Greenhouse Pots, potting and composts FAQs

I am told I should not grow crops in the greenhouse soil. Why is this?

It is certainly best to avoid doing this if possible. For edible crops, tomatoes in particular, it is extremely risky to use the ground for more than one year when the greenhouse is first put into use. Most gardeners know how important it is to rotate crops whenever possible. If the soil is constantly employed for the same crop it will become ‘sick’ and various disease organisms may take hold. Far better results can be obtained by growing crops in containers or growing bags containing potting compost.

Are different types of compost available for different purposes?

Yes. The seed composts and potting composts used for sowing and growing plants in containers differ from each other, and are carefully formulated and prepared mixtures with the required texture, the right balance of plant nutrients, and relative freedom from pests, diseases, and weed seeds. They are quite different from the garden compost made by rotting down vegetable waste—this should never be used for pot plants. Instructions for making your own composts can be found in many gardening books, but most people now prefer to buy them ready-made. The John Innes composts were the first to be introduced and are still excellent, but they are based on special loam now not easy to come by. Most modern proprietary composts are based mostly on peat.

Which are better, clay pots or plastic pots?

The advantages of plastic pots are that they encourage hygiene because they are easy to clean and to sterilise; they are not so easily broken as clay pots; and they are lighter in weight and easier to store. The fact that plastic pots retain moisture better than clay, and that plants growing in them tend to need less-frequent watering is also an advantage, although for people used to clay pots there is the danger of overwatering. Plastic will usually deteriorate with age, so it is still a wise investment to buy large clay pots, especially if they are to be stood outdoors and exposed to sunlight, which causes many plastics to become brittle. Finally, of course, most people would agree that clay pots—especially the larger sizes—are better looking than plastic ones.

Can you outline potting procedure?

Always use a pot size in correct proportion to the size of the plant to be potted, moving on into a slightly larger pot at intervals as the plant develops; a sequence called ‘potting-on’. This keeps the compost fresh and lessens loss of nutrients. An approved potting compost is essential. It must be nicely moist but not soggy before use. With modern composts, potting does not have to be too firm, and usually ‘crocking’ is unnecessary unless the pot-drainage holes are very large. (Crocking involves covering the holes with some clean stones or pieces of clay pot to ensure that the holes do not become clogged). After potting leave a gap of 25 mm (1 in) or so between the compost surface and the pot rim; this is the ‘watering space’. It helps to assess quantity when applying water, and also prevents the water from running off the surface.

Is there any difference between potting-on and re-potting?

Yes. Potting-on, which involves annual plants and perennials in their younger stages, is carried out as the plants increase in size . Perennials to be saved for future years, however, will need re-potting, which should be carried out when the plants are dormant or just about to make new growth. Re-potting involves removing a plant from its pot, reducing the size of its root-ball, and returning it to its pot, or to another of the same size, which is partly filled with fresh compost. Very large plants do not lend themselves to this treatment. Deal with them by removing the top layer of compost and replacing it with fresh or with a dressing of balanced fertiliser. This treatment is called ‘top-dressing’.

What is a ‘plunge-bed’?

It is used for starting hardy bulbs and similar storage organs into growth. It is made from a container or a pit with very good drainage filled with a clean, moisture-retaining material, usually peat or sand. The plunge-bed should be outside the greenhouse, but covered to protect it from waterlogging. Planted-up containers are put into the plunge-bed until the plants are well rooted (usually after about 8 weeks), before being taken into the greenhouse. When making a plunge-bed bear in mind that it must be sufficiently deep that the pots will be covered by 50-75 mm (2-3 in) of the moisture-retaining material.

Am I likely to be troubled by weeds in my greenhouse?

Modern seed and potting composts are relatively weed-free, but they can become contaminated with weed seeds if left about exposed to the air. Ground not used for growing can be treated with a total weedkiller such as a mixture of paraquat, diquat and simazine applied with a watering can and dribble bar. Hormone weedkillers should not be used in the greenhouse: they may become airborne, and even in minute traces are extremely damaging to plants.

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