Half Hardy Annuals Growing Guide

Like hardy annuals these complete their cycle of growth in one year, but, unlike them, they cannot be sown outdoors in early spring because of the danger of frost. Most must be raised under glass, but there are borderline varieties, nearly but not fully hardy, that may be sown outdoors in late April or early May. All can be grown outdoors without protection during the summer. Half-hardy annuals are also grown as pot plants for greenhouse decoration.

Soil and Situation. The same as for hardy annuals with, if anything, more emphasis on good drainage and a sunny position. Nicotiana is one of the few exceptions which likes shade.

Sowing. Seed of most varieties can be sown in a warm greenhouse towards the end of February or in an unheated greenhouse or frame in March-April. Temperature of about 60° is required for germination. Seed is sown in well-drained pots, pans, or boxes in any good seed compost. It should be scattered very thinly, covered lightly with a sprinkling of finely sifted soil and then each receptacle covered with a sheet of glass and another of brown paper. The paper must be removed at the first signs of germination. A day later tilt the glass slightly with a pebble or stick, and two or three days after this remove altogether.

half hardy and greenhouse plants

Soil must be kept moist. This is best done by holding each pan for a few moments almost to its rim in a bucket of water with the chill off. When the rising water darkens the surface of the soil, remove the pan and allow it to drain.

Pricking off. When the seedlings have made two or three leaves each, they must be transferred carefully to other trays or pans prepared in a similar manner. Lift carefully with a pointed stick, divide into single plants with as little injury to the roots as possible and replant with a round-ended dibber about as thick as an ordinary lead pencil. The seedlings should be planted about 2 in. apart each way. Make the soil firm round the roots, water freely overhead through a fine rose and return to the greenhouse or frame. Shade from strong sunlight for a day or so until growth resumes. Subsequently, give full exposure to sun and ventilate as freely as possible, consistent with a minimum temperature of 55° by day, 500 by night.

Hardening off. The seedlings must be gradually accustomed to the outdoor temperature. Those raised in the warm greenhouse should be transferred to a frame about mid-April. Ventilation must be further increased as weather permits until by mid-May lights are removed entirely except when frost threatens, as it may on clear, calm nights.

Planting. In some sheltered gardens half-hardy annuals can be planted out early in May, but in most localities it is not safe until the end of May or even until early June. Remove seedlings from the boxes or pans with as little soil disturbance as possible. This is most readily done if they have been well watered a few hours previously. Divide into separate plants and replant with a trowel. Make the soil thoroughly firm around the roots and water in freely. Small varieties should be 6 in. apart; those of medium height 9 or 12 in., large-growing kinds, 1½ to 2 ft..

Culture. The same as for hardy annuals. The trailing growths of Phlox drummondii should be pegged to the soil. Floppy varieties can be supported with bushy twigs.

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