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Germination and Pricking Out - Thompson & Morgan

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Germination to Pricking Out

Ian Hodgson concludes his series for beginners by considering the stages from seed germination to pricking out the seedlings.

Germination need not be a lottery or game of chance. Success simply hinges upon creating the right conditions and for seeds this means exposure to a subtle blend of warmth, air and moisture. Whilst light can also stimulate germination in many seeds, for others it is not important until the seedlings actually start growing. Although seeds from the world's climates have their own particular requirements we gardeners strive to create conditions of uniformity. This has the obvious aim of encouraging the maximum number of seeds to germinate and works to perfection with highly bred man-made plants such as F1 hybrids. Their seeds are so identical, that they all germinate simultaneously. Excellent news for the impatient seed-raiser.

Seeds taken from the wild however, will probably not have this uniformity. Patchy or erratic germination may not be an indictment of your lack of skill but merely the plant not putting all their eggs in one basket. Thicker seed coats, varying sensitivity to temperature or a host of other mechanisms may allow successive germination over a period of time. An insurance of success in an uncertain, hostile world. The outcome of this is not to become disheartened but to persevere with your efforts for as long as your patience will hold out.

Seeds will not germinate without first absorbing water. At no other time is a plant so vulnerable than when it first starts to germinate. If seeds dry out during this critical period they will almost certainly perish. Ideally both compost and seeds are soaked in water immediately after sowing. Left unprotected in direct sunlight, the compost surface may soon start to dry out. To prevent this from happening, cover the rim of the pot with a sheet of glass or clear polythene. The best protection is obtained from a propagator. Mist over the surface of the compost from time to time with water. If the compost persistently continues to dry out, re-soak the containers in a bath of water. For tropical plants always use tepid water. Cold water direct from the tap may chill seeds and check germination.


Temperature exerts a profound influence on seeds. All have a specific band of temperature within which they will germinate. Those which are consistently low or even too high will hold the seed in a state of dormancy. A small fluctuation of temperature can enhance the speed of germination in a variety of plants.

Without any form of artificial heating compost temperature will be dictated by the surrounding air or direct exposure to sunlight. For hardy plants this is usually quite sufficient but for temperate or tropical species this may be quite inadequate. They require the warm, protected conditions such as that found in a greenhouse or windowsill indoors. The airing cupboard can also be used if you have no other form of heating, but the pots and containers require constant attention and the young seedlings must be removed from the darkness the moment they appear to prevent them from growing long and spindly. High temperatures can be obtained by standing pots closer to heaters both indoors and in the greenhouse. If you anticipate raising a large number of seeds that require warmth, particularly during the early months of the year, it will pay dividends to invest in a propagator. They enable more effective control over conditions required for germination and the nurturing of young, vulnerable seedlings. They vary in complexity from just a plastic tray with clear hood through to the state-of-the-art electrically heated models with thermostatic control. Heating units warm through the compost to within a specified temperature.

Thermostatic units offer more precise control, allowing temperature to be pre-set to any desired limit. This allows for more flexibility and also allows lower temperatures to be maintained once the seeds have germinated. It is also useful for gradually acclimatising young plants to the surrounding room temperature before they are potted off.

Heating cables or heating mats also provide a gentle bottom heat and are an option which can easily be taken up by any greenhouse owner.

It's good practice to always install a thermometer when germinating seeds so that checks can always be made that the desired temperature is being achieved. The most accurate method is to insert the bulb of a small rod thermometer just below the surface of the seed compost. Reading the thermometer may require you to remove the hood of the propagator which may temporarily release the bubble of warm, moist air which has built up beneath the canopy.

Alternatively a small round aquarists thermometer with suction disc can be positioned on the inside of the propagator canopy. Although this will prevent disturbance, it will only measure the air temperature inside which may differ slightly from that of the compost.

Check the temperature on a regular basis and make a note of the time it takes and the temperature required to germinate each type of seed, particularly those which are from wild sources or saved from your own garden. This information will prove indispensable when planning future seed raising efforts.

Although we take sufficient steps to maintain hygienic conditions by using clean pots and sterilised compost, the coats of seeds can harbour harmful fungal diseases which may rot the seed even before it germinates. Whilst commercial sources endeavour to maintain seed cleanliness, that collected from the wild or from our own gardens may not be quite so clean, particularly that from sticky, juicy berries. Any material adhering to seeds can provide a food source for fungi which can quickly overwhelm emerging seedlings. To prevent this happening take the precaution of watering the seeds with a copper-based fungicide solution.


