This section is from the book "Culinary Herbs", by M. G. Kains. Also available from Amazon: Culinary Herbs, Their Cultivation, Harvesting, Curing and Uses.
Most herbs may be readily propagated by means of seeds. Some, however, such as tarragon, which does not produce seed, and several other perennial kinds, are propagated by division, layers, or cuttings.
Flat of Seedlings Ready to Be Transplanted
In general, propagation by means of seed is considered most satisfactory. Since the seeds in many instances are small or are slow to germinate, they are usually sown in shallow boxes or seed pans. When the seedlings are large enough to be handled they are transplanted to small pots or somewhat deeper flats or boxes, a couple of inches being allowed between the plants. When conditions are favorable in the garden; that is, when the soil is moist and warm and the season has become settled, the plantlets may be removed to permanent quarters.
If the seed be sown out of doors, it is a good practice to sow a few radish seeds in the same row with the herb seeds, particularly if these latter take a long time to germinate or are very small, as marjoram, savory and thyme. The variety of radish chosen should be a turnip-rooted sort of exceedingly rapid growth, and with few and small leaves. The radishes serve to mark the rows and thus enable cultivation to commence much earlier than if the herbs were sown alone. They should be pulled early- the earlier the better after the herb plantlets appear. Never should the radishes be allowed to crowd the herbs.
By the narration of a little incident, I may illustrate the necessity of sowing these radish seeds thinly. Having explained to some juvenile gardeners that the radish seeds should be dropped so far apart among the other seeds that they would look lonesome in the bottoms of the rows-not more than six seeds to the foot-and having illustrated my meaning by sowing a row myself, I let each one take his turn at sowing. While I watched them all went well. But, alas, for precept and example! To judge by the general result after the plants were up, the seedsman might justifiably have guaranteed the seed to germinate about 500 per cent, because each boy declared that he sowed his rows thinly. Nevertheless, there was a stand of radishes that would have gladdened the heart of a lawn maker! The rows looked like regiments drawn up in close order and not, as was desired, merely lines of scattered skirmishers. In many places there were more than 100 to the foot! Fortunately the variety was a quick-maturing kind and the crop, for such it became, was harvested before any damage was done the slow-appearing seedlings, whose positions the radishes were intended to indicate.