When my father gave me a few new plants last year as I moved into my new apartment, he told me to water them in proportion to their leaf surface, since it is the leaves that breathe, and hence exhale water. I’ve stuck to that bit of advice and it has served me, or rather the plants, well.
One of the plants, however, grew quickly and I soon needed to trim it. At first I simply cut off entire branches that stuck out too far, or trimmed their tops if they had grown too tall. But the branches never grew back, leaving gaping bare sections, and each trimmed top grew two new sprouts, weighing the branch down even further until it broke under its own weight. So I attempted a new strategy: i began to cut the branches off mid-way. Now the plant looks less bare, and instead of doubling the tips, new leaves grow along the full length of the shorter branches.
The traditional moral of this story might be that in practical matters, since we cannot understand every element of complex systems, we must resort to rules-of-thumb, and tinker at the edges. However, i whish to suggest another conclusion: in horticulture as in ethics — or politics, or economics — we must not define rules or laws, however approximate; instead, we should develop principles, which need only be adequate and exact, not approximate and true. And since such principles are not hazy laws, which must be applied with art and guesswork, they can be followed exactly, and steadily improved through experimentation.