be rooted in the greenhouse.

When taking cuttings, select small vigorous shoots – about a finger length is long enough. Use a sharp blade such as a razor, so not to bruise the tissue, and cut the shoot at the base of the stem, just below the point where the leaves are borne. The leaves, just above the cut, must then be pulled away. It is from this point that the roots will eventually emerge. The cutting is then inserted into a cutting compost consisting of a mixture of clean grit and peat. A proprietary cutting compost can also be used. Several cuttings may be inserted around the edge of a pot, but seed trays can be used instead if there are many cuttings to be rooted. Small bags, like mini growing bags, are available for rooting cuttings. Finally, put the cuttings where they will receive the correct amount of warmth and will not lose moisture during rooting. Covering the containers with clear sheet polythene or a polythene bag may be adequate, or use a covered propagator to provide extra warmth and maintain humidity.

You can dip the base of the cuttings in a hormone rooting preparation before insertion into the compost. This sometimes hastens rooting, and most of these preparations also contain a fungicide to prevent rotting. Some cuttings of greenhouse plants, such as nerium and tradescantia, root very easily just standing in water, for example. Others, such as camellia, take quite a time. The cuttings and the compost must not be allowed to dry out whilst rooting. The method known as ‘mist propagation’ automatically keeps cuttings sprayed with a fine mist of water to keep them at the point of maximum water uptake. Small units for this method are now available for the keen amateur. As soon as a reasonable root system has developed, the cuttings must be potted into a proper potting compost.

A very simple procedure for rooting cuttings without the aid of a propagator in summer and early autumn is to put a little cutting compost at the bottom of small polythene bags and insert the cuttings in this. The bags are then closed and hung up in the greenhouse. When rooting occurs it can be seen through the polythene.


This method can be used for a large number of favourite greenhouse plants, particularly for the gesneria and begonia families. It is especially applicable to plants with bold leaf veins. Leaf cuttings can be propagated in several ways. The simplest is to take a leaf and make slits with a razor blade in the veins at intervals on the under-surface. The leaf is then placed flat, slits downward, on the surface of some cutting compost. A few clean pebbles can be used, if necessary, to keep it flat and in close contact. Roots will grow from the slits and tiny plants form that can be separated and potted. Leaves can also be cut into small triangles with a vein at the apex of each triangle. The point with the vein is then inserted into the rooting com post in the same way as a stem cutting. If plants have elongate leaves these can usually be cut into sections, and each inserted in a similar manner. Some small-leaved plants, such as African violets and peperomias, can be propagated by merely detaching a leaf with a piece of stalk attached and inserting it as far as the top of the stalk in a rooting compost. Leaf cuttings usually need warmth, and the maintenance of humidity is essential.


This is the simplest and quickest way of propagation and can be done with any plants that form a clump of roots. First remove the plant from its pot, then, using a sharp knife, cut down through die clump to make several pieces. Pot each one in the usual manner. Division is best done with most plants just as growth is about to begin. Make sure the compost is moist, initially, but after potting be sparing with water until the plant is seen to be actively growing.

In the case of tuberous plants, division should be done after the tubers have started into growth, when well-defined shoots can be seen. Divide them up so that there’s a shoot to each piece, then dust the cut surfaces with powdered charcoal to check sap loss before potting.


This method is used mainly for climbers and trailers but can sometimes be used for other plants if their stems are flexible enough. A length of stem is led into a pot of rooting compost so that a short portion is immersed. This is then weighted in position. You can either make a slit in the stem where it dips into the compost, or remove the leaves at that point, as widi stem cuttings. When roots have formed, the stem is cut from the parent plant and potted into a potting compost.


This can be useful for plants that become ‘leggy’ and lose their lower leaves. It is most applicable for plants such as the rubber plant (Ficus elastica). First make a slit in the stem where a new root system is required and insert a tuft of peat dusted with a hormone powder. Then fasten a ball of moist peat and sphagnum moss around the slit, wrap it around with clear polythene sheeting and secure with string or florists’ wire. When roots can be seen through the polythene, cut the stem just below the point from which they emerge and pot in the usual way. Moderate warmth over several weeks is essential for the success of this method, so it is best to start it during late spring.


Greenhouse bulbs and corms may produce small ‘bulblets’ around their sides as they reach maturity. These can be isolated and potted individually. Such small storage organs may take several years to become large enough to flower. Some rhizomes and tubers, such as achimenes and gloriosa, also reproduce themselves. The offspring can be separated during the dormant period and may flower well in the first year of potting.

Some plants, bromeliads, for example, send out shoots from the base, which form into young plants. These can be carefully cut off close to the parent stem, which eventually dies, and potted separately.

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03. September 2013 by admin
Categories: Featured, Garden Management, Greenhouse Gardening, Propagation, Top Tips | Tags: , , , , | Comments Off on Greenhouse Propagation


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