For the very finest sweet pea blooms for cut flowers and exhibiting we are always recommended to sow in the autumn. Bernard Jones explains why and describes the ideal technique.
The sweet pea, Lathyrus odoratus, known traditionally in the form of the tall-growing English Spencer type as the 'Queen of Annuals', is, of course, grown from seed. Sowing is done during spring or autumn, but the latter produces far superior results for the slower growth through the winter fosters sturdier plants and stockier shoots far more able to withstand any hard weather after planting out. Likewise, the longer life in the pots produces a prolific root system and, in turn, more vigorous growth throughout the longer flowering season. This in turn gives longer stems, extra size and quality of bloom - and in good time for the early shows if exhibiting. The disadvantages of autumn sowing are merely the chores of the winter care, actually a routine of great interest.
A cold frame, based on soil and in an open position facing the sun, is ideal for raising the plants. It should be shallow to encourage short-jointed growth, say 10in (25cm) at the rear and 8in (20cm) at the front, stout walled and employing Dutch lights - an extra set is an advantage during severe weather. Essential for replacing each light whenever off is a wooden frame holding a taut network of black cotton or plastic mesh for protection against attack from birds. Also necessary are wooden blocks for propping the lights open to allow ventilation and padding material, e.g. old carpeting, for extra protection during frost; a boon, too, is a sheet of medium grade polythene large enough to cover the whole frame and extend a few feet outwards overall.
An average sowing date is early autumn, October 7-14th in the UK. If soil of the flowering quarters is light or medium use 5in (2.5cm) pots (preferably clay ones), cleaned well and finally rinsed in a weak solution of disinfectant two weeks beforehand. If the soil is heavy and it is difficult to plant bare rooted plants in, sow in boxes or trays 3in (7.5cm) deep with suitable small pots to hand for potting-on singly into later, this is to simplify planting out from with ball of soil intact; the old 'long tom' clay pots, 2¾in (7cm) diameter and 4in (10cm) deep, are ideal if obtainable, but long bottomless tube-type containers, which disintegrate after planting, are excellent and widely available.
For sowing, use a reliable soil based seed compost such as John Innes Seed Compost. Divide this into two parts, two thirds as the rooting phase, and one third the germinating, the latter with a good mixture of a soil-less compost well mixed in. This provides the perfect friable compost for healthy seed germination. Ensure both composts are moist at sowing time by building each into a heap several days beforehand, watering gently layer by layer with a fine rose on the watering can and allowing the water to thoroughly permeate and drain; this produces the ideal moist yet crumbly condition (i.e. not wet and soggy).
Always sow plenty of seed to allow ample selection at planting time. Seeds are broadly of four types:-
Now the sowing itself, firstly if using 5in (2.5cm) pots and say batches of six to eight at a time. Crock each pot and fill a generous two thirds full with the moist rooting compost, tap to settle, then give a really good soak with water (the pre-moistening already advised ensures a good take up of this watering) and allow to drain. This lower reservoir ensures moisture rise as needed and the avoidance of drying out of the compost above if warm conditions occur during germination. Now fill to the top with the moist germinating compost, level across and make six pencil holes ¾ inch (2cm) deep in a circle 1 inch (2.5cm) from the rim and drop one seed in each (ensure none are caught half-way down!). Fill in and firm down gently but decidedly with the bottom of a small pot, this will produce a useful ¾ inch (2cm) space at the top of the pot. Label each pot and place in a pre-planned order in the frame, cover with newspaper, slide on light(s) snugly and apply protective covering at night if cold; if sunny by day, shade with sacking to avoid overheating. Essential from the word go are slug-bait between the pots and an occasional sprinkling of paraffin around the outside of the frame will repel mice.
Examine the rear pots (usually the earliest) daily from the fourth day after sowing and at the very first signs of soil lift remove the paper but leave the glass on for a day or so to encourage an even germination, then prop the frame light open to allow air at back for a further day or two before removing glass completely; but replace the glass for a further few nights to encourage any laggards.
