In fact, though ivy does cling by aerial roots, it can do little harm on a sound, well-pointed wall. The risk is that on old, soft-mortared walls ivy can open up cracks which then let in water. This can then freeze and loosen the mortar and eventually stones, too.
In its youth, ivy produces long shoots with aerial roots on one side, by which it clings to whatever will take it up to the light. Once it gets to the top, it loses its upwardly mobile tendencies and falls victim to middle-aged spread. The shoots are shorter and fatter. Leaves sprout on all sides of the stem. The aerial roots by which it elbowed its way to success abruptly cease, and it begins to think about how nice it would be to have babies. Flowers appear.
This mature flowering growth has a different effect on walls. It may not invade, but it builds up into dense bushy masses, whose sheer weight, especially under snow and 20 years of old birds' nests, can produce enough leverage to bring down a weak wall. Whenever I see those 18th-century paintings of Rome in ruins, I cannot help wondering if ivy was as much to blame as the Vandals and Visigoths for its downfall.
But ivy under control is an asset few gardens can afford to do without. There are few enough evergreen climbers, and fewer still which are self-supporting. None is as hardy as ivy.
If you want to grow ivy on a house wall, then do so; but keep it in hand. The easiest way is to shear it all over in February or March, before growth begins. You can take it hard back to the wall, until barely a leaf remains. Off will come all those shoots which have decided to begin flowering rather than clinging.
This is the time, too, to cut back all those shoots worming their way behind the gutters and investigating the roof-space. After a few years of this treatment, you will have a solid green wall from top to bottom, and the leaves will all be larger and more succulent. It is rather like pollarding a tree; all the energies of the plant are pushed back into the main stems instead of the extremities, and you get fast, lush growth.
Fancy ivies given this treatment show themselves off to maximum advantage. 'Parsley Crested' is a tough old cultivar which, with clipping, will make emerald crimped leaves four and five inches across. I can never bear to cut mine until the last minute, at the very end of winter. But if cutting is left too late, the shelter of the old leaves brings on a soft growth of new leaves underneath. Clip it then, and the cold nights and winds can spoil the newly exposed, unfurling leaves.
Another good clinger is 'Glymii', a rather smaller plant but valuable for its winter foliage, which turns with increasing cold to shades of deep bronze and black.
A wall clad entirely in ivy looks attractively rustic on a single-storey building, and can obviously be kept in hand with very little work. On two-storey buildings the idea begins to seem less attractive,because you can see what effort it will demand.
Not all ivies are good clingers. Those varieties with thin, wiry stems tend to be the least successful. When planting an ivy to cover a wall, I like to shock it into action by cutting it off at 3in above the soil. This is not the waste it may seem, especially since you can always make cuttings of the left-overs.
The reason for doing this is that, once an ivy stem's aerial roots have come unstuck, they rarely regain a hold. If you buy a 2ft ivy from a nursery, you may find that the pressure of wind on those stems will stop them ever getting a good hold. They will lash around against the wall, rubbing off the old aerial roots and getting nowhere. If, however, you cut them down and plant very close to the wall, they can spread up from the ground with next-to-no wind resistance, and will shoot tightly up the brickwork. (The same holds true with Boston ivy and Virginia creeper; cutting them down saves you time in the long run.)
To produce a wide-based plant, you can, of course, peg the stems along the foot of the wall instead of cutting them off. Their sideshoots can then run upwards.
Ivies with large leaves, such as Hedera colchica 'Sulphur Heart', are always prey to sneaky winds around buildings while they are getting established. It pays to cut off any stems that have not taken a really firm hold in the first couple of autumns, so that they cannot flap in the winds and peel off other more successful growth. Once they are established on a firm framework, you can allow them their head, and perhaps even encourage some interesting bulges of flowering growth.
As well as climbing ivies there are bush ivies; that is, ivies propagated from the mature flowering growth. They have a charm all of their own, and are much neglected. Over many years they will make 4ft mounded bushes.
They are slow to propagate and slow to get away, especially the variegated forms, but they are worth waiting for. There are the usual glossy ivy leaves, rather more wavy at the edges; flowers bloomat the very end of the season; black berries provide the decoration in late winter. The form Hedera 'Poetica Arborea' even has orangey-yellow berries. You can use them (instead of the inevitable conifers) as a change of texture among heathers. I have seen bush ivy used to create topiary, trimmed in spring with secateurs after the flowers are over. It is certainly a lively antidote to topiary pieces in yew or box, which are at their tightest in winter. I even know of an 'armchair' made of bush ivy, surely the last word in flowery loose covers.