Crouched deep within a shaded bed to weed last week, I came across an unusual sight: a small, self-sown 'busy Lizzie' (Impatiens walleriana, ht 30-60 cm) with a red flower. I recalled how this garden bed - which has a theme of red flowers - used to contain many busy Lizzies of this colour. Over the years, I had only allowed red-flowered ones to stay, so generally most of the seedlings came up in this colour after a while.
Busy Lizzies were plants I always had liked ever since I was a child - so colourful, so easy to grow and propagate: a cutting could develop roots in a glass of water! They came in a range of jewelled colours (including white, pinks, reds, orange, lilac and purple) and were so useful for shaded areas. They did self-seed, but were easy to pull up. They had amazing explosive seed capsules like coiled springs, which were fun for little children to play with. Though often grown as annuals, they would survive for several years in frost-free gardens, and flowered for many months of the year. They could grow a bit straggly after a while but if they were pruned back they would get a new lease of life. Compact cultivars were introduced over the years that kept a good shape. Some had double flowers, which resembled little roses. I loved all the colours but the white one was a favourite in shaded borders to echo white-variegated foliage.
All of a sudden, a few years ago, my busy Lizzies began dying - losing their leaves so that all that was left were sad stalks, which eventually died. No seedlings popped up in spring in my garden. I thought at first they had been eaten by caterpillars but it seemed an unlikely explanation for so many of them dying. I thought it was only in my garden, then I discovered that other people were having the same problem. It no longer even seemed possible to buy them in nurseries. It seems that busy Lizzies have become vulnerable to 'Impatiens downy mildew', caused by the pathogen Plasmopara obducens, which causes the symptoms I had noticed in my plants. This pathogen, related to Phytophthora, has been also found in the UK, Europe and the USA. There is apparently no chemical control available for home gardeners.
It's sad to think that a plant that we probably took too much for granted may never return to our gardens. The very few seedlings that have come up in my garden, like the one I discovered last week, generally don't survive very long, as they are soon afflicted by the fungus. Such plants should be thrown into the garbage bin, rather than composted.
The good news is that other species and cultivars of Impatiens are not affected by the pathogen. The New Guinea Impatiens (Impatiens hawkeri and its hybrids) are very attractive plants, with larger flowers in a range of lovely colours. Some also have interesting dark-coloured or variegated leaves. They do reasonably well in shade but they don't self-seed, and eventually need to be replaced by cuttings when they get tired - but they should last a few seasons, except in very cold suburbs. They shouldn't be pruned back until September, even though they may look scruffy during winter.
I used to grow quite a tall, shrubby species, called Impatiens sodenii (ht 1-3 m), which had soft pastel lilac flowers - I hadn't seen it for years but caught sight of it in a Blue Mountains garden last week. At one stage I also had a strange one with curved red and yellow blooms - it was called Impatiens niamniamensis 'Congo Cockatoo' (ht 1 m) and was quite a novelty. I also grew Impatiens balsamina (ht 75 cm) years ago - it had tall stems with hooded single or double flowers (the latter resembling tiny Camellia japonica). It used to self-seed quite freely but it hasn't come up in my garden for a few years and I wonder if it is also affected by the fungus.
We can also choose other shade-loving fillers instead of Impatiens - bedding (or 'wax') Begonia (Begonia semperflorens) are wonderful plants for this purpose and come in colours of white, pinks and reds. They are also very useful in pots. They do self-seed, but I have never minded this. Other suggestions for shaded positions include the many and varied bromeliads, low-growing Ruellia and Justicia species, rhizomatous Begonia and coleus.
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