Compost For Seed Raising
After a large number of experiments with all kinds of seeds, the John Innes Horticultural Institute has reported that the ideal seed-raising compost is this mixture, which is right for practically every kind of seed raised in gardens :—
2 parts good garden loam, sterilized.
1 part clean sand.
1 part good moss peat (or sterilized leaf-mould, if peat is difficult to obtain).
This compost is, of course, for seeds raised under glass. It provides a medium free from pests and diseases, able to hold moisture, and at the same time to hold air, and friable enough (i.e., it crumbles easily enough) to offer no serious obstruction to the growing seedling.
These conditions allow for free and even germination. A seed, to begin growth, must have three things. First moisture, the immediate action of which is probably purely mechanical. Water is absorbed by the seed, which swells, and bursts its hard outer covering. This allows moisture and the second essential, air, to be received by the tiny root, formerly dormant inside the seed.
SEEDS NEED WARMTH
In addition to moisture and air, warmth is necessary for proper development. The amount of warmth varies according to the kind of seed—tropical plants frequently need a very high temperature to start them on their way, while some plants seem to need a temperature but little above freezing point. Watch how weeds germinate and grow in winter. What concerns the gardener in this connection is that a temperature too low is likely to retard growth for so long that the seed decays instead of germinating properly. It is because temperature is so important that gardeners usually prefer to raise expensive seeds under glass, where conditions can be better controlled than in the open.
Seed raising is a simple operation, but failures are frequent, and their cause is often to be found in small items that have escaped the raiser’s notice. To prepare a box of compost, and scatter seeds over it, finishing with a dusting of fine soil, is the ordinary procedure. Water is given, usually overhead from a fine rose can, and sometimes shading is also provided.
Probably seventy-five per cent of the seeds sown in this rough general way will germinate and produce a sufficient number of seedlings to satisfy the ordinary gardener. Any that do not are forgotten, or the seedsman is blamed. However, with the object of lessening the proportion of failures, the following summarized hints are given, under separate headings for easy reference.
SOWING IN THE OPEN
We will take first open ground sowing. To raise seeds in the open the soil must be well dug, well drained, and raked until there is a fine surface tilth, i.e., a surface 2-in. Layer of fine crumbly soil free from large stones.
- Seeds can be sown broadcast, i.e., scattered freely over the soil surface, or in drills, i.e., in shallow depressions drawn with a pointed stick, hoe or rake, and running parallel to each other.
- Seeds to be transplanted early can be broadcast, but seedlings that remain for some time in the seed bed should always be in drills, so that a hoe can be used to keep down weeds and aerate the soil.
- A light dust covering, or no covering at all, is the rule for fine seed. Larger seeds can have twice their own thickness of soil over them.
- A row of small twigs pushed in on either side of the seed rows protects young seedlings. Portable cloches are an even better protection. A cloche is a small “ tent “ of glass, easily taken to pieces and packed away flat in winter months.
- Avoid watering immediately after the seed is sown; it is far better to sow the day after the top spit has been well soaked with rain or hose water. Overhead watering may cake the surface and seal the seeds in, so that they do not germinate freely.
- Always thin out ruthlessly, but preferably in several stages, removing the weaker seedlings if they are to be discarded. Particularly in the case of autumn sowings, leave the seedlings comparatively thick in the rows, and only remove them when they appear to crowd. They give each other a little protection, if they are not too drastically thinned at first.
Let us now deal with seeds raised under glass. Although the compost given above (2 loam, 1 sand and 1 peat or leaf-mould) is good for nearly all seeds, a great deal depends on the texture, that is the fineness and distribution of the ingredients. A common mistake is to use too fine soil throughout, with the result that the contents of the seed box or pot become a consolidated mass. A good practice is to chop the loam (decayed turf) and store it dry for a time before use. Then, when sifting, keep the lumpy pieces for the lowest layer, and use the finest compost for the surface.
Fine seed and very small seedlings need a compost more uniformly fine and sandy, as it is difficult to transplant small seedlings if their roots penetrate lumpy turf.
Seed boxes and pans, which are of course used in the open as well as under glass, need drainage holes. A layer of pieces of broken pottery or other material at the bottom of the box prevents the soil from clogging these holes. Boxes with loose-fitting sides, however, do not often need special holes for drainage holes.
Charcoal and sphagnum moss are both useful over the bottom of pans where seeds that germinate slowly are to be raised. They prevent souring of the soil. Decayed leaves or spent hops can also be used; both hold moisture and also feed the roots of the young plants. Too much leaf-mould is undesirable when raising seedlings that tend to damp off.
Seed boxes should be filled to the rim loosely with prepared compost, after the moss, or leaf-mould, has been laid in position. Light even pressure over the whole surface, from a flat board, should then be used to prepare a flat level seed bed. The seeds are sown in very shallow depressions, about ¼-in. deep, made in parallel lines across the box with a point such as a wooden label.
The finest possible dusting of soil over this, followed by a thorough moistening of the soil, if necessary by partial immersion, completes the operation.
A sheet of glass over each seed box, with brown paper over that, to exclude light until such time as the seeds have germinated, is generally advisable. Seeds raised in a cold frame do not need this extra sheet of glass if the frame is filled with one kind of seed, as the light can be excluded by shading over the whole frame; but different seeds need different amounts of shading.
