Annuals: As Pot Plants

There are a number of annuals which make good pot plants for the greenhouse, providing welcome colour from April until July and sometimes August.

Annuals need growing coolly and a temperature near but not below 4°C. (40°F.) suits them well. Too much heat must be avoided or the plants will become drawn and weak. The seed should be sown, with some exceptions, including celosia, exacum and nicotiana, in September and early October. These exceptions, too, have different temperature requirements.

Seed of those annuals included in the main autumn sowing should be sown in boxes of seed compost and germinated either in a cool greenhouse – on a shelf near the glass-or in a cold frame, preferably the latter. Water and ventilate the seedlings with especial care for the greenhouse can soon become cold and clammy at that time of year and damping off may occur.

The seedlings should be pricked out in 3-in. pots as soon as possible using John Innes No. 1 Potting Compost. The pots should be kept on a shelf near the glass during the winter, for good light is essential, and both high and low temperatures must be avoided. Move into 5-in. pots as necessary.

For a spectacular display, three, four or five seedlings can be planted in one large pot but they may become too large for the average greenhouse.

Some annuals are better when stopped, the tips of the shoots being pinched out when they are about 4 or 5 in. high to encourage the plants to develop a bushy habit, but most are best left to grow naturally.

My choice of annuals for pot cultivation is as follows:

Carnations (Annual)

Among the annual carnations, strains of Dianthus caryophyllus, are the Chabaud Giants, with fringed, double flowers in an attractive range of colours which include shades of red, pink, yellow and white. They are sweetly scented. Pinch out the tips of the shoots when 4 or 5 in. long to encourage the plants to form a bushy habit.


The celosias (Cockscombs) are splendid pot plants for greenhouse decoration. Celosia cristata, with crests of tightly packed flowers, grows 9 to 12 in. tall and there are dwarf strains like Jewel Box Mixed, only 6 in. tall. Celosiaplumosa has feathery plumes of flowers which are very decorative. Dwarf Red Plume and Dwarf Golden Plume are only 9 in. tall, but there are other varieties which reach 18 in. Colours include various shades of red, orange and yellow.

The seed is sown in seed compost in March and germinated in a temperature of 18°C. (65°F.). The resulting seedlings are pricked out into seed boxes of John Innes No. 1 Potting Compost, spaced 2 in. apart, when they are about 1 in. tall and are later moved into 3-in. pots of a similar mixture. A temperature of 16 to 18°C. (60 to 65°F.) is needed at this stage. By June they are ready for moving into the 5-in. pots in which they will flower. Watering must be done with care, particularly when they are young, and they must not be allowed to become dry or over-moist. The foliage should be syringed twice a day and the plants fed once a week with a liquid or soluble fertiliser when the flower buds first appear.

Centaurea cyanus

This plant is the popular Cornflower and there are dwarf strains which make good pot plants. These grow from 1 to 1-1/2 ft. tall and include Polka Dot Mixed with a wide colour range and the blue Dwarf Double Jubilee Gem. Pinch out the tips of the plants when they are 4 to 5 in. long so that they form a bushy habit. Cornflowers like sunshine and dry atmospheric conditions. Grown three or four in an 8-in. pot, they are very effective in bloom.

Clarkia elegans

The double-flowered varieties, 2 to 2-½ ft., arising from this species are excellent pot plants. Their colour range embraces purple, mauve, shades of red and pink and white. They are usually very successful when grown three to a 6-in. pot. Pinch out the tips of the shoots when 4- or 5-in. Long to encourage the plants to make a bushy habit. Ventilate the greenhouse freely whenever possible.


The South African Star of the Veldt is a delightful annual with daisy-like flowers in orange, apricot, salmon, and yellow and white. The aurantiaca hybrids, 12 in. tall, are beautiful and popular; others include the large-flowered Goliath, orange with a green centre, 15 in., and the dwarf Glistening White, 9 in. These also can all be grown three to a 6-in. pot.

Exacum affine

This is an attractive greenhouse plant, up to 12 in. high, which bears fragrant pale blue flowers.

It requires warm, moist conditions, with good drainage and shade from hot sun. Although a biennial, seed can be sown in March in seed compost in a temperature of 16 to 18°C. (60 to 65°F.) to provide plants to flower in late summer and autumn. Larger plants can be obtained by sowing in August and overwintering the plants in 3-in. pots. These are then potted on into 5-in. pots the following spring. Such plants need a temperature in winter of 16°C. (60°F.).


These are excellent pot plants, providing a display over several months in the early part of the year. They include dwarf varieties and strains of 1 to 1-1/2 ft. in height – especially suitable for pot cultivation – and others growing to 2 to 3 ft. Both double and single varieties are available in colours from mauve and red to pink, salmon, rose and white. The tips of the shoots should be pinched out when 4 or 5 in. long to encourage a bushy habit of growth.


Included amongst the annual delphiniums are varieties of Delphinium ajacis, the Rocket Larkspur, 2 to 4 ft., and D. consolida, the Branching Larkspur, 3 to 4 ft. The colour range includes blue, lilac, pink and white. The smaller growing D. chinensis (D. grandiflorum) includes lovely blue varieties like Blue Butterfly and Blue Mirror. These are strictly speaking perennials, but are treated as annuals.


This plant, botanically Reseda odorata, is one of the best fragrant plants for greenhouse cultivation. Seed should be sown in August using John Innes No. 1 Potting Compost in the 6-in. pots in which the plants will flower, the resulting seedlings being thinned to about 1 in. apart as soon as they can be handled. Grow on as detailed for other annuals. Provide very little water in winter but water freely when the plants are coming into bloom. Staking is necessary to keep the growths upright. A spring sowing will provide flowering plants for the cool greenhouse in summer and autumn.


The nicotianas or Tobacco Plants are attractive, fragrant flowers which make good pot plants. The smaller varieties of Nicotiana affinis are especially suitable for this purpose. Colours include red, white and green.

Seed should be sown in February and germinated in seed compost in a temperature of 16°C. (60°F.) and the resulting seedlings potted separately in 3-in. pots of John Innes No. 1 Potting Compost as soon as they can be handled. These are grown on in a temperature of 13°C. (55°F.) and moved to 6-in. pots when their root development makes this necessary.


The 2-ft. tall Salpiglossis sinuata is a very effective pot plant. It has large, tubular-shaped flowers in many colours. Including crimson, scarlet, blue, purple and white with gold markings providing a striking contrast. The plants need staking early as they tend to flop and pinching out the tips is desirable to induce a shorter, more branching habit.


The Sweet Scabious, Scabiosa atropurpurea has double- and single-flowered varieties of many colours from reds and pinks, to purples, blues, lavender and white. The foliage is deeply divided and admirably complements the flowers on their long stems. The double-flowered dwarf strains of about 1-½ ft. in height are especially suitable for pot cultivation. If the plants are grown four to a 6-in. pot they will soon make a very bright and attractive display.

01. March 2012 by Dave Pinkney
Categories: Featured Articles, Greenhouse Gardening, Plants & Trees | Tags: , | Comments Off on Annuals: As Pot Plants


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