Looking after an established garden is rather like keeping the peace, making sure the right plants thrive and the thugs don't take over. The same principle applies if you look at the pattern of adjoining gardens which make up so much of the urban and suburban landscape.
Plants are supposed to stick within the boundaries of their intended garden. The trouble is, some of them refuse. Some of them have serious territorial ambitions, and always want to spread into next-door's garden.
Have you ever had the feeling that it will not be the seeding sycamore or the shading leylandii which will one day inherit the earth, but the runners and suckerers: the underground movement?
Gardeners should think hard about planting such species on their boundaries. It is not a kind thing to do. Sometimes a suckering plant seeking levels of moisture or sunlight more suited to its needs will actually try to move out of one garden altogether, to establish itself in happier territory next door. You can hardly blame it. If you have planted your running bamboo in dry shade and it can sense a sunny bog garden just under the fence, what's a bamboo to do?
The Deep Tunnelers
Plants invade and cross boundaries by a number of underground mechanisms. Some burrow fast and deep, like torpedoes. Bamboos are a case in point. For every well-behaved clump-forming species, there are many more runners.
The genus Sasa, for example, spells trouble. Sasa palmata is undoubtedly gorgeous, easy to propagate, and therefore still widely sold into small gardens. But plant it on your boundary and your neighbour will forever be digging it out, learning the hard way that wherever there are shoots, the roots will be a yard further into his garden. Dig it back to the fence and it reappears the moment you turn your back. A heavy-duty barrier inserted deep into the soil, such as sheets of corrugated iron, helps to restrain it.
Less unwelcome but still an invader is the Scotch flame flower, Tropaeolum speciosum, whose succulent white roots move slow but deep through cool acid soils, searching for new hosts around whose necks to throw their arms.
The Shallow Tunnelers
Some plants tunnel less quickly but on a steady front, backed up by an all-invading mass of underground roots. Mahonia aquifolium, Hypericum calycinum, Gaultheria shallon and Pernettya mucronata are some of the most indomitable, moving through other plant's roots and under fences like one of those Roman shield formations, the tortudo, which was so effective in battle. These are all good plants, of course, in the right place. God forbid we should have gardens full of polite, feeble doers. But they do need placing with care.
The Surprise Approach
Of all methods of invasion, perhaps suckering is the most troublesome to neighbours. Certain trees look harmless enough (what could be more delicate than golden Robinia 'Frisia'?) but have roots which, when damaged, throw up suckers. If that root happens to be 5m into your neighbour's garden, then he will not be pleased. Digging makes matters worse, and the answer is simply to keep pulling off the suckers before they become so woody that they have to be cut off below ground where they join the main root. (Cutting them short only encourages a proliferation of shoots.)
Some shrubs sucker too. The deciduous, sweetly scented Elaeagnus 'Quicksilver' will do it. So will sea buckthorn (Hippophae rhamnoides). But Aralia elata is perhaps the worst. Everyone loves that vast, dramatic pinnate leaf and pyramids of flower balls. The variegated form is even more sought-after, expensive, and actually quite frustratingly slow. But it is always grafted onto plain green aralia whose roots, once established and once damaged, love to throw up spiny suckers.
The SAS Approach
Finally there are plants which seem to defy nature by burrowing through apparently solid objects. The Californian tree poppy, Romneya coulteri, I have seen pass through an old lime-mortared wall in search of sun. The fat white roots of Acanthus will do the same, and also Clerodendrum bungei: and so, surprisingly, can wisteria. It only happens very occasionally, but what a testament it is to the strength and determination of a powerful plant when cornered.
The answer, when using all these attractive but ambitious plants, is to put them where they will be happy with their conditions without wanting to stray, or where they can stray unhindered. Put them where they will perhaps already have some kind of natural confinement, from walls or even water, and somewhere they will be undisturbed by too much digging. Take them on with tactics rather than brute force, and hope no-one ever plants them on your boundary.
On the other hand, supposing you had a neighbour determined to flog his garden for housing development, might you 'set a thief to catch a thief' by planting a few of the above on your boundary? It's an interesting thought, although the expression 'shooting oneself in the foot' springs to mind.