British gardens overflow with a cornucopia of plants from all corners of the world. We have no shortage of reference books to give us basic descriptive information and to advise us on what to grow where. But once plants start to interact, to compete with each other (or with weeds), things get complicated. This is when horticulture merges into ecology – a place where outcomes are much less predictable. With many more gardeners making naturalistic gardens where plants grow cheek by jowl and local authorities and community groups trying to manage ornamental plantings on minimal resources, I thought it would be interesting to try to apply some plant ecology science to familiar garden perennials. And so…. I began a part-time research PhD with the Department of Landscape at Sheffield University.
Like most researchers, I have not found out anything really new. But instead I feel I have been able to clarify and systematise some of the vast body of anecdotal and unrecorded knowledge that many experienced gardeners have. In particular I feel as if I am in a good position to predict the performance of anything unfamiliar I come across. My task has been largely a lonely one, sometimes engaging in activity which others find bizarre – drying leaves in the oven and weighing them to an accuracy of 0.01 gram, drawing schematic diagrams of leaf and stem connections, planting out hundreds of tiny plug plants in geometric blocks. Trips to Sheffield are rare, but intensely social. In particular I have loved meeting overseas researchers, and have already been on a Mexican lecture tour as one result of a friendship.
So what can I pass on to other gardeners? Perhaps the most useful is the realisation that perennials can be grouped into a rough series of categories based on several factors which revolve around their garden performance: lifespan, spreading ability, time at which growth begins in spring, persistence of dead foliage. Above all, I would stress the importance of closely observing your plants and trying to make connections between their physical appearance and their behaviour over the year.
Perennials are not always perennial
The perfectly satisfactory plant that suddenly drops dead is one of the mysteries of gardening, but a lot of perennials are more correctly ‘short-lived perennials’. A key distinction is between perennials which have spreading shoots which root as they grow (eg. most hardy geraniums), so ensuring a potential for infinite constant increase, and those which just have one single point of connection between their roots and above ground growth (eg. the popular dark red scabious Knautia macedonica). The latter indicates firstly that the plant may live for only three, or five years; it may live more, but it will not live for ever; and in any case, this one connection point between stem and root gives it an inherent vulnerability to gnawing vole or misplaced boot.
Some perennials spread like crazy, but this is not always so bad.
A lot of less experienced gardeners look with horror on plants like Euphorbia cyparissias that after one year in the border start sending out underground runners that pop up some way from the parent. Relax. The fact is that most plants with what ecologists call ‘guerrilla spread’ cannot penetrate the clumps of established plants. The species in question can be a real bonus filling in the gaps between larger plants.
If a running habit is combined with height, as in the old cottage garden favourite, the yellow loosestrife, Lysimachia punctata, then a real ‘thug’ is in the making. How useful! These plants are ideal for filling difficult to maintain or out of the way spaces.
Timing is everything
British springtime is a drawn-out affair; some perennials start into growth really early, others very late. Perennials from Mediterranean regions tend to get going around Christmas – at least in our current run of mild winters. If their foliage is attractive they can make a great contribution to the spring border: globe artichoke (Cynara cardunculus) and species of Acanthus are two. Those that start late include many from continental climates where spring is short and intense. The increasingly popular grass Panicum virgatum is one of these, and it can suffer as a result, crowded out by earlier-growing perennials or mistaken for a weed grass.
Dead leaves have their uses.
Ecologists have discovered that plants which dump lots of dead foliage around them in autumn can suppress the growth of other plants around them, especially of seedlings. In other words these plants are self-mulching. Gardeners however tend to clear away dead foliage. Leave it, and you may well find that weed seed germination around these plants is greatly reduced. Geranium species do this, and even more effectively, Iris sibirica. This iris is one of the great survivors in neglected borders, its leaves take a long time to rot down, and until they do they carpet the ground in a thick layer of mulch around the core of the plant.
Big plants may not be survivors
Generally speaking, large size equals an ability to dominate, in wild plant communities, as well as the boxing ring. But not always. There is a clear distinction between perennials with large basal leaves, which tend to flower before mid-summer and those with tall, upright stems, with lots of small leaves which flower later. The latter, such as asters, heleniums and rudbeckias, may be tall and vigorous, but they do not shade out competition around the base. Species of Achillea and Geranium may be a lot shorter, but their combination of early season growth and sideways-spreading leaves ensure that over time they will shade out and suppress other plants around them.
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