People call both of them ivy and plant them interchangeably, then are surprised a year later when one has covered a whole wall and the other is still thinking about it. They are not close relatives: common ivy is in the aralia family, Boston ivy is in the grape family, and the differences show up in the leaf, in the winter, and in how each one holds onto the wall.
How each one attaches
This is the difference you can check in ten seconds. Common ivy sends out stem roots, short hairs along the underside of the stem that dig into mortar and rough bark. Boston ivy has no stem roots at all. It puts out tendrils whose tips swell into tiny adhesive pads that stick to the surface like a suction cup, which is why it will climb smooth stone, render and even a painted wall where ivy cannot get a grip.
The consequence for a building is worth knowing before planting. Those pads leave a mark, and on a delicate or old surface the removal is worse than the growth. On a plain brick or concrete wall, neither is a problem.
Growth, leaf and the winter test
Ivy is an evergreen woody climber, normally running to three or five metres, with leathery dark leaves and soft young tips clothed in fine scales. It flowers late, from about May to August, in small umbels of yellowish or greenish-white, and follows with round yellow-to-red berries from September into November. Out of leaf it is still green and still screening.
Boston ivy is deciduous and far more vigorous, with stems reaching eighteen metres or so. Its leaf shape varies on the same plant, from three-lobed to entire, which is a good clue to identity by itself. The flower clusters are small and yellow-green in June, and the fruit is a blue-black bloom-covered berry in September or October. In autumn the whole plant turns red before it falls, and the bare wall is a trade-off you should decide about in advance, not discover in December.
Where each one will grow
Ivy is the shade plant. It tolerates a north-facing wall, it is happy in cool, humid conditions, it is genuinely hardy, and it will grow on most soils provided they are not waterlogged. It suits the difficult corner and it also behaves well as an indoor pot plant, which Boston ivy does not.
Boston ivy wants more light and takes it: it is not troubled by full sun, is cold-hardy, stands drought better than ivy, and in mild winters keeps some colour rather than going fully bare. It dislikes heavy, wet soil and it resists air pollution well, which is why it is so often seen on urban walls and buildings.
Geographically both are old garden plants in China. Ivy is native across Europe, Asia and North Africa and is cultivated from the coast to the south-west; Boston ivy is native to eastern Asia and the Himalayan region, with relatives in North America, and is planted across almost every province.
Which one to choose
Plant ivy where the wall is shaded, where you want the screen to hold all winter, or where it will eventually come indoors. Plant Boston ivy where the wall is in open sun, where you want height fast, and where autumn colour is a feature rather than a season you would rather not look at.
Two lookalike climbers confused in exactly this way, this time indoors, are dealt with in tree philodendron and monstera, and the same genus-difference problem in a smaller plant is in golden buckwheat against common buckwheat.
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