Some plants are like some drunks: they start off very amusing, slip into being a bit wearisome and end up so extremely dull that you wish you had never met them in the first place. Also like people, however, the crashing bores are often difficult to identify early enough to avoid disaster. There are some extreme examples: in America it is the kudzu vine, which was brought over from Asia as fodder but, if left to its own devices, is quite capable of swallowing an entire house within a very short time. Over here it is the bounty of an intrepid planthunter who staggered home wearily after many uncomfortable months to announce, “Darling, I’ve found a wonderful plant called Japanese knotweed. Let’s plant it and give some to all our friends.” That plant has now colonised not just gardens but miles of canal bank, graveyards, fields and railways: it will not die without indecent helpings of poison.

There is a tightrope to be walked here: at what point does a self-seeding plant cease to be something delightful and begin to become a pest? The dandelion, if viewed from a position of horticultural naivety, is a charming plant: smiley, many-rayed flowers, an attractive, edible (though very bitter) leaf, easy to grow, difficult to kill and an absolutely delightful fluffy seedhead that provides hours of amusement for chubby-fingered children. What’s not to love? The simple fact that each of these seedheads contains approximately 200 seeds and that during a season each plant will produce about 2,000 offspring, each one of which will happily send down an enormously strong taproot exactly where it is not required. The thistle is the same, and no matter how many times the old chestnut about a weed being a plant in the wrong place is chanted, nobody wants either of them in their gardens.

What we want from our self-seeding plants is a certain restraint. We do not want them to spread their seed all over the place like a pirate on shore leave in Sodom and Gomorrah. We would like them to be easily guided and well behaved: sadly, however, plants have their own agendas and, I’m afraid, no interest in doing what we ask.

There are a few plants that are reasonably easy to keep in check, and their advantages should not be underestimated. They produce lots of free plants which, provided you are relatively organised, can be transplanted when young in order to bulk up existing borders (or to give away to friends and church fêtes). All this without the trouble of seed trays, pricking out, glasshouses or remembering to do the watering. You also know that these seedlings, being raised – as it were – in the wild, are going to be hardy and strong. However, sentimentality has no part in this arrangement. Often the most perfect seedling will choose to grow in the most inappropriate place: in the middle of a gravel path, for example. No matter how sweet it may look (and baby Alchemilla mollis are particularly cutesy) it will grow up to be a pest, so you must harden your heart and dig it out immediately.

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Finally, some of the finest plant combinations have been created by the serendipitous drifting of seeds from one part of the garden to another. The rule in these cases is never to admit that such things are accidental, but always to take the credit. It is your garden after all.

One drifter you might offer a welcome to is fennel (Foeniculum vulgare). Yes, it is a little wild, but anything with foliage that looks like feather boas can be forgiven at least some bad behaviour. The flat-headed flowers are acid yellow and add good structure to many of the more giggly planting schemes.

A plant without which every garden would be a lesser creation is Verbena bonariensis – one of the very best self-seeders. The colour (a sort of clear mauve) is a perfect companion to every other colour; it never requires staking; it flowers for at least three months; and then it rewards us (and the birds) with hundreds and hundreds of seeds.

One of the best plants for late May and June in my garden (when it is going through its pink period) is Geranium psilostemon. It is a fairly large plant, forming a hemisphere about a metre across that seems keenest on settling itself in gravel paths where it can cause the greatest inconvenience to anybody wishing to pass. However, the flowers are such a deep and intense pink that forgiveness is easy.

Nigella damascena (a name that immediately conjures up visions of chocolate puddings) is a gorgeous annual that is, perhaps, one of the simplest plants to grow. Rake some soil, chuck on seeds, and go away.

And then there’s Alchemilla mollis: a classic edging plant with crinkle-cut leaves, acid-green flowers and a way with raindrops that can melt the stoniest of hearts.

The princess among these plants, though, is the opium poppy, Papaver somniferum. She has the most spectacular flower, subtle bluish green leaves, seed pods like sultans’ turbans and a dying moment of supreme elegance. Even better, there is an element of chance: will the flower be blood red, claret mauve, palest sugar pink or (if you are unlucky) a muddy double lilac? You can reduce the chances by a bit of basic eugenics, but you can never be absolutely sure until the last minute.

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