Greenhouse Cultivation


Keeping a collection of greenhouse plants in a healthy and successful way requires the same sort of attitude as caring for a small zoo of animals. I find it helpful to establish a routine otherwise I can always find something more important to do than go into the greenhouse.

The best time to do most of the checking is in the morning. The first task is to check round any equipment that you rely on and make sure it is all running. Read the temperature on the thermometer and note the maximum from the previous day and the minimum night temperature. If you have time, it is interesting to note these down for comparisons with other years, or if a plant suddenly goes into a decline this can frequently be linked to a below or above average temperature. Be sure to reset the thermometer ready for it to register the new maximum.

Be as aware of the weather as possible and set the vents and, if necessary, shading, to cope with the anticipated weather for the rest of the day. During the warmer summer months it must be realised that ventilation, shading, watering and damping down may have to be checked as many as three times spread out during the day. For enthusiasts who are out at work all day it helps to plan at an early stage for some automatic systems to cope with this. I am not saying it is impossible to do without these checks but it makes the difference between getting by and really doing well with the plants.


Cleaning the plants is more important than most people realise. The eggs of pests and spores of harmful fungi linger in the dead leaves and debris and hatch out to infect the plants again if they are not cleared away. Take off dead leaves and flowers, pull away any weeds from the pots and take any rubbish away from staging or floor. Sweep or rake the floor and remove the rubbish from the greenhouse. I take this opportunity to thoroughly inspect my plants as I clean them up. I notice signs of attack from pests or fungal diseases, especially mildew and damping off disease on seedlings.

Whilst going through the plants I find I can really appreciate them, noticing how much growth each has put on and checking their progress. If you use the greenhouse as a holding and growing on place for plants that are being used in the house or conservatory, this is a very good time to change any sorry looking plants around. Bring the ailing specimens back so that they can be sorted out and replace them with fresh plants when the routine is completed.

When more time can be found it is often necessary to give the plant leaves a more thorough clean. If there have been problems with pests leaves tend to become covered with sticky ‘honeydew’ which is secreted by the insects. This gives rise to the growth of sooty mould which lives off the honeydew. While not attacking the leaf, it masks out the sunlight that the leaf needs to absorb and looks most unsightly. I have never been able to find a chemical which would kill the sooty mould and enable it to be washed off the leaf. The best, although rather laborious, method would seem to be to add a tiny drop of washing up liquid to water and gently wipe the mess off with some dampened kitchen paper. Plants that have become dusty can be washed in the same way. A good idea is to brush the dust off first with a small soft paintbrush which prevents a paste of dust and water forming. Hairy-leaved plants are cleaned effectively by brushing alone. I dislike all forms of proprietary leaf shines and cleaners as they can harm some plants, especially the new leaves. I would not risk using them on any of my collection.

Watering, Humidity and Damping Down

Having cleaned the plants up it should be part of the morning routine to check each plant for water. It is particularly important that plants are not overwatered during the winter. A combination of cold and damp makes them far more vulnerable to low temperature. If you decide that they must be watered it is best to do this in the morning so that the plant and its surroundings have a chance to dry out before temperatures drop at night. During summer when the temperatures are high try and anticipate the watering requirements of the plant for the whole day so that it is unlikely to be under the stress of needing water while you are at work or otherwise occupied. The only sure way to see whether a plant needs water is to look at the surface of the soil in the pot or border. During summer and warm weather if the soil is crumbly and no water can be squeezed between the fingers then watering is necessary. During cold weather the top 2.5 cm (1 in) or so of compost can be allowed to dry out between waterings. A plant in fairly dry soil is much more able to withstand frost than the same plant in wet soil.

Having decided that watering is necessary make sure that the plant gets plenty of it. Dribbling a few drops into the pot will result in only the top 2.5 cm (1 in) or so becoming watered, while the roots in the bottom of the pot slowly die. The biggest danger is overwatering. Very few plants can survive in soil that is continuously wet. The roots need to breathe so there must be a chance for air to get back into the spaces in the soil before they are filled with water again. Failure to do this results in the roots suffocating, dying, rotting off and no longer being able to absorb water. As a result of this the whole plant will then collapse and die, believe it or not, from lack of water. The only cure for an overwatered plant is to let it dry out again as much and as fast as possible without the plant shrivelling. New roots will then grow. The temptation is to repot the plant into fresh soil. In my experience, however, this almost always kills the plant off. Quite possibly the few remaining minute live roots are torn off in the efforts to repot.