If sowing in a box or tray for potting-on later, ensure drainage then follow the subsequent routine step by step exactly as just described when using pots, but sow the seeds 1½in (4cm) apart in rows as much apart, finally filling in and firming down with a flat wooden block. Place in the cold frame and again follow the pot routine exactly, removing paper at first signs of soil lift. When the plants are 1½in (4cm) tall, with first pair of leaves still unfurled, lever each up carefully, nip off the top of the still single tap root (to induce a well-branched root system) before potting-on singly. If bottomless pots they should be already stacked together on a layer of peat in a suitable box. The pots should be previously filled with the moist rooting compost - a deep pencil hole simplifies the transplanting. Arrange the seed at its previous depth, and ensure a firm planting with a ½in (13mm) gap above the compost. Finally, place potted seedlings back in frame, lightly water with a fine rose on the can and keep the glass on (propped up if not cold) for a few days to foster prompt rooting.
Whether in 5in (2.5cm) pots or singly in smaller pots, grow on slowly and hard within sensible limits, i.e. glass off day and night except when weather forecast (a daily must) indicates night frost. As a broad guide, if temperature drops to 37-40F (3-4C), leave the frames open: down to 33-35F (1-2C), glass on but air allowed: 32-30F (-1 to 0C), glass on snugly: 29-27F (-2 to -3C), double lights (or single plus covering). Finally, if lower temperatures are forecast, increase the covering and lay the polythene sheeting over the whole frame stretched outward and secured with a few bricks on all four sides, to keep the insulating covering dry and provide further useful insulation.
If the frost is merely overnight, remove glass and any covering in the morning, replacing it at night if necessary; if persisting by day, maintain protective material and polythene until conditions improve. On rare occasions, persistent very severe frost may even penetrate the protective barrier and affect plants and compost. In that case maintain the covering fully until plants and compost are quite thawed out (test by inserting a long pin into a few random pots). Sudden exposure to air and sunlight can be disastrous to hard-frozen plant tissue and a slow thaw, even taking several days, is good insurance. Likewise, as good policy, ensure the compost is always moist - for plants then withstand hard frost far better than if the pots are on the dry side. As planting time (and spring) approaches and nights shorten, protection can be gradually relaxed - within sensible limits, and plants allowed to experience the odd degree or two below zero more freely.
Now an important cultural point. The first shoot produced rarely grows on, and it is the side-shoots produced at the two joints below the bottom pair of leaves which build up to the flowering plant. Growers are sometimes tempted to stop their plants during November, i.e. remove the growing point above the second pair of leaves to divert sap flow to stimulating side-shoot development. However, such early side-shoots become over-tall, ungainly and hard put to withstand wind and frost after planting out in March. So be patient and if side-shoots are appearing by the New Year leave well alone and even do not stop at all, for the main shoot will peter out anyway. But if any plants are showing no or negligible side-shoot evidence by late January, stop them, when shoots will soon appear and grow on satisfactorily.
Two further points on the frame phase. Firstly keeping the plants well up to glass level (fostering sturdy growth) will, in turn, entail sinking the pots a few inches into the soil two or three times during the winter to avoid shoot and glass contact. Secondly the space advised at the top of the pots earlier allows topping up (after pricking over the surface) with a richer soil-based compost such as John Innes Compost No. 1, a month before planting out.
At planting time itself, the plants should be a warming sight, with the aim and routine of autumn sowing fully achieved - short-jointed plants, hard grown and well rooted, and each with two or more sturdy side shoots. Two further points merit mention; firstly, when selecting for planting, ensure that the neck of each plant, i.e. the white seed stem below soil level, is free from a brown mark or collar, unlikely in our case but if an odd one is suspect do not use, for early collapse is highly probable; secondly, the proper depth when planting is with the bottom side-shoot at soil level.
Planting time generally means early spring from the autumn sowing and the method of growing, if just for cut bloom, may be the natural, bush or free style system, whereby the plants are grown without restriction as clumps or rows on reasonably prepared ground and given bushy pea-sticks or netting for support during upward climb. If grown for highest class cut bloom or exhibition, however, the plants are grown on richly prepared ground and on the cordon system, whereby each is trained up a bamboo cane and restricted to one main stem by removing the side-shoots (also the tendrils as they are not needed). This entails a degree of attention, often daily, securing the plants to the canes, maintaining the cordon routine, and seeing to watering and feeding, but has been justifiably described as the most fascinating and rewarding pursuit in horticulture.
Bernard Jones is the breeder of many fine sweet pea varieties and author of The Complete Guide to Sweet Peas.
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Source of article
Growing From Seed - Summer 1987 Vol. 1 Number 3
© The Seed Raising Journal from Thompson & Morgan