The regular biennials may be treated practically as annuals; that is, seeds may be sown every year, and after the first year, therefore, a seasonal succession of bloom may be had. Of such are adlumia, Canterbury bell, lunaria, ipomopsis, Œnothera Lamarckiana; and foxglove, valerian, and some other perennials would better be treated as biennials.
Most annuals will bloom in central New York if the seeds are sown in the open ground when the weather becomes thoroughly settled. But there are some kinds, as the late cosmos and moon-flowers, for which the northern season is commonly too short to give good bloom unless they are started very early indoors.
If flowers of any annual are wanted extra early, the seeds should be started under cover. A greenhouse is not necessary for this purpose, although best results are to be expected with such a building. The seed may be sown in boxes, and these boxes then placed in a sheltered position on the warm side of a building. At night they may be covered with boards or matting. In very cold “spells” the boxes should be brought inside. In this simple way seeds may often be started one to three weeks ahead of the time when they can be sown in the open garden. Moreover, the plants are likely to receive better care in these boxes, and therefore to grow more rapidly. Of course, if still earlier results are desired, the seed should be sown in the kitchen, hotbed, coldframe, or in a greenhouse. In starting plants ahead of the season, be careful not to use too deep boxes. The gardener’s “flat” may be taken as a suggestion. Three inches of earth is sufficient, and in some cases (as when the plants are started late) half this depth is enough.
The difficulty with early sown seedlings is “drawing up,” and weakness from crowding and want of light. This is most liable to occur with window-grown plants. Vigorous June-sown plants are better than such weaklings. It must be remembered that very early bloom usually means the shortening of the season at the other end; this may be remedied to some extent by making sowings at different times.
The “hardy” annuals are such as develop readily without the aid of artificial heat. They are commonly sown in May or earlier, directly in the open ground where they are to grow. Florists often sow certain kinds in the fall, and winter the young plants in coldframes. They may also be wintered under a covering of leaves or evergreen boughs. Some of the hardy annuals (as sweet pea) withstand considerable frost. The “half-hardy” and “tender” annuals are alike in that they require more warmth for their germination and growth. The tender kinds are very quickly sensitive to frost. Both these, like the hardy kinds, may be sown in the open ground, but not until the weather has become settled and warm, which for the tender kinds will not commonly be before the first of June; but the tender kinds, at least, are preferably started in the house and transplanted to their outdoor beds. Of course, these terms are wholly relative. What may be a tender annual in Massachusetts may be a hardy annual or even a perennial in Louisiana.
These terms as ordinarily used in this country refer to the northern states, or not farther south than middle Atlantic states.
Some familiar examples of hardy annuals are sweet alyssum, ageratum, calendula, calliopsis, candytuft, Centaurea Cyanus, clarkia, larkspur, gilia, California poppy, morning-glory, marigold, mignonette, nemophila, pansy, phlox, pinks, poppies, portulaca, zinnia, sweet pea, scabiosa.
Examples of half-hardy annuals are: China aster, alonsoa, balsam, petunia, ricinus, stocks, balloon-vine, martynia, salpiglossis, thunbergia, nasturtium, verbena.
Examples of tender annuals: Amarantus, celosia or coxcomb, cosmos, cotton, Lobelia Erinus, cobea, gourds, ice-plant, sensitive-plant, solanums, torenia, and such things as dahlias, caladiums, and acalypha used for bedding and subtropical effects.
Some annuals do not bear transplanting well; as poppies, bartonia, Venus’ looking-glass, the dwarf convolvulus, lupinus, and malope. It is best, therefore, to sow them where they are to grow.