When my mother started her first garden, more than 70 years ago, she took a cutting, whilst on a walk around her village, of 'a very pretty blue-flowered vine' to grow on a trellis. All these years later, remnants of it still haunt that garden, self-seeding, layering and twining energetically around any structure, despite decades of trying to eradicate it. It was, of course, morning glory (Ipomoea indica), one of the scourges of Sydney gardens, often innocently planted by those seduced by its flamboyant blooms, which are amongst the bluest of all flowers.
I too have been prey to some attractive-looking plants - some from friends' gardens, some bought from nurseries - that have turned out to be monsters that have tried to completely take over my garden. Sometimes we are misled by gardening books from overseas, where these plants are quite well behaved (or even hard to grow, requiring nurturing in glasshouses to survive) in cooler climates than ours, and where they are promoted as desirable specimens for any discerning gardener. This is another aspect of climate-specific gardening that I have come to be aware of over the years.
The bad plants I have known fall into several categories of takeover tactics (and some plants use more than one strategy to ensure their domination, and somehow you just have to grudgingly admire their amazing will to survive): excessive self-seeding; proliferous ability to reproduce bulb offsets; a capacity for tenacious layering of roots from stems that touch the ground; and the ability to multiply by fleshy underground suckers, roots or tubers, the slightest bit of which is left behind will form a new plant. It's probably the latter plants that are my worst enemies, because they are just so hard to exterminate and much of the infiltration is going on stealthily underground before one realises. Interestingly enough, these plant thugs often have ravishing flowers that enchant hapless gardeners, as if this is all part of their plan to conquer the universe.
As a cautionary tale to others, I will outline some of these latter plants that I have battled in my own garden. One is the toad lily (Tricyrtis hirta). I fell under its spell when I was still a victim of English gardening books and trying to grow the plants featured in them. The toad lily is a cherished plant in northern hemisphere woodland gardens and its flowers are indeed exquisite: beautifully formed and speckled with purple dots. A friend (who shared my passion for such plants) had obtained a specimen and brought me a tiny piece wrapped in tissue paper, which I reverently planted - in rich soil in a special place in the garden. I did get some of the gorgeous flowers, but also found that it spread like wildfire by underground stolons. I am still digging it out.
Several Salvia plants that I acquired when I first got interested in this genus, more than 35 years ago, also proved to be fiends in my previous garden in Ryde. I was captivated by the gorgeous azure flowers of bog sage (Salvia uliginosa) when I first clapped eyes on it, and carefully planted it in pride of place in my front garden. It was only when it started to come up metres away from this spot that I realised something was amiss, and upon digging it up, I found a tangled mass of suckering roots going in all directions; I ridded myself of it only by moving to a new house. Salvia guarantica (and its cultivar 'Black and Blue') were other plants that I was enthralled by: they had stunningly rich blue flowers. I soon discovered that they developed large, woody tubers that multiplied with alarming speed and sent up new plants, and it was very hard to get rid of every last one of them. Luckily, most other Salvia do not behave like this and are worthwhile garden plants. The plant that I later bought under the name Salvia guaranitica Large Form, has been well behaved for a number of years and it has the same stunning flowers.
Another very pretty flower that caught my eye in my early gardening days was called the Chinese wind poppy (Eomecon chionantha). It looked the most innocent of blooms: very much like a beguiling white poppy, with interested scalloped-edge leaves. It was said to like shade: another great advantage, thought I, as I lovingly planted it in good soil in a shaded bed near my hellebores. Alas, it too proved to have a hideous rhizomatous root system that sent up new plants within a 2m-radius, getting in amongst other plants so that it is very difficult to dig up completely. I still fight it, 18 years later.
Many years ago I was given a plant called 'New Zealand Christmas bell', with a variegated leaf (Alstroemeria psittacina - not from New Zealand at all but native to Brazil). The flowers looked very vaguely like the Australian Christmas bell (Blandfordia grandiflora) and I thought this could look good as a groundcover under a tree. I soon discovered that the large tuberous roots multiply with great speed and are almost impossible to get rid of. I dig up hundreds of the claw-like tubers each year and yet up they still pop the following spring. The plain-leafed version is just as rampageous. The larger-flowered Alstroemeria hybrids have a similar root system, and although I do grow and enjoy these plants because of their stunning flowers, so useful for vases, Sydney gardeners should be aware that these too can form huge clumps over time and are probably just as hard to get rid of as their invidious cousin. The compact-growing forms (often sold as 'Princess Lilies') now often seen in nurseries are possibly not as invasive but should be watched with care, or grown in pots to be on the safe side.
Another wicked plant that I was charmed by was Clerodendrum bungei. I already had several lovely Clerodendrum plants in my garden, including Clerodendrum wallichii and Clerodendrum ugandense and I thought this specimen, with its big clustered heads of pink blooms, like a giant Pentas, would be a fabulous shrub. However, it turned out to be one of the worst suckering plants I have had yet in my garden, sending up woody stems metres from the original plant. Other plant thugs I have known and fought include the pink evening primrose (Oenothera speciosa), some forms of Artemisia (including a particularly noxious one known as 'Oriental Limelight'), Lysimachia ciliata 'Firecracker' (which had brilliant burgundy foliage) and a few Achillea.
The best advice I can give is to observe any unfamiliar plant carefully and remove it (into the garbage bin) if it shows the first sign of spreading aggressively. It might even be a good idea to keep new plants in pots for a little while to see if they have any bad tendencies. The ideal time to tackle them if they are going berserk in your garden already is when the soil is moist after rain (like right now!). I rarely use herbicides, but in some very recalcitrant cases, it may be necessary. There is an organic-based one available now based on plant oils that works pretty well. Sometimes it is possible to harness bad plants: in areas where nothing much else will grow, or spots where you really don't mind something taking over. Some people have told me that they have bog sage growing in wild parts of their garden, and they love it! In rich, moist soil, Japanese windflowers are on the borderline of being invasive, spreading by underground stolons - however, if you have an area that can be bounded off by a path, that you want filled with a groundcover that gives truly glorious autumn flowers, they come up trumps, and I have them in my garden in just such a position, roaming amongst Hydrangea shrubs - and looking absolutely fabulous at the moment ...
Blog first posted 18 March 2012; updated 13 March 2022.
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