Weaning Seedlings

Ian Hodgson adds a postscript to his series on seed sowing for beginners.

It's easy to think that once seeds have germinated that most of the hard work is over. Whilst this is true in part, seedlings are at their most vulnerable during the first few days of life and are very sensitive to sudden changes in their growing environment. We strive to produce conditions that are ideally suited for germination but these requirements frequently differ from those normally required for the more mature stages of their lives. Our own growing conditions may differ even more from that ideal, with lower temperature and fluctuating humidity, that would spell instant doom if seedlings were immediately transferred from propagator to the big, wide world. Young plants must gradually be accustomed to any change to give them the best chances of survival. This process is called 'hardening off or 'weaning' and is a skill which is just as important to acquire as any other in the seed raising game.

The speed at which seedlings require to be weaned will depend upon the growth rate of the plant concerned. Fast growing subjects, such as annuals will require to be weaned off within the first few days of germination whilst slower growing plants such can be left in the propagator for longer periods of time, until they reach a size where they may be safely transferred. Slow growing plants weaned too soon may be unable to acclimatise sufficiently well to the surrounding conditions and are likely to perish. The timing of weaning will also depend upon the size of the propagator, the amount of successive sowings for which the propagator is required and the time of year. The process will necessarily be tailored to your own specific conditions and the type of plants you raise. With the simplest of equipment, weaning is a very imprecise affair. With basic unheated propagators, the transparent lid can be propped open with a piece of dowel, notched at either end. The period of ventilation can start with just a few hours each day, closing it again at night until the canopy can be completely removed. More advanced propagators have their own mini-ventilators that can be adjusted at first but the canopy lid will require to be raised and eventually removed once weaning gets underway.

Electrically heated propagators should have the temperature reduced gradually by having the unit turned on for decreasing periods of time. Those more advanced models with variable thermostatic control can have their settings gradually turned down to the ambient room or greenhouse temperature so that seedlings gradually become hardened off. Again, ventilation should be increased in tandem with lowering temperature. To help keep a check over conditions that the seedlings are experiencing, install a small thermometer in the propagator and make a routine of checking the seedling temperature with the surrounding air temperature adjusting your technique accordingly as dictated by the prevailing conditions.

Weaning techniques will differ from the time of the year. Conditions that seedlings experience in spring will vary quite markedly with those that will be experienced in high summer. Seeds raised early in the year will be subject to cool temperatures, lower light levels and generally less favourable conditions for growth. The rate of growth will depend upon just how high a temperature you can heat your greenhouse. The more tender the plant, the higher the minimum temperature will have to be. Those who do not heat their houses should wait until the danger of severe frost is past before embarking on their seed raising efforts.

If you have a large number of seedlings which require frost protection early in the year, you may consider installing heating cables in a deep bench to produce a little bottom heat. It is well established that plants will endure lower air temperatures as long as their roots are warm. A variety of cable sizes can be bought from manufacturers which have differing heat outputs. They usually come with fitting instructions, depth they should be buried and spacing requirements to suit all types of bench. Fitted with a thermostat the temperature can be more finely controlled. As an alternative, certain manufacturers produce heated mats which comprise of a heating element sandwiched between two skins of PVC. To conserve heat, the mats are laid on a sheet of polystyrene foam. By installing sides to produce a shallow tray and a clear polythene canopy supported on wire hoops, you have a ready made heated tunnel in which to wean your seedlings properly. Ventilation is increased as the seedlings are weaned.

Early in the year, seedlings should be kept moist and never waterlogged, as this will increase the chances of them rotting. Maintain humidity but keep the air circulating to prevent the build-up of diseases such as grey mould or damping off disease. As a precaution, water seedlings with a fungicide like Cheshunt Compound . If you cannot open the ventilators an electric fan heater will help keep air circulating.

In summer the problems are more or less opposite. Temperatures are more favourable and it is direct sunlight and drought which are the main problems. Make sure that the seed raising area is adequately shaded even if you choose not to shade the rest of the house. Use either shading paint or fabric. In the height of summer extra shading may be required.

Keep seedlings moist at all times and maintain humidity by spraying the greenhouse floor with water. Seedlings can be misted over directly with water but never do this in full sun. Water droplets act as lenses and scorch young leaves and stems. Misting should be carried out early in the morning or in the evening when the sun is not so powerful. Keep the seedlings cool by keeping them well ventilated but avoid letting extreme temperature build-up and then suddenly give full ventilation as this too will give seedlings a severe shock which may also cause them to perish.

Ian Hodgson trained at Kew and is now Technical Editor of Practical Gardening magazine.

Source of article:
Growing From Seed - Spring 1989 Vol. 3 Number 2
© The Seed Raising Journal from Thompson & Morgan