How to Feed and Water Greenhouse Plants
Even today, when modern scientific knowledge should be making the issues clear, greenhouse gardeners find themselves frustrated by numerous incorrect and illogical recommendations on the subject of feeding and watering plants. Some of these are nothing more than primitive customs that have not yet been abandoned in favour of common sense.
Most pot plant failures are caused by overwatering and, to a lesser extent, by overfeeding.
Your first important step is to learn the difference between dry, moist and waterlogged soil or compost. Moist conditions should be your aim. If the soil or compost feels moist to the touch, adheres slightly to the fingers but does not ooze free water when pressed, then it is moist. However, do not press hard when testing as this can lead to the soil becoming compacted.
How much water?
The water requirements of a plant vary — often from one day to the next. For instance when the weather is cool or dull (and also in winter when growth slows down) they need far less water — sometimes hardly any if they are dormant. When, as during summer, they have warmth and lots of light, they drink enormous quantities by comparison.
The needs of a plant are also governed by its size and vigour, and whether it is flowering, fruiting or growing vigorously. For these reasons you cannot give fixed doses of water to a plant all year round. The question ‘how much water should I give my plant?’ cannot be answered by a simple sentence. You must take into account the time of year, the nature of the plant and the prevailing conditions, and then decide.
When in doubt err on the side of less rather than more water. Most plants are able to survive drought to some degree and the symptoms of slight wilting are not serious; they will soon recover when watered. Excess water has a far more serious and insidious effect. The main symptoms are usually slow, sickly growth, yellowing foliage and wilting. But by this time the roots may be in an advanced state of rot, and it may be too late to save the plant by reducing watering. Note that wilting can be a symptom of both overwatering and underwatering, so test the compost.
Desert and water-loving plants You must, of course, consider the nature of each plant and its natural habitat. But, strangely enough, this is not always a reliable guide for providing the best artificial conditions, for in the wild plants often have to struggle to survive. Many cacti and other succulents, for example, will grow vigorously if given far more water than they would get growing naturally.
On the other hand, aquatic plants need an abundance of water all year round. Not many of these are cultivated as pot plants, but if you are growing them they must be kept standing in water so that the a potting compost is always wet.
A waterlogged soil or compost is usually badly aerated. This encourages species of bacteria and fungi that cause rotting of plant roots. Plants with fleshy roots (like bulbs and tubers) are prone to rot if kept wet during their period of dormancy. But modern potting composts are usually based on peat; this means they will hold plenty of water without actually becoming waterlogged, and can still maintain excellent aeration. For some greenhouse plants (like columneas, orchids, bromeliads and certain palms) plenty of air with moisture is vital. You can help aeration by adding sphagnum moss, charcoal or polystyrene granules to the compost.
Testing for moisture
Nowadays plastic pots have become very popular. They are convenient and hygienic, being easy to clean. They check evaporation of water, so that plants growing in them need less than in clay pots. The time-honoured method of assessing whether a clay pot needs watering is to tap it with a cotton reel fixed to the end of a cane. If the compost is moist, the pot will emit a dull thud; the sound will be a higher-pitched note if the soil is dry.
Unfortunately this does not work very well with plastic pots, and a better way is to assess the moisture content by feeling the weight of the pot.
Recently, electronic devices have become available that give a rough idea of the water content of compost. They usually consist of a probe which is inserted into the compost, and a meter (or arrangement of tiny neon lights) that indicates the moisture content. With one design the scale is numbered and a booklet is supplied with the meter so that the numbers can be related to the moisture requirements of different classes of plant. As you gain experience you will find that the ‘dry’, ‘moist’ and ‘wet’ indications on a scale are a perfectly adequate guide.
In spite of these aids, practical experience is the best way to become confident about watering. You will eventually be able to tell all you need to know by feeling the compost with your fingers and by letting the appearance of your plants tell you what they require.