While in the greenhouse watering this is an excellent time to check humidity, which is the amount of water vapour in the air. In the winter this will not be such a problem except in very tropical houses. However, during the summer a greenhouse can be a very arid place. You will know when the humidity is low instinctively and should damp down by splashing water all over the floor, staging and even the plants themselves (although not in full sun as this will lead to scorch). In very hot weather the watering should be checked again and the house damped down in the middle of the day and at the end. With a big collection of plants it may be worth installing electronic under-bench misting devices and capillary matting which can be kept moist by an automatic system.

If everybody is like me they will not be able to resist having a mixed collection of different types of plants from different parts of the world; obviously they will have different requirements. Cacti, succulents, Pelargonium, Sansevieria and bulbs, for instance, will not need to be checked more than once a day at the most for watering and humidity. However, more tropical plants are much more demanding. At the most extreme are the epiphytes. These are plants which naturally grow by attaching themselves to trees rather than the ground. They are usually found in areas of very high natural humidity and can find all they want by absorbing water and nutrients from the atmosphere or drips from the trees. Most orchids are epiphytic although they can mostly be grown in pots in a special compost. Air Plants, however, which are mostly in the genus Tillandsia, will not grow in any kind of compost and need a very difficult balance of humidity with good air circulation.


Assuming that the fresh potting compost used contained a good balanced fertiliser, a newly potted plant will not need feeding for about one month. As soon as the roots begin to reach out to the sides of the new pot and the plant is taking off again it should be fed. I find it helps to designate a feeding day otherwise I would forget altogether, and while plants would probably not die without being fed, they are certainly not able to do as well as they could. The best method of feeding is to liquid feed by diluting the recommended amount of fertiliser into a can and watering it in. It is unwise to feed either a dry plant or a very wet plant, so try to manage the watering so that by feeding day the plants are exactly right to take it up.

During spring and summer while most plants are actively growing feeding should be weekly. For normal growth a well-balanced, slightly high nitrogen feed is best. Choose one which contains trace elements for better results. There are many different fertilisers on sale but they are remarkably similar. Look on the bottle or packet and there will be an analysis of the main constituents. The main three are nitrogen, phosphates and potassium shown as NPK and they will show the ratio of these to each other: 9:3:7 indicates that there is more nitrogen than anything else. Nitrogen is good for leafy growth. However, if a plant is building itself up to flower, potassium will help as it promotes flowering and fruiting.

As autumn and winter draw on change the feed to higher phosphate and potash. During colder weather lush leafy growth could be easily damaged by frost or grow drawn and long in the lower light levels. As watering is less frequent, feeding is reduced to only every three to four weeks. The only exceptions are for those plants which are actively growing at this time of year such as Cineraria.

There are other methods of feeding. Slow release feeds are good if you are too busy or absent minded to remember weekly. One application of granules, tablets or sticks should last at least six weeks. Foliar feed is very quick as the fertiliser is absorbed through the leaves. Not surprisingly this is best applied as a spray. Top dressing is mostly for plants which are in such a large pot that they cannot be potted on any more. Instead, the surface soil is very carefully removed, even removing the plant from its pot to do so if necessary. This old soil is replaced by a strong new mixture.

For large collections of plants it is possible to apply liquid feed without having to dilute each lot into a watering can. They are expensive, but the Cameron diluter will attach directly to a hose. A concentrated solution of fertiliser is carefully released into the stream of water which makes the operation a lot more direct and speedier.

Potting and Potting Composts


The first question concerning potting is usually ‘which compost shall I use?’ Dealing first with compost that can be bought ready-made there are basically two types; peat-based and loam-based. Peat-based composts are very reliable, being composed of peat mixed with fine sand and fertiliser. While suitable for a wide range of plants I feel that there is often not enough drainage material in the compost to assist the passage of water through it. Sharp sand or grit, perlite or vermiculite are the additions which enable a compost to be referred to as ‘well-drained’. I prefer to use these composts for tropical plants but find that for plants which I want to withstand cold temperatures in winter, a loam-based compost is favourite. This is because I like to keep these plants very dry when the temperatures are low and this is easier to do when there is loam in the soil.

There is really only one main loam-based compost on sale and that is based on the John Innes recipe. The loam that is specified for this is moderately heavy prepared by stacking turves grass side downwards for from six to twelve months and riddling the resulting wonderfully fibrous loam through a 10-mm (½-in) sieve before use. This loam should then be steam sterilised. Steam is passed through the soil at a temperature of nearly boiling which kills off any nasties in the loam. It must be noted that chemical changes in the soil at this stage are poisonous to plants, this effect wears off and the effects are counteracted by the additions of superphosphate in any case.

