There is one remarkable difference in the way seeds of hardy perennials germinate. Some are quite happy to be harvested and kept for several years, if necessary, before sowing, still retaining their ability to grow when the right conditions come along; others lose their power to grow, their viability, very quickly, and give best results when sown almost immediately after harvesting.
Typical of the latter is the whole Primula family. Hundred of self-sown seedlings can often be found coming up in the autumn, around a cowslip or a polyanthus (both members of the family), just where they have dropped only a week or two before. Yet if this same seed had been gathered and kept packeted for sowing in spring, only a low percentage would have germinated. Their viability would have been lost.
Some of the long-keeping seeds have to experience winter temperatures and be frozen before they will germinate well. Hellebores and Hemerocallis (day lily) for example, are best sown in the autumn in pots or pans, and left outside through the winter to germinate when the weather changes in spring.
The majority of hardy perennials, however, can be sown from March mini June. The aim must be to produce plants in that first summer, big and strong enough to come unharmed through their first winter, and sowing time should be geared to this.
Notoriously slow growers are best sown in March in a warm greenhouse. Quicker growers can be sown in a cold house or in frame in April or May, or even outside in June.
The method of sowing is the same as for half-hardy annuals. A good seed compost, well moistened in pots, or boxes, thin sowing, and very fine covering if seeds are small, with a sheet of glass and paper on top.
Seedlings are pricked out in boxes when big enough to handle, or, more likely with hardy perennials, potted up individually. Then they are gradually introduced to outside conditions as they grow, and planted in their permanent positions when convenient, either in the autumn or the following spring. Some, such as delphiniums, will even produce the odd flower in their first autumn, but the small plants will grow bigger and better if this is not allowed to develop and it is pinched off as soon as seen.