How to water
A common recommendation is to water freely, when you do water, and then leave the plant alone until the pot is almost dry again. This is a helpful guide for some plants — but not all. For example, erratic watering of this kind will not do for tomatoes as it will certainly lead to blossom end rot or cracked fruit; and many ornamental plants (such as fuchsias and begonias) may react by dropping buds or flowers.
Never give so much water that large quantities run from the drainage holes of pots or trays. This will soon carry away any soluble fertilizers in the compost. (A good way to water seed trays or pots is to use a fine-mist sprayer. This will thoroughly penetrate the compost without washing away any nutrients or loosening the seedlings.
During warm weather many plants enjoy an overhead spray. This cleans the foliage and aids plant respiration. Once again you must use your common sense as to how much water to apply. Do not spray open blooms because the tender petals can easily be damaged and brown rot or grey mould may occur. Also, in very sunny greenhouses, avoid getting water on the foliage. The droplets act like lenses to focus the sun’s rays on the leaves and may cause burning.
The importance of clean water Be careful what sort of water you use on your plants. Some people advocate rainwater; if the rainwater is clean, all well and good. Unfortunately it is often collected from roofs and stored in open tanks or butts, which rapidly become stagnant pools of pests, diseases and weed seeds. As you are probably using sterilized potting composts in your greenhouse, it is obviously foolish to water these with dirty rainwater. Mains drinking water is safer — even if it tends to be hard and limey. There are numerous greenhouse plants that object to lime, but they will suffer much more from the dangers inherent in dirty water.
If you do need soft water for special lime-hating plants, put out clean bowls just after it starts to rain, and store the rainwater in clean, closed vessels. Water that has been boiled for some time and then allowed to cool and settle will also be softer than tap water.
Avoid using animal manures in the greenhouse or for potting composts —unless they are a modern, sterilized, proprietary type. Crude manures can introduce many pests and diseases that can be dangerous to humans as well as to plants. Moreover, they often have few of the nutrients that plants require.
One of the main advantages of manures is that they improve soil texture, but modern seed and potting composts have body built in, usually by the addition of peat. Most pot plants need a higher ratio of nitrogen than plants grown in the soil outdoors; all proprietary potting composts take this into account. If you use these products, no feeding is needed until the plants are well advanced.
The need for feeding is assessed in much the same way as watering — taking into account the vigour of the plants and the time of year. None is necessary when plants are dormant. But when plants are producing buds, flowers and fruits, some extra feeding is essential. Use a proprietary balanced fertilizer according to the maker’s directions. Never give more than is suggested. ‘Little and often’ is a good feeding rule.
Modern feeding now takes into account the preference of some plants for acid or alkaline soils. In alkaline soils (or composts) iron, magnesium and trace elements essential to plants become `locked in’ and therefore unavailable to the plants.
These nutrients can now be restored in a special form (known as sequestered or chelated) which is easily absorbed. Avoid using hit or miss mixtures or additions of what are known as ‘straight’ fertilizers that supply only one or two of the basic nutrients (such as sulphate of ammonia or sulphate of potash). Properly formulated feeds for plants like tomatoes, carnations and chrysanthemums that need special nutrient ratios can be bought in proprietary form.
You can apply the basic plant nutrients (nitrogen, potassium and phosphorus) as foliar feeds — in specially formulated mixtures. A very recent product of this type includes vitamins and trace elements as well. It is particularly useful for plants that have not yet made an extensive root system, and can also be used as a general plant tonic. Seedlings, newly potted plants and cuttings often react dramatically to such feeds.
Plants can only use liquid solutions of nutrients. The quickest-acting are soluble feeds that dissolve readily in water; use these for all short-term, fast-growing plants, such as annuals and biennials. Greenhouse perennials will also benefit from liquid feeds when they are making active growth.
You can also top-dress perennials by mixing a little of a balanced, slow-acting, solid feed (such as John Innes base fertilizer or Growmore) with the top layers of the soil or compost. These will dissolve a little at each watering. Specially formulated proprietary feeds in tablet form are also obtainable.