Thus it can be seen to be a fairly lengthy process. With this knowledge in mind we can be sure that most manufacturers of large quantities of John Innes are not going to go through with all of it. Topsoil is used instead of the prepared loam and sterilised in the same way. This makes the loam far less fibrous and more variable depending on where the topsoil has come from. Sand for the compost should be coarse, clean and contain particles up to 3 mm (1/8 in) in diameter. I sometimes feel that the sand used is too small or too large in particle size. I prefer Chichester grit or Cornish grit but unfortunately these are not easy to obtain in small quantities. The final main ingredient is peat, either moss or sedge; I find that a mixture of the two is excellent, moderately coarse in texture. The source of this is not a problem. These three constituents are mixed together in a ratio of seven parts of loam to three of peat and two of sand by bulk. Before mixing, John Innes base fertiliser is added. There are different strengths of compost so that John Innes (JI) No. One is half the strength of JI No. Two, which is, in turn, two-thirds of the strength of JI No. Three. lime is also added to adjust the pH.

This system makes these composts useful in that small young plants can be given a weak compost, whereas large, strong-growing types such us Fuchsia or Chrysanthemum can be given the strongest.

Clearly either of these two different sorts of ready prepared composts are adequate. However, I find that I get better results by adjusting them to my own liking. I prefer a peaty fibrous compost with some loam and good sharp sand to improve drainage. This can be achieved either by mixing a home-made compost or by adding extra peat and sharp sand to a John Innes compost. Five parts JI to one of peat and half of sand is good. Remember that this must be well mixed to distribute the fertiliser from the JI evenly throughout. It also pays to remember that the JI will then be diluted. Either start off with a stronger type or add more base fertiliser.


Potting up is the term which refers to the first time the plant is put into its own pot from a seed tray, pot full of cuttings, jam jar of water or any of the variety of propagating containers there are. Whether a seedling which has been pricked out or a rooted cutting the young roots will be fragile so great care must be taken to handle the plant carefully. Use a small pot which is just a little bigger than the new roots is required and the compost should be a JI No. One or Two or a universal or multi-purpose peat compost.

Potting on is the term describing the moving of a plant, that is already established in a pot, to another pot which is usually, but not always, larger. All plants will, after a time, outgrow their pots and soil. Unfortunately, the time varies not only with different plants but with the same types grown in different conditions. A plant will almost tell you when it is ready for repotting. Signs to look for are roots growing through the bottom of the pot, the pot feeling very tight about the roots, vast amounts of growth in relation to pot size and finally the leaves turning yellow and having a starved appearance. Hopefully you will have noticed before this. If a plant is in the process of being grown on to full size or to flower, potting will need to take place as soon as the roots have reached the sides of the pot. It is less easy to tell this without experience. If in doubt, up-end the pot carefully and, supporting the plant and pot surface with one hand, gently but firmly knock the rim of the pot against a steady surface so that the pot-shaped root ball will come out enabling the roots to be seen. If a plant has reached full size then avoid repotting more than necessary as you will run out of space for the ever increasing size of container.

Nevertheless the plant must still have something to live off and must be fed and top dressed. This involves removing the top few cm (in) of soil without disturbing the roots too much and replacing that soil with fresh strong compost, preferably containing a slow release fertiliser such as Vitax Q4. Sometimes it is necessary to knock the plant out of its pot to do this. The process is also known as ‘shouldering off which describes it rather well.

The best time to repot is during spring or summer so that maximum warmth and light are available to assist the root and shoot growth which should follow. However, shading may be necessary for a while directly after potting. Avoid repotting between October and February unless you have warm tropical winter temperatures. I have learnt by experience that it is not a good idea to repot and severely cut back a plant simultaneously. I prefer to cut back, wait for new shoots to develop and begin to grow and then repot.

A repotting bench is not essential but makes life a lot easier. A small portable wooden structure which can be placed on the greenhouse staging is ideal as it can be stored between use and takes up no room. It may take a bit of time to make, but speeds up the potting process no end. Load some compost on to the bench and add a selection of washed pots. My reasons for disliking potting into dirty pots are not just aesthetic; the old soil may contain pests and diseases, especially root mealy bug and the old soil inside sticks to the new soil in such a way that when it is knocked out for the next potting it never comes out clean, often tearing roots which stick to the sides. Other equipment includes a watering can, knife, stakes, string and labels.

