'Nelly Moser' Clematis is a large flowered clematis hybrid. It bears single, traditional clematis flowers which are striking because of their color: Rose or lavender with pink or white stripes.
The size of this unusual clematis flower makes it even showier. Each bloom is 6-8 inches in diameter. Sort of a "dinner plate" clematis flower.
I don't know why, but some 'Nelly Moser' Clematis vines will display rose striped blooms while other plants will have lavender stripes. Perhaps two different strains have arisen. This is an older variety, introduced in 1897.
The flowers, which tend to lighten as they mature, are not fragrant.
Because 'Nelly Moser' blooms on both new and old wood, the flowers arrive in two flushes: the first in late spring, the second in late summer, on 10-12 foot vines. This gives you a great deal of flexibility in pruning it.
Option One: You can cut it back to 18 inches in early spring and it will grow to at least 8 feet by fall, blooming in late summer.
Option Two: You can wait until after the late spring flush to prune it. Gardeners who live where the growing season is short will be forfeiting the second flush of flowers (which is not as heavy as the first flush) by doing this. In the more southern end of the plant's growing range, the vine will have time to produce enough new growth to support a second flush before frost.
Option Three: You can leave 'Nelly Moser' unpruned or just trim it back mildly to neaten it. This option results in the greatest flower production, but may not work in small space gardens where the size of the vine must be strictly controlled.
This plant will flower more profusely in future years if you prevent it from blooming in its first season. You would have to pinch or shear off any flower buds that form.
This is a very difficult thing for most gardeners to do.
But if you have the iron will to do it, it will force the vine to direct its energy into establishing a stronger root system which will support more top growth and flowers the following year.
'Nelly Moser' can be grown from semi-hardwood cuttings.
Like most other Clematis varieties, it can be grown from seed. There is a catch though.
It will not come true from seed. You may get different flower colors.
Personally, I regard this as a benefit, not a drawback.