Once young seedlings have appeared, light becomes just as important as temperature and moisture. To maintain strong, healthy growth, seedlings should be exposed to light coming from all directions. This is normally the case with seedlings being grown outdoors or in a greenhouse. Indoors, however, seedlings may be being grown on a windowsill or darkened room. All plants grow towards the light and their stems will extend or flex, so that their leaves obtain the maximum exposure. In poor light, the stems of seedlings will become long and spindly. The speed with which they do this will depend upon the vigour of the plant. Those which grow quickly and demand most light will quickly respond and kink towards the light from a window or extend to the point of collapse. Slow growing plants such as cacti will take far longer to respond to adverse light conditions. To prevent kinking, turn the pots of seedlings round every couple of days.

If poor light continually causes spindly seedlings, lamps can be used to supplement natural daylight. Ordinary light bulbs do not produce the right kind of light. Use mercury vapor lights or tubes whose light most approximates that of natural daylight. They have a low surface temperature and so can be placed close to plant leaves without fear of scorching them. When installing lighting always follow manufacturer instructions and obtain the services of an electrician if you are at all uncertain.

Conversely, young seedlings should be protected from exposure to strong, direct sunlight. Although seedlings require good light for healthy growth, strong sunlight can damage or scorch tender, young leaves. Thin leaved or shade loving plants such as ferns are particularly susceptible. Odd pots can be moved temporarily to a shady position or placed beneath the bench in the greenhouse. Propagators or the tops of pots can be covered with sheets of newspaper or green mesh shading fabric. If seedlings require watering or misting with water to maintain humidity, this should not be done under the glare of full sunlight. Water drops act as lenses and cause scorching and burning if they lodge between leaves. Water your seedlings under shaded conditions or if possible wait until early evening or morning when the sun is weaker.


When temperatures become too extreme, seedlings should be ventilated. Besides reducing temperature, ventilation helps replenish and re-circulate air. Damp stagnant conditions such as those experienced during the early part of the year can allow fungal diseases such as grey mould or botrytis to gain a foothold. The technique is not to change the air too dramatically but allow the process to continue gradually. Most propagators have circular ventilator portals which can be opened by varying degrees to increase the air flow. Larger propagators may be fitted with louvre vents. Canopies can also be propped open with a cradle made from wire or a short length of stick wedged between the rim of the lid and the rim of the tray.

By gradually increasing the amount of ventilation, you can gradually accustom your seedlings to tolerate the general conditions under which they are to be grown. This is known as weaning or hardening-off. Done properly it will help prevent severe checks to growth when the seedlings are potted on.

Pricking out

If all goes to plan and seedlings continue to grow healthily, they will soon require to be lifted or pricked out into individual pots. But how do you know when they should be pricked out? The timing will depend upon how closely the seeds were sown and the growth rate of the plants. Slow growing types such as cacti can be left for a considerable time until quite crowded. Fast growing annuals may require pricking out only a matter of days after germination. As a general rule when the first true leaves have started to form then the seedlings can be pricked out. Remember, there is little or no fertiliser in seed compost. Those remaining for any length of time may require an occasional watering with very diluted fertiliser solution.

Space sown seedlings can remain undisturbed until large enough for potting on.

Lift your seedlings in shade. Never pot on dry seedlings, water them thoroughly first. Have pots and compost ready and waiting. Pots 2½-3in (6.2-7.5cm) are ideal for starting off most plants. Containers are a matter of choice between plastic and terracotta. The latter tends to dry out more quickly but is ideal for use with many alpines. Square pots optimise more bench or windowsill space than round pots. Seedlings can also be pricked out into plastic cell modules before potting on into their final containers. Fill pots loosely to the rim with a Soil-less or soil based potting compost, such as John Innes No 1.Then water the pots or cell trays before you start pricking out. Make a hole in the centre of the compost with a dibber or piece of dowel. Remove seedlings by inserting a dibber or spatula beneath the roots and gently prize upward. Once loosened, lift out by holding the seedling by one of the seed leaves. Avoid holding the stem to prevent crushing. Tugging out seedlings is a sure method of severely damaging them.

Transfer the seedlings and place the root ball into the hole made in the compost. Position the seedling so that the roots are buried below the surface. Only pot up as many seedlings as you need at any one time, selecting the strongest and most representative examples unless you are looking for something different. After potting up seedlings from batches with erratic germination return the pots to the propagator to await the appearance of more seedlings.

Place the pots in a semi-shaded position and out of cold draughts. With care, all seedlings should survive but count on 10% failure when estimating the amount of seedlings you require. The first few days are critical, and once the seedlings have started to grow away, the hardest part of the whole seed raising business is over.

Ian Hodgson trained at Kew

Source of article
Growing From Seed - Winter 1988-89 Vol. 3 Number 1
© The Seed Raising Journal from Thompson & Morgan