Seed boxes are best stood in moist ash, or moist coconut fibre, either of which will keep the temperature more even than if the boxes are surrounded by air.
When germination has taken place, the maximum amount of light is needed, and as much ventilation as the plants can stand, according to the weather. On no account should seedlings be exposed to full sunshine in a closed frame—they will almost certainly scorch. Moreover, the hardier the plants are encouraged to be, the better they will succeed when at last they reach the open ground. Seedlings that have been kept too close become tall, thin, pale in colour, and generally weak. Seedlings that are grown in cool, well-ventilated frames are short jointed, thick, and dark green in colour.
The first leaves to appear are generally the seed leaves—that is, they are the leaves that actually existed inside the seed, packed with food, and they merely grow by raising themselves on stalks through the soil that covets the seed. Almost at once other leaves begin to appear, and when two or three of these “ true “ leaves have developed, the seedlings can be pricked out. Pricking out, by the way, is the same thing as transplanting, but applied to tiny seedlings that are sometimes too small to be actually handled. The seedling is lifted in a cleft stick from its bed of fine, crumbly soil and held in position in a prepared hole, while soil is levered against it.
The greatest care should be given to the work of pricking out; for a seedling carelessly handled, and bruised, will not only fail to grow well, but will often become diseased, and may even be a source of danger to other seedlings.
Tender plants are generally pricked out either into other boxes of similar compost to that of the seed box, or into separate pots, which are then placed under glass. Hardy plants raised from seed are sometimes set out direct in the open ground from the seed box.
Seeds germinated in the greenhouse, can be raised from seed. These often take some years to reach flowering stage, and for that reason bulbous plants are more frequently introduced to the garden in the form of dormant bulbs.
If seeds are sown, they should be sown as soon as they are fully ripe. They are best placed in pots, sown thinly, so that transplanting is not needed for some time. It should be possible to leave the plant undisturbed until a bulb has been formed, when any desired transplanting can take place during the dormant season.
Bulbs generally appear first as small “grass-blades,” and the novice should remember this. I have heard of cases where bulb seedlings (iris, narcissus, etc.) were all pulled out, under the impression hat they were weeds !
FERNS FROM SEED
Comparatively few gardeners raise ferns from seed, though the operation is really quite simple, and can be carried through successfully even in a living room. The seeds or spores, to be strictly accurate, are to be found on the underside of the frond. The spores are too minute to be visible individually to the naked eye, but can be seen as brown dust.
They must be sown when fully ripe, and the best way to do this is to wait until the “ dust “ is ready to drop when the frond is touched, and then to take pieces of the frond and lay them, right side up, on prepared seed pans.
Use deep pans or pots, half full of drainage material, and then fill with the ordinary seed sowing compost, with perhaps rather more sand than usual, and made very fine on the surface. If brick or flower-pot, broken to pea size or smaller, can be used as drainage material, the pan can stand in an inch of water until the spores germinate. After this there is no difficulty in potting out and growing the young plants, provided a compost similar to that needed for adult plants is used, with a little emphasis on the amount of sand in it.
The question of heat depends of course on the ferns to be raised; hardy ferns need no artificial heat, while most of the ferns usually grown in rooms need a little bottom heat to start them on their way.
THE FUNCTION OF BERRIES
A great many plants, particularly shrubs and trees, grow their seeds in the form of berries. Shrub berries generally present little difficulty in raising, but they cannot be hurried too much. The natural way with such seeds is for the berry to be carried some distance by birds. It may then be dropped, or the seeds may be discarded, or they may pass through the bird undigested, and so
most freely when raised over heat of some kind. It is common to stand the seed boxes over hot pipes, since bottom heat is so generally acceptable, moving them to shelves near the glass roof as soon as the seedlings are through the soil.
A great many of the plants we loosely call “ alpines “ are merely dwarf herbaceous or annual plants that do not require any special treatment when raised from seed. Others, such as a number of the saxifrages, androsaces, ramondias, etc., need special compost, and special precautions concerning drainage.
DRAINAGE PRECAUTIONS IN COMPOST
For these plants the ordinary seed compost can be made to serve by adding to it a quantity of old mortar rubble, well crushed, so that it will pass through a fine sieve. If mortar rubble is unobtainable some crushed flower pot, burnt soil, or even crushed sandstone can be substituted.
Sift this material, and use the coarse siftings to fill 5-in. pots two-thirds of the way up. Special alpine pans can be obtained which are provided with rather more drainage holes than are in the ordinary pot or box; use these only half filled with coarse siftings.
The surface should be specially fine and porous, and all watering should be done by partial immersion, holding each pan in the water until the moisture rises to damp the surface.
Early pricking out is desirable, specially with seedlings that seem slow to germinate, as the removal to fresh soil acts as a stimulant.
If only one or two seeds seem to have germinated, do not throw away the seed pan. Remove the germinated seedlings carefully, fill their places with sand, and replace the pan in the frame or alpine house. Other seedlings will often appear even months later.
Never use much, if any, heat to raise alpines; in fact, the better way to encourage germination is to pile snow over the pans during the winter months. The action of the snow has been proved to speed up germination among “ difficult “ subjects.