Having knocked the plant out of its old pot and decided that it definitely needs potting, gently tease some of the outside roots away from the old root ball which will point them in the direction of the new compost. Choose a new pot that is not too much bigger. There should be just enough room for the new soil to be pushed down the sides. Pot sizes are measured according to the diameter of the open top of the pot. Generally a plant from an 8-cm (3-in) pot would go to a 13-cm (5-in), a 10-cm (4-in) to a 15-cm (6-in), a 13-cm (5-in) to an 18-cm (7-in), a 15-cm (6-in) to a 20-cm (8-in) and so on. Never think that you will save time in bypassing one of these stages. A plant in a new pot is surrounded by wet soil and the air that the roots need to breathe and live has a long way to travel to get to the roots. By putting a plant in a pot too large for it you are creating conditions in which the roots are liable to suffocate in the same way as with overwatering.

Place a little compost in the bottom of the new pot and set the old root ball on this. Make sure that the surface of the old compost will be at exactly the level you want at the top of the new pot, allowing for a watering gap. Make sure the stem of the plant is central and that the plant itself is upright. This is a good opportunity to correct a leaning plant. Carefully feed in the new soil around the old and firm gently. If there is not sufficient room, use a stick to make sure that all the spaces around the old root are filled. This is vital. Finish off by tapping the new pot sharply on the bench which will give an even finish to the surface. Place the pot on the floor and water in thoroughly using a can with a rose. Subsequent watering should be careful. Wait until the surface is just beginning to dry before watering again.

Staking and Tying

This is a very important aspect of plant care. Even shrubby plants that do not climb frequently sometimes need a cane to keep the main stem straight while they are growing and to support the weight of flowers and fruits. Climbers up against a greenhouse wall benefit from wires even if they can self-cling. However, the problems start when climbing plants have to be free standing. With flimsy shoots such as Bomarea, Gloriosa and young Passiflora shoots a good method is to attach string to the pot or base of the plant and wind it around the stems. Draw it up tightly and tie to the greenhouse structure above the plant. Always have a good supply of different lengths of canes handy, and string. This should be proper green gardeners’ string or raffia—preferably not a thick rope of pink nylon. Most climbers can be secured to canes. Two canes in a pot will allow the shoots to be trained in a circular fashion, even in a double circle as is often seen with Stephanotis and Bougainvillea. Three canes pulled together and tied at the top make a tripod around which shoots can be trained. Always tie the shoots in using a neat reef knot as this will not pull undone. Cut off the ends neatly. Remember that stakes and ties should eventually become invisible. For tropical climbers which produce aerial roots, moss poles are excellent. These can either be bought or made. Take a length of plastic tubing available from plumbers’ shops. Wrap handfuls of moist sphagnum moss around the tube and secure it by tying round and round with fine nylon fishing line. Leave the base of the tube bare as this end will be potted into the pot along with the plant. Keep the moss as moist as possible and the aerial roots of Monstera, Philodendron or Scindapsus will grow into it. Lengths of plastic or wooden trellis work are useful either against walls, as greenhouse dividers or stuck into pots.


I feel that pruning is something that many owners of greenhouse plants are scared to tackle. The same person who gladly prunes a Buddleia or Forsythia is wary of doing the same for an Hibiscus or Datura. With almost all shrubby or climbing greenhouse plants pruning is desirable. Not only does it keep their size in check but renews old growth and gets rid of all the pests and diseases that may have built up on the old leaves. It is occasionally necessary to prune climbers down to clean the glasshouse wall behind them. Always make a clean cut above a node (place on the stem where a leaf grows). Most shrubs and climbers are pruned quite hard to get a complete renewal of growth from near the base of the plant. However, a few plants will not shoot readily from very old wood. Cuts are thus made on newer wood within a bud or two of the joint with older wood. When a climber is being asked to cover a large space it is usual to train a network of shoots against the wall or trellis. These growths are allowed to become old and woody with annual pruning consisting of current year’s shoots being cut back to within short spurs of the older wood. This is the main principle behind the pruning of grape vines but is applicable to a wide range of plants which flower on current year’s growths.

Avoid potting or heavy feeding and watering at the time of pruning. Wait until the plant has started into growth before taking any action.

29. May 2013 by admin
Categories: Greenhouse Equipment, Greenhouse Gardening | Tags: , , | Comments Off on Greenhouse Cultivation


Get the Facebook Likebox Slider Pro for WordPress