Oenothera.—­The Evening Primroses are most useful and beautiful plants, well suited for ornamenting borders, beds, edgings, or rock-work.  All the species are free-flowering, and grow well in any good, rich soil.  The annual and biennial kinds are sown in the open in spring.  The perennials may be increased by dividing the roots, by cuttings, or by seed, the plants from which will flower the first season if sown early in spring.  They bloom in June and July.  Height, 6 in. to 4 ft.

Olearia.—­These evergreen shrubs thrive in peat and loam, and may be increased by division of the roots.  O. Haastii has foliage resembling the Box, and a profusion of white, sweet-scented flowers in summer:  a chalk soil suits it admirably.  Height, 3 ft. to 4 ft.

Omphalodes Verna.—­A hardy perennial which may be grown under the shade of trees in ordinary soil.  It produces its flowers in March, and is increased by dividing the roots in autumn.  Height, 6 in.

Oncidium Sarcodes.—­Plant these Orchids firmly in well-drained pots, using equal parts of live sphagnum and fibrous peat.  Give one good watering as soon as the potting is finished, and stand them in a light, warm part of the greenhouse.  They will require very little more water until the roots have taken hold of the soil—­only sufficient to keep the pseudo-bulbs from shrivelling—­and during the winter months scarcely any moisture is needed.  They flower in August.  Height, 1-1/2 ft.

Onions.—­Require a deep, rich, heavy soil.  Where the ground is not suitable it should have had a good dressing of rotten manure the previous autumn, and left in ridges during the winter.  Level the ground, and make it very firm just before the time of sowing.  The seed should be sown early in March for the main crop and for salad and pickling Onions, and in August for summer use.  Thin out to about 6 in. apart, excepting those intended to be gathered while small.  The Tripoli varieties attain a large size if transplanted in the spring.  The Silver-skins do best on a poor soil.  For exhibition Onions sow in boxes early in February in a greenhouse; when about 1 in. high prick out, 3 in. apart, into other boxes; give gentle heat and plenty of air, and when they have grown 6 in. high put them in a cool frame until the middle of April, when they must be planted in the open, 1 ft. apart.

Ononis Rotundifolia (Round-leaved Restharrow).—­A charming hardy evergreen of a shrubby nature.  It will grow in any ordinary garden soil, and is increased by seed, sown as soon as it is ripe.  It is most effective in clumps, and blooms from June to September.  Height, 1-1/2 ft.

Onopordon.—­Half-hardy perennials of a rather interesting nature and of easy cultivation.  Sow the seed any time between March and June.  They require the protection of a frame or greenhouse during winter, and produce flowers in July.  Height, 6 in. to 8 ft.

Onosma Taurica (Golden Drop).—­This hardy herbaceous plant is very pretty when in flower, and suitable for rock-work.  It requires a well-drained vegetable mould, and to be planted where it can obtain plenty of sun.  It is increased from cuttings taken in summer, placed in a cucumber frame, kept shaded for about a fortnight, and hardened off before the winter.  The flowers succeed one another from June to November.  Height, 1 ft.