A note of caution: Before embarking on this 'how to grow', it is important to note that laburnums are toxic plants and the toxins appear in every part of the plant.
The main toxin, Cytisine, can cause severe diarrhoea and convulsions. The seeds are particularly toxic - so this isn't a tree to plant where children play - because they may be attracted by the pea-pods of seeds. The unpleasant taste would deter most children - but I cut one down to avoid the danger.
Where to plant
Both species of laburnum grow on well-drained sloping sites in the wild, so this tree prefers good drainage and lighter soils.
However, laburnums have proved adaptable and seem to able to grow in a wide range of situations.
Laburnums do not like being waterlogged, it will kill them. They are not completely hardy in colder parts of Scotland, but do survive in most places in Britain.
How to train and when to prune laburnums
Pruning a laburnum on an arch: this is tackled in early winter whilst the tree is dormant. At Bodnant the trees are taken off the arch and laid flat on the ground. Then the whippy, young growths are taken back to the spurs (in a similar way to wisteria pruning). This encourages more flowers.
Small trees rarely need pruning, but if needed this should be done in summer.
The classic combination of the tall allium 'Purple Sensation' and the golden yellow laburnum flowers is hard to beat - and both flower together in May.
Laburnums are members of the pea family, or Leguminosae, and there are only two species.
Laburnum alpinum, Scottish laburnum, is found naturally in central and southern Europe on limestone in woodland or scrub. It was introduced here in 1596.
Laburnum anagyroides is also found in central and southern Europe. It grows on stony hillsides.
The tree used on laburnum arches is Laburnum x waterii 'Vossii', and this award-winning laburnum is the best tree to plant because it has the most stunning flowers of all. It is a hybrid between Laburnum anagyroides and Laburnum alpinum and it occurred naturally in the Tyrol (where both parents are endemic) and was first noted in 1856. Nine years later another appeared at Waterer’s Nursery in Surrey and then more became commercially available through Waterers. This hybrid is more child-friendly too - it does not produce much seed.
The hybrid 'Vossii' is a small, short-lived tree that may survive for between thirty and forty years. The wood is very hard and brittle and generally laburnum trees tend to lose branches in very exposed sites. The trunks are highly prized by wood turners because this yellow wood is one of the hardest and shiniest.
Did you know…?
Despite their toxicity laburnums are hugely popular. The laburnum arch at The National Trust’s Bodnant Garden in North Wales is world-famous. It inspired the late Rosemary Verey to create her own mini version at Barnsley House in Gloucestershire. There, the cascading flowers were flattered by purple alliums and this image has appeared on countless cards and calendars and lots of gardeners have copied the idea.