"Friend or foe?"

If grown, these plants need firm control!
Sunday, 30 October 2022     

Calceolaria mexicana with nasturtium

This week a friend asked me about cute, unusual, yellow-flowered annual that was popping up everywhere in her garden and between pavers. The plant appeared to be Calceolaria mexicana, a winter/spring-flowering annual I also have in my garden, though I had deliberately planted mine, from a seedling from a keen gardener. It certainly does like to self-seed (when it decides it likes your garden) but I actively encourage this by shaking the spent plants around in the hope some will come up next year. The flowers are such an unusual shade of clear yellow and, joyously, they will grow in shady parts of the garden, illuminating the area with their neon colour. The little ferny seedlings are easy to pull up in autumn if there are too many of them. It got me thinking about plants that might be seen as weedy foes but which we can harness as friends - as long as we maintain an upper hand.

Many self-seeding annuals fall into this category: I am looking at you, forget-me-not, Primula malacoides, Amaranthus species, Cleome hassleriana, Nicotiana langsdorffii, Euphorbia coralloides, nasturtiums, heartsease, bedding Begonia, Browallia americana, honesty, Verbena bonariensis and Queen Anne's lace! In fact, I have spent much of my gardening time over the past week pulling out the excess seedlings of the summer-blooming ones from my borders. I actually rely on some of these to fill out my summer/autumn borders, but I have to remove about 99% of the self-seedlings, otherwise they would have no room to develop into the statuesque plants they have the potential to be. The smaller ones bring a soft, wild informality to other parts of the garden, and I look forward to seeing them each year. These sorts of plants do involve some work. I probably will get to the point one day when I can't cope with them anymore but, at the moment, I remain in their thrall.

Many rhizomatous or running perennial groundcover plants have the capacity to take over the garden - and even escape into nearby bushland - and we need to be very careful about using them at all. However, I have to confess that I do use some notorious ones in truly hopeless, dry, shady parts of my garden under big trees where literally nothing else will grow. An extremely careful eye has to be kept on them - and they should never be planted in 'good' sections of the garden or where they can infest bushland. All can be safely grown in pots!

Stripey-leaf versions of Cholorphytum comosum (spider plant, ht 30 cm), the plant that got me hooked on gardening, can be pretty aggressive once set free from a pot but grows very well in dry, shaded areas where other plants struggle, making a weed-proof mat. The long, thin flowering stems (appearing now) on which plantlets develop after blooming, should be removed to help rein it in. The cultivar I use is called 'Ocean' and it has crisp, white stripes on its foliage that can be used to create colour echoes. I have it alongside a path opposite some white Agapanthus and I love that combination when the aggies are in bloom.

Persicaria capitata (Japanese knotweed, ht 15 cm) is another plant I have a perverse affection for, probably because it grew with abandon in the garden of my childhood. It has bright green foliage with distinctive purplish V-shaped markings. It seemed to be always sporting its strange, pink, bobbled flowers. This can spread widely by runners, stealthily spreading alongside paths and cascading over walls, but it is a useful plant for any spot where you need the soil covered in dry shade where nothing much will grow.

Syngonium podophyllum is a very vigorous and not-to-be recommended climber that can, with great caution, be used as a groundcover, spreading by runners. The yellow- and lime-variegated versions are useful for very difficult, dry areas under trees. It is crucial not to let this plant start climbing trees, as it becomes very aggressive if it does this, and like ivy, can completely change its personality into an unstoppable monster. The small-leaved versions of Syngoniumcan be used with greater confidence, and there are some beautifully coloured ones in hues of pink, silver or lime/gold.

Trad relative Tradescantia zebrina (ht 20 cm), with its seductively striped leaves of silver and green, with their purple undersides is very keen to take over your garden with its long stems that take root as they spread - but I do use it in very dry, shady spots where little else will grow, and it forms a lush carpet, and its very shallow roots needs minimal water. It can make an alluring backdrop to dark- or silver-foliage plants. It is easy to pull out when it strays (which it will definitely do).

Viola riviniana 'Purpurea' (purple-leaf dog violet, ht 5 cm), with its tiny, purple flowers and purple-flushed, heart-shaped foliage is useful for difficult, dry shade, including full shade, but can become invasive as it self-seeds abundantly as well as sending out stolons, so needs monitoring. Viola hederacea (native violet, ht 10 cm) is endemic to Sydney (and elsewhere) and will colonise far and wide when unleashed, and indeed can become a menace. In a place where little else will grow, it can provide a lush carpet and a profusion of tiny, purple-marked blooms. I have recently seen it used as a lawn substitute in a shady garden.

I also occasionally use some taller, spreading plants in very challenging spots in dry shade. One is Neomarica northiana (walking iris, ht 60 cm), with arching fans of lush, long leaves and small, three-petalled, blue-and- white flowers (out now), which open only in the morning, and are often never noticed! It sends out long stems with baby plants on the end, which is how it takes over, so it's best to remove these. Ctenanthe are also very insistent plants, but I do use the attractively foliaged Ctenanthe setosa 'Grey Star' (ht 1.2 m) in very difficult dry shade under trees, as it can stealthily spread far. Both these plants need to be kept under a stern eye.

These plants are certainly not for everyone and must be kept in bounds, and certainly never, ever allowed to run amok! I quite understand if you feel appalled by my suggestions!

