If you happen to have a cold frame available, that may be used; but it is better to make a frame especially for your rock plant seedlings.
If this is constructed against a fence or a wall, three feet will be a convenient width, as all the reaching will have to be done from one side.
A half sash or "pony" frame, with sash three by three feet to fit it, may be purchased complete; or both frame and sash may be home-made.
The former is easily constructed with one-inch boards, held in place by light posts.
The latter may be light wooden frames, covered with plant-protecting cloth, or cello glass, which is light, convenient to use, not brittle, and perfectly suitable for keeping out storms and keeping in moisture.
If the sash are hinged at the back, to fasten up with a hook out of the way when not wanted, it will be found a great convenience.
The frame commonly used for starting these types of plants is of the standard six-foot width, but facing or sloping to the north, instead of south, so as not to catch but to deflect, the sun's rays.
In addition to the glass sash or cello glass sash, for keeping out stormy weather, there will be needed also some form of shading, which is quite as necessary as the other.
Light wooden frames, of a size which will just cover either one or two of the glass sash, may easily be made from one by two inch strips, and covered with ordinary plastering lath spaced their own width apart.
Or, consider garden statuary that can provide shade as well as decoration.
Soil Perfect drainage is just as essential in the frames as in the rock garden itself.
Dig out six or eight inches of soil, and fill in with a six-inch layer of gravel or cinders; fill in over this with a mixture of one-third each of sand, granulated peatmoss, and light garden loam.
Then top off with two inches, one-half sand and one-half peatmoss, screened to make it loose and fine.
For lime-loving varieties, agricultural lime, at the rate of a quart or two to the bushel, may be added.
Sowing the Seed The seeds of many of the rock plants in nature remain in the ground over winter, and start, almost under the melting snows, in the spring.
They may be planted, therefore, in the autumn (late October or November), covered with leaves, and left until March, when the bedding is removed in time for them to get up.
Others should be planted just as early in the spring as possible; so as to allow a full growing season before their first winter; for they must be firmly anchored by their long roots to prevent "heaving out" in climates where there is danger of alternate freezing and thawing-a danger which they are not subjected to in their native haunts.
One of the difficulties in growing alpines and rock plants from seed is that many of the varieties quickly lose their vitality.
It is best to order seeds of these some time in advance, so they may be sent to you by the seeds man or the collector immediately the fresh crop is available.
Some sorts should be sown as quickly as possible after they are gathered.
Directions, which usually accompany the seeds, should be carefully followed.
Several hours before you expect to sow the seed, give the bed or frame a very thorough watering.
Garden water falls or some sort of irrigation system may be a smart decision.
If the soil is not quite dried off by the time you are ready to sow, an additional sprinkling of the prepared soil, a quarter of an inch or so in depth, will permit the beginning of operations, and will absorb moisture from the soil below so that it will not be necessary to water on the surface while it is absolutely dry, and will not soak up the moisture quickly where it falls.
Most of the seeds are so small that they will need covering barely from sight, with fine sand.
The very smallest should be merely pressed into the surface.
Those of measurable size should be covered to a depth of about their own thickness.
While the soil should never be allowed to actually dry out, it must be remembered that these plants dislike water on the surface of the soil as much when they are in the baby stage as when full grown.
If outdoor water features are to be included in the rock garden, make sure their functions are carefully controlled.
Particular care should be taken to water the small seedlings only on bright days, so that the foliage will have a chance to dry out thoroughly by evening.
The ordinary American can is not at all suited to watering delicate seedlings; the oval English pattern, with a very fine spray and a long spout, making possible the application of water without having it fall from a distance above the plants, is better.