Greenhouse Borders: Soil Preparation
Preparing the soil
When starting with a newly erected greenhouse which has not been put on a solid concrete plinth, you may be lucky enough to have good quality soil already on the floor. In this case, all that is needed for the first season is the normal routine digging and the same application of a base dressing which would be carried out on an outside plot.
It is unlikely that the foundations for the outer walls of the greenhouse will be deep enough to impede the natural drainage of the soil inside, but on very poorly drained sites it is advisable to incorporate a drainage layer of coarse grit while double-digging.
Plants grown under the protection of glass or plastic tend to respond by growing more quickly than in the garden. In so doing, they take up nutrients rapidly from the soil — often depleting ordinary soils well before the end of the growing season. You must take special care to keep the greenhouse border soil enriched. Well-decayed manure, garden compost or leaf-mould should be dug into the soil at a rate of one bucketful per sq m/yd at few weeks before planting.
Before planting, water the border thoroughly with a hosepipe and when completely drained through, scatter a general-purpose fertilizer over the surface at a rate of one generous handful per sq m/yd and rake it in.
If the greenhouse has been built on poor soil, dig out the soil entirely to one spade’s depth and replace with fertile garden soil. Better still, use sterilized compost, such as John Innes or a similar
potting compost available from most garden centres and nurseries — delivery can usually be arranged within a reasonable radius, often without charge.
On a site where the greenhouse border soil is poorly drained, an alternative to digging the soil out and replacing it is to construct a raised bed over the broken-up subsoil. Build an outer framework of wooden planks — well treated with a proprietary wood preserver — or use brick or concrete to form the sides. A height of about 23-30cm (9-12in) should be adequate for most plants.
Infill the bed with sterilized garden soil to within about 2.5-5cm (1-2in) of the top. You can fill a small bed with potting compost —John Innes No.2 or No.3 potting compost, for instance.
Planting and aftercare
The principles of growing plants in a greenhouse border are identical to those for growing outdoors, except that you must take extra care with watering and feeding —regulation of the greenhouse environment relies entirely on your constant attention. Soil dries out very quickly under glass and it may be a good idea to install some form of automatic watering system, especially when growing salad crops which tend to run to seed when they are hot and dry.
Pests and diseases must be kept under strict control — minor outbreaks can turn rapidly into a major epidemic in a confined environment. Many of the chemical pesticides and fungicides recommended for garden use can be used in the greenhouse (check the manufacturer’s instructions), but be careful not to breathe them in when spraying or dusting. It is best to wear a face mask and to leave the greenhouse afterwards until the chemical has settled.
House plant pesticides can also be used, and these tend to be much safer, though they may not be as efficient in controlling the more persistent pests. For a thorough clearance of above-ground pests, use a proprietary greenhouse fumigant smoke. These are sold in a canister like a firework and give good control of aphids, whitefly, red spider mites, mealy bugs, leaf miners and many other pests. You must follow the safety instructions on the container to the letter — close all windows and vents to contain the smoke, then evacuate the greenhouse immediately after lighting the touch paper and do not return until after the manufacturer’s specified time.
Biological pest control is more environmentally friendly than chemical sprays and is often a better remedy as some greenhouse pests, notably red spider mite and whitefly, have become resistant to pesticides available to the amateur gardener.
Control of soil-borne pests and diseases can be more of a problem in the greenhouse border, since they build up unnoticed and the few chemicals which are effective against them are not recommended for use by amateurs — they are far too dangerous. For this reason, it is important to use only sterilized soil in the greenhouse border and to maintain scrupulous hygiene at all times throughout the house.
Tar acids formulated as an emulsion are used to clear moss and slime from paths and can also be applied to the glass, framework and border soil as a sterilizer. Soil disinfectants like Jeyes fluid give some control of soil pests and are relatively safe to use.