The tiny dust-like seed of plants like begonias and calceolarias can be especially difficult to deal with. Alan Toogood describes how to go about germinating seed you can hardly see.
Many plants, both hardy and tender have extremely fine dust-like seeds. They are produced in copious amounts to ensure that at least a few germinate, for if they become covered with soil, 'fall on stony ground' or are dispersed into other unsuitable conditions they may fail to grow.
This is what happens in the wild - as gardeners we are faced with only minute quantities of seeds, perhaps a mere trace of dust in a small cellophane packet! Therefore we have to handle and sow them extremely carefully, which is not the easiest of tasks. After sowing we have to ensure perfect conditions for germination. Having germinated our seeds we still need deft fingers to prick out or transplant the tiny seedlings to give them room to grow.
It is worth bearing in mind that fine seeds have a lower germination and survival rate compared to larger seeds and therefore should not be kept for long periods. The best advice is to sow as soon as possible. If you collect fine seeds from plants in your garden, such as hardy primulas, meconopsis, gentians and rhododendrons, then sow them immediately after collection, when they should germinate well.
As we are usually dealing with only small quantities of seeds it is sensible to use small containers for sowing. It is wasteful of compost and propagation space to use a full-size seed tray for a mere pinch of seeds. Plastic or clay pans or half pots, 4-5in (10-12.5cm) in diameter, make useful containers. If desired you can place a few crocks (broken clay flower pots) in the bottom to cover the drainage holes, although this is not now usually considered essential. The containers must be scrupulously clean to avoid the risk of seedling diseases. So thoroughly wash and dry them before use.
For the majority of subjects you can use a soil based compost such as John Innes Seed Compost or a soil-less (peat-based) seed compost. Fill the container to overflowing with compost, then lightly firm it with your fingers, including particularly the area around the edges. Now scrape off the surplus with a straight piece of wood till it is level with the rim of the container. Finally press lightly with a wooden presser, which should leave a perfectly level, smooth surface. A wooden presser is easily made with a flat piece of wood, such as plywood. It should fit exactly inside the particular container, so you will need several for pans of different sizes. A small block of wood nailed in the centre of the presser will act as a handle.
Seeds must be sown very evenly over the surface of the compost. To make them easier to handle it is the usual practice to mix them with some very fine dry silver sand. The amount will be something like a heaped teaspoonful for a 4in (10cm) diameter pan.
There are several different ways of scattering the seed/sand mixture over the compost surface, but whichever you use sow half the quantity in one direction and the other half at right-angles, as this helps to ensure very even sowing. If you have sown well, the surface of the compost should be lightly and evenly dusted with the silver sand.
You can sow from the seed packet. Hold it an inch or two above the surface of the compost and slowly move it to and fro across the surface, at the same time gently tapping it to slowly release the seed/sand mix. This technique should ensure it scatters evenly over the compost. You can use the same technique but instead place the seeds/sand on a square of paper folded down the middle; or you can sow from the palm of your hand. But make sure your hand is perfectly dry, otherwise seeds may stick to it.
Fine seeds must not be covered with compost otherwise they will not germinate. But to ensure they are in close contact with the compost, and are therefore able to absorb moisture, lightly press them into the surface, using the wooden presser. This must also be perfectly dry to prevent seeds sticking to it. There are a few seeds which are on the borderline of very fine and small, such as impatiens, nicotiana and digitalis. These can be covered with the merest dusting of compost, using a very fine - 1/8 inch (3mm) - sieve to scatter it. A kitchen sieve is useful for this.
The compost should now be watered, but not overhead as this will disturb the seeds. Instead stand the containers in a deep tray or bowl of tepid water, almost up to their rims, until the surface of the compost become moist, then remove and allow to drain. It is recommended that you add some Cheshunt Compound or other copper based fungicide to the water, as this prevents the dreaded seedling disease, damping off, which can rapidly destroy a complete batch of seedlings.