 Reader Comments

1/9  Janice - 2069 (Zone:10 - Warm Temperate) Monday, 31 October 2022

Hi Deirdre, thankyou for the timely reminder, as I have many of these plants, which are both a help, but can be very vigorous! Janice 2069. Thanks, Janice. We do need to stay in charge of them! Deirdre

2/9  Gaynor - 5044 (Zone:10 - Mediteranean) Monday, 31 October 2022

So much of this article rings true. I have Nigella, Honeywort and Calendula as well as annual Valerian (Kiss-me-quick) as volunteers in my garden. The birds and bees love them. I also struggle with Silene vulgaris (bladder-wort?) and Oenthara (primrose?) Without them my garden would be dull. Sorry I am not sure of the botanical names of many of these, I hope you know the ones I mean. Yes I know them! Some don't do as well in Sydney due to our humidity. Deirdre

3/9  Lloyd - 4060 (Zone:11A - Sub-tropical) Monday, 31 October 2022

Hypoestes - 'freckle Face' - is my nominee. Spreading amongst flagstones and into garden beds having been used as a cheery colour in some hanging baskets and small pots. Have grown to 30-40 cms in height at the base of some bamboo palms. Deep rooted and an effort to remove them. So I will just keep an eye out and get them before they clump up. The contrast with the grey basalt flags is quite attractive. But even 'genuine' plants like Jasmine can be a problem once they set off across the ground. Yes that freckle-face really does set seed, yet I love its leaves and it is useful in shade. I try to pull out most of the seedlings and also try to cut off the flowers as soon as I see them. Deirdre

4/9  Mary ann - 2064 (Zone:10 - Warm Temperate) Monday, 31 October 2022

As a volunteer bushcarer, I can no longer look at trad, agapanthus, or liriope the way I used to. They are all really aggressive and volunteers spend years trying to rid the bush of these weeds that have escaped from gardens. I totally understand that. It is horrible when these plants get into the bush and gardeners have to be very careful about growing them. Deirdre

5/9  Valerie - 2121 (Zone:10 - Warm Temperate) Monday, 31 October 2022

I have to admit to growing a few of the above, mostly as filling plants for difficult areas. I did try to grow the Persicaria but it didn't take. The other one that seems to self seed everywhere is Seaside Daisy or Erigeron. I like the look of drifts of it in spring but afterwards it has to be tamed. Oh well, can't have perfection in every direction in our largish garden. We used to have Jasmine hedge meeting Honeysuckle hedge across the ground of the garden at the back. Now that was invasive! Yes that daisy just came up in my garden - maybe it was in compost I brought from my previous garden. I am wary of honeysuckle and jasmine, even though I loved jasmine when I was a kid and we had a huge bower of it over a porch. Deirdre

6/9  Pamela - 2158 (Zone:10 - Warm Temperate) Monday, 31 October 2022

This list made me laugh Deidre as I have EVERY one of those plants in my garden!! In large gardens we need the fillers to cover big spaces but some just take over very quickly. Persicaria Red Dragon is super tough and will grow from a clipping dropped on the ground, Freesia laxa spreads wildly, some Plectranthus can take over very quickly and Strobilanthes Goldfussia pops up everywhere here. Orlaya,Cynaglossum & yellow Corydalis seeds prolifically but I love them. Don't plant that blue Ruellia!! I do love self-seeders but one has to be ruthless to stay on top. It is interesting how some plants self-seed in one garden and not another. And I had only one Orlaya this year - I normally get hundreds! Maybe due to all the rain we have had. Deirdre

7/9  Margaret - 2122 (Zone:10 - Warm Temperate) Tuesday, 01 November 2022

I also have many of the plants listed growing in my garden, and enjoy them, but, Deirdre, you are correct, you have to be in charge of them! Like the calceolaria, nigella and poppies, which arrive every year. Have no luck with growing Q.Ann's. L, or Orlaya. It is so strange how certain ones self-seed in one garden and not another. I don't get any poppies! Deirdre

8/9  Leveena - 2099 (Zone:10 - Warm Temperate) Wednesday, 02 November 2022

Ah yes-when we moved into our place 5 years ago (North Curl Curl), syngonium had been planted under two large palms...and had already established a dense woody vine almost 30 ft high! At this stage I don't mind-the palms are cocos, and both provide a shady spot for my alpinias, gingers and Medinellas. Native voilet though-let it grow in a shaded spot, and now I'm trying to tame it in the sunny part of my lawn! It is fascinating how that Syngonium changes personality when it climbs - like ivy. I have the Syngonium as a groundcover and it does a good job in a really difficult area. Deirdre

9/9  Jean - 4035 (Zone:11A - Sub-tropical) Wednesday, 02 November 2022

Hi Deirdre Just a question about Salvias. Do salvia grow to 7ft? I moved a moderate shrub which hindered other flowers on bank. It did nothing in that spot so a year later I moved it back to the sunny bank but where it would not other plants. It flowered a lovely pink and then from the centre these tall strong shoots have shot up 7ft high! So now it has 2 levels - what is that? Some of the big salvias can get that tall - eg Salvia karwinskii, 'Timboon', 'Pink Icicles', S. wagneriana. One way to manage them is to prune hard twice - once after flowering then again around January. This helps a bit. I don't really grow many of them now as they took up too much space but they are wonderful in full bloom. Deirdre

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