This section is from the book "Culinary Herbs", by M. G. Kains. Also available from Amazon: Culinary Herbs, Their Cultivation, Harvesting, Curing and Uses.
No more care is required in transplanting herbs than in resetting other plants, but unless a few essentials are realized in practice the results are sure to be unsatisfactory. Of course, the ideal way is to grow the plants in small flower pots and when they have formed a ball of roots, to set them in the garden. The next best is to grow them in seed pans or flats (shallow boxes) in which they should be set several inches apart as soon as large enough to handle, and in which they should be allowed to grow for a few weeks, to form a mass of roots. When these plants are to be set in the garden they should be broken apart by hand with as little loss of roots as possible.
But where neither of these plans can be practiced, as in the growing of the plants in little nursery beds either in hotbeds, cold frames or in the garden border, the plants should be "pricked out," that is. transplanted while very small to a second nursery bed, in order to make them "stocky" or sturdy and better able to take care of themselves when removed to final quarters. If this be done there should be no need of clipping back the tops to balance an excessive loss of roots, a necessity in case the plants are not so treated, or in case they become large or lanky in the second bed.
Leading Forms of Trowels
In all cases it is best to transplant when the ground is moist, as it is immediately after being dug or plowed. But this cannot always be arranged, neither can one always count upon a shower to moisten the soil just after the plants have been set. If advantage can be taken of an approaching rainfall, it should be done, because this is the ideal time for transplanting. It is much better than immediately after, which is perhaps next best. Transplanting in cloudy weather and toward evening is better than in sunny weather and in the morning.
Since the weather is prone to be coy, if not fickle, the manual part of transplanting should always be properly done. The plants should always be taken up with as little loss of roots as possible, be kept exposed to the air as short a time as possible, and when set in the ground have the soil packed firmly about their roots, so firmly that the operator may think it is almost too firm. After setting, the surface soil should be made loose, so as to act as a mulch and prevent the loss of moisture from the packed lower layer. If the ground be dry a hole may be made beside the plant and filled with water and when it has soaked away and the soil seems to be drying, the surface should be made smooth and loose as already mentioned. If possible such times should be avoided, because of the extra work entailed and the probable increased loss due to the unfavorable conditions.