Although sweet peas all belong to the same botanical species, Lathyrus odoratus, sweet pea seeds vary considerably in colour size and shape. A small wrinkled seed with dark sunken areas would be an immediate reject in most pink varieties, but would represent a normal healthy seed for some lavender or fancy sweet peas.
Generally there are about 10 sweet pea seeds to the gram, 280 to the ounce, but this is only a guide figure and individual seed samples can be very different. Pink and picotee spencer varieties tend to have large seeds, typically 8 to the gram, while many lavender and blue varieties have very poor looking seed, with 12 or 14 to the gram. The grandiflora, or heirloom sweet peas have rather smaller seeds than the Spencers, averaging about 12 seeds to the gram, blue and lavender varieties again being smaller than the average.
Sweet peas are classified as being 'black' or 'white' seeded. Neither epithet should be taken too literally. 'Black' seeds can be grey or brown, while 'white' seeds are cream or fawn in colour. White seeds seem to be more prone to rotting under unfavourable conditions, but the flowers of white seeded sweet peas often display clearer colours.
Many blue and lavender varieties have mottled seed. Viewed with a hand lens the darker patches can be seen to be slightly sunken. This is entirely normal, and all attempts to isolate a pathogen from such seed have failed.
Like the edible garden pea, sweet peas can have wrinkled or round seeds. Sweet peas with striped flowers such as Mars and Lilac Ripple tend to have large wrinkled seeds. The difference is of no consequence to the gardener as both sorts perform equally well.
Sweet pea seeds have a 'scar' or hilum which marks the point of attachment of the funiculus, the short stalk which connects the seed to the inside of the pod, or legume. Although the dried funiculus is normally shed by the ripe sweet pea seed, in some varieties it is persistant and remains as a pale crescent shaped fragment of tissue covering the hilum. This may cause concern to gardeners who think that the seed has started to germinate and that the funiculus is a dried root.