When the Folklore Society conducted its survey of inauspicious plants early in the 1980s the belief that placing red and white flowers together led to, or foretold, death accounted for approximately 10% of the material collected. It appears that this belief is still widespread today and most days users of this website seek information about it.
The earliest known version of the belief is given in the Sunlight Almanac of 1896, where it stated that to dream of red and white flowers foretold death, and there seem to be few records of the belief before the mid twentieth century, when it was well known, particularly in hospitals. Typically:
Red and white flowers should never be put together, this foretells death [Sandiway, Cheshire, October 2004].
The bad luck can be averted by adding flowers of another colour to the vase.
The usual explanation is that such flowers represent blood and bandages. Alternatively it has been suggested that there may have some connection between red and white flowers and the read and white stripes on a traditional barber’s pole. According to J.K. Melling, Discovering London’s Guilds and Liveries, 1981: 28:
The present-day barber’s pole … represents the wand used in venesection, the two bandages and the barber’s blood-letting dish.
It appears that the belief is restricted to the British Isles.
Red and white flowers are traditionally used in church decorations on Whit Sunday, when they are said to represent ‘the fire and the wind of the Holy Spirit’. It is said that as red and white are the colours of the City of London red and white carnations are often used when there are events at the Guildhall, and that Chinese brides in Britain often carry bouquets of red and white flowers, combining the tradition wedding colours of China and Britain.

Note: According Margaret Bradley of Wolverhampton, writing of her childhood in Banffshire in January 2014: red and white flowers represented ‘red for blood and white for purity’.

Image: Tooting, London, January 2014.