Seeds must be provided with optimum warmth and this is best achieved in an electrically heated propagating case in the greenhouse or on a windowsill indoors. Heat is provided from below so the compost quickly becomes warm. For seeds which do not need high germination temperatures there may be sufficient warmth on the bench of a heated greenhouse, or on a windowsill in a warm room. In these instances the containers are best covered with a sheet of glass to prevent the compost from becoming dry. Many people germinate seeds in a warm airing cupboard, again covering the containers with glass. But bear in mind that some seeds need light to germinate so do not place these in a dark cupboard.
You must check containers daily and if the compost is becoming dry stand them in tepid water as described above. If compost becomes dry it will delay or often prevent germination altogether.
As soon as germination occurs the containers must be placed in maximum light to prevent the seedlings becoming weak and spindly. However they must be shaded from the sun, which can scorch and dry them up. This can occur very rapidly in a greenhouse. The part of the greenhouse in which the seedlings are growing could be shaded with a piece of shading netting, if you do not want to shade the entire house.
Various germination temperatures are called for, depending on the subjects being sown. Seeds of some plants need quite cool conditions to germinate, and may fail if the temperature is too high. Others need very high temperatures and may not germinate, or germinate erratically, if there is insufficient warmth. Germination conditions and times for some popular subjects are given below:
Before the seedlings start to become overcrowded in their containers they must be pricked out or transplanted to other containers to give them room to grow. Seedlings from fine seeds are very delicate, proportionately small and not too easy to handle.
Containers used for pricking out should be chosen to suit the plants. For example, if you are raising summer bedding plants like lobelia and begonias you can prick out into deep seed trays, or half-size trays for small quantities. Pot plants could be pricked out into individual containers like Jiffy 7s or 3in (7.5cm) pots, and hardy plants like foxgloves and meconopsis are best pricked out into trays.
If you used soil based John Innes Seed Compost then prick out into John Innes Potting Compost No. 1, but if soilless compost was used for sowing then use a soilless potting mix for pricking out. Prepare the containers in the same way as for seed sowing.
Now we come to the problem of how to handle the tiny seedlings. Firstly remember you must never hold them by the stems for they are very delicate and easily damaged - always hold seedlings by the leaves. Only lift a few at a time to prevent roots from drying out. An old table fork is a good tool for lifting seedlings, inserting it well underneath them.
Although extremely tiny, lobelia seedlings are perhaps easiest to handle, for they are pricked out in 'patches' of three or four. So bear this in mind when lifting them. Other seedlings have to be separated.
A useful tool for handling seedlings is a small stick with a notch cut in the end. This notch is placed under the lower leaves and then the seedling can be moved. You may find this a lot easier than trying to pick up seedlings with your fingers.
It is important to make a planting hole deep enough to allow the roots of the seedlings to dangle straight down. Make this either with a pencil, or with a small wooden or plastic dibber (like a blunt-pointed pencil). Then hold the seedling over the hole so that its roots hang straight down and the lower leaves are just above compost level. With the dibber gently firm the compost around it. On no account use firm pressure as this can damage roots and stem. With soilless compost you do not even need to firm - simply push a little compost around each seedling to hold it upright.
When pricking out into trays, space the seedlings evenly each way. They are normally planted in rows: say five rows of eight seedlings, to give you 40 per standard-size tray.
After pricking out water in the seedlings with a watering can fitted with a very fine rose. This will help to settle the compost around them. Hold the can to one side of the container, and when water starts spraying through the rose move it to and fro over the container. When enough water has been applied move the can away before stopping the watering. In this way you will not disturb seedlings with sudden gushes of water, nor indeed with heavy drips. Always add Cheshunt Compound or another copper based fungicide to the water to prevent damping-off disease, for seedlings are still prone to it even after pricking out. Return the containers to a position in maximum light but shaded from the sun.
Source of article
Growing From Seed - Winter 1986-87 Vol. 1 Number 1
© The Seed Raising Journal from Thompson & Morgan