Mr. B. Thompson surveys the various methods of germinating this ever-popular plant
The criterion for the germinating of a sweet pea is similar to any other dormant seed, it needs moisture and warmth to stir it into life, it then goes on to form a seedling followed by a fully grown plant which will eventually flower.
The average gardener will probably sow a packet of mixed sweet peas containing possibly twenty or thirty seeds. Some of these will germinate, others will not and rot away. The percentage that do germinate will almost certainly be colours or varieties which germinate naturally without too much trouble, and the end result of this might be a far from satisfactory combination. It may also follow that some varieties which do not germinate are those which are heavily scented and so it could turn out to be a double disappointment. However, with just a little effort and possibly a slight change of method or just a few helpful hints, a much improved rate of germination can be achieved. The following methods are all tried and tested by both exhibitors and by natural growers who grow for cutting and garden display, but every gardener has his own special way of sowing seeds, the proof of which is in his own results.
Regardless of the strain of sweet pea you have chosen to sow, e.g. Spencer, Old Fashioned, Snoopee, Bijou, Jet Set or Galaxy to name but a few, first of all decide if the seeds need chitting. Chipping is the term used by sweet pea experts and means removal of a very small part of the hard outer seed coat. This is done on the side opposite the 'eye' and some form of sharp instrument is necessary, either a knife to take off a small slice, or a file to file down a small area and rub a piece of the coating off. The reason for chipping is that some of the seeds have a very hard coating and the moisture which the seeds need to be able to absorb in order to germinate cannot penetrate some of the harder coatedseeds. The way to decide which seeds need chipping and which do not, is first of all to soak them all for a few hours in a table-spoon or two of tepid water. By and large you will soon discover that the darker the colour of the sweet pea flower, the harder the coat is, and therefore even after it has been soaked you will find little difference in the size of the pea, whereas the lighter coloured varieties will swell rapidly and become soft. In fact frequently these light coloured flower seeds need neither chipping nor soaking, but unless you know the colour of the sweet pea you are sowing it is better to give them a short soak for a few hours which will do no harm as long as they are patted dry afterwards on tissue paper or kitchen roll.
Having soaked the seeds and then chipped those which have not softened and/or swollen up, you are now ready to sow seeds, but prior to this a good sowing compost must be chosen depending on which of the following methods you wish to try.
The first method is one whereby once the seeds are sown they will not be disturbed again until they are actually planted out. Usually when this method is chosen the seeds are sown in autumn (mid-October). A compost such as John Innes 2 or 3 which is soil based loam and is a good choice or alternatively a peat-based mixture with perlite or vermiculite added to it. It needs the addition of one of these in order to make the peat less compact as otherwise the seeds may suffer from too much moisture clinging to the seed and causing it to rot away.
Any 5 or 6in (13 or 15cm) pot may be chosen for this method. They should be filled almost to the top with the compost of choice and the seeds evenly spaced around the pot and planted to a depth of about ½in. (1.3cm), usually 5 or 6 seeds per pot of this size. The pots should then be placed in a cold frame, or cool greenhouse and be left to grow to an approximate height of about 4-6in (10-15cm) by the following spring.
A few degrees of frost is necessary for autumn sown seeds, as it is nature's way of stopping the growing tip, this then encourages side shoots to develop and since the flowers will form on the side shoots, the more side shoots the greater the abundance of flowers should be. If the frost is not severe enough to stop the tip from growing, then sometime between mid-February and early March it will be necessary to pinch this tip out gently with finger and thumb. On the other hand, should the winter be particularly severe or prolonged, some form of makeshift protection on the cold frames or a very low heat to keep it just above freezing may be necessary in the greenhouse.
The second method may be used for sowing sweet pea seeds in either the autumn or in January to March. This way enables you to check which seeds have germinated before potting on so that there is no wastage in the pots if you are short of space for growing. First take a length of kitchen roll and fold it in half to double the thickness. Wet it gently under a tap and carefully squeeze it out so that it is evenly damp all over. Lay the double folded paper flat on a shallow dish, saucer or any shallow container depending on the number of seeds you are sowing. Spread the seeds well spaced out from each other and cover with a similar piece of damp kitchen roll. The container should then be kept either in a dark place or covered with brown card or paper in a temperature not exceeding 70F (21C). After 3 days check the container for signs of germination and at the same time make sure the kitchen roll remains damp (but not wet). From 3 days onwards, once the seeds have about Oin. (1.3cm) tip and root showing, they are ready for planting on.
The last method is again for either autumn or January to March sowing, and again is a way of ensuring that only germinated seeds are potted on. However, it is a method which requires at least a basic propagator, although it certainly does not have to be anything sophisticated as long as it can maintain and be regulated at a constant temperature, never exceeding 70F (21C), approximately 58-62F (14-17C) is best. The use of a propagator is to ensure that the seeds receive even bottom heat. Once the seeds have been soaked and chitted gently, dry them in tissue paper and roll them in seed dressing, taking care that it is only a light dusting adhering to each seed. The seeds are then placed at evenly spaced intervals in a seed tray filled with a proprietary seed compost. The compost should be damp but not wet, and the seeds should finally be covered with another ½in. (1.3cm) of compost. The propagator should be kept away from the light, or covered to exclude the light for the first 3-4 days or until the first sign of growth has reached ½-1in (1.3-2.5cm), each plant should be carefully prised - using something equivalent to a lollipop stick or the back of a teaspoon, pricked and placed in pots.
For the second and third methods use a John Innes No. 3 or a proprietary peat-based compost, again mixed with a little perlite or vermiculite, to encourage good root growth, and depending on the space available, either individual pots or larger pots containing 5 or 6 plants may be used for potting on. At this stage the plants will require very little warmth if sown in the autumn, as they will be sufficiently hardy to survive the winter in all but the most severe frosts. They should therefore after a couple of weeks, be placed outside -preferably in a cold frame to protect them from the wind. If they are January to March sown seeds, some early protection will be required, especially if the temperature drops below freezing.
The ideal is to keep the seeds in temperatures above freezing but below 50F (10C) so as an alternative to a cold greenhouse, a windowsill in a cold unheated spare room would be adequate, but do remember to keep turning the pots to avoid them growing towards the light.
The January to March sown plants will require the growing tip pinching out carefully after the first two pairs of leaves are fully open. The pots are unlikely to get too dry kept at a cool temperature but should be kept moist as opposed to wet.
These late sown seeds will need careful hardening off before planting in April/May.
Mr. Thompson is one of the leading growers of sweet peas.
Find more helpful information and advice about growing sweet peas on our helpful hub page.
Source of article
Growing From Seed - Winter 1989-90 Vol. 4 Number 1
© The Seed Raising Journal from Thompson & Morgan
© 2024 Thompson & Morgan. All rights reserved. A division of Branded Garden Products Limited.
Sign up for exclusive offers!