There is considerable confusion about the two terms grandiflora and grandiflorus when used in the context of sweet peas or other Lathyrus species. Their similarity can lead people to assume that the terms are synonymous, and the problem has been confounded by gardening 'personalities' carelessly misusing the terms. Some popular seed catalogues further compound the confusion by offering their grandiflora sweet peas under the erroneous epithet Lathyrus grandiflorus.
The two species
To start with, we need to draw a distinction between the two species involved. There are many species of Lathyrus distributed in both the old and new worlds. Several are widely cultivated, and others are proving to be useful garden plants as they become more widely available. The two we need to consider in this context are Lathyrus odoratus and Lathyrus grandiflorus.
This is the true "Sweet Pea". It is an annual, and is native to southern Italy and Sicily. It was first described in 1696 and was first cultivated in northern Europe in 1699 after seed was sent to England and Holland. Several different colour forms appeared over time, but it was not until the late C19th that much work was done to improve the flower quality. The most famous of the sweet pea breeders of this period was Henry Eckford, who produced a great number of excellent varieties with larger and more impressive flowers than had previously existed. These varieties were accordingly named the 'grandiflora sweet peas' or simply 'grandifloras'.
As the flower size of these grandiflora varieties is quite small by modern standards, they are now frequently listed as 'heirloom', 'antique' or 'old fashioned' varieties. These more general terms are also used to include other old varieties such as 'Cupani Original' which, of course, predated the true grandifloras by nearly 200 years. These varieties are famous for their strong spicy fragrance and come in a huge range of colours and patterns.
A perennial species from southern Europe, known as the 'everlasting Pea'. A spectacular plant with good sized unscented flowers carried in twos and threes over a long season. Can be encouraged to climb, or left to sprawl naturally. The flowers are a rich magenta with a deep red keel. Lathyrus grandiflorus was introduced into the UK in 1814. Seed is rarely produced in cultivation and is not commercially available. It is, however, readily propagated from its spreading tuberous roots, and plants can often be found in garden centres. Otherwise check the RHS plant finder.
It is very disappointing to see supposedly knowledgable gardening "experts" confusing these two very distinct species, and even more worrying that some widely distributed seed catalogues serve only to increase the confusion. There is no such plant as Lathyrus grandiflorus 'Matucana'.