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.

Self-seeded Ammi majus in my summer border with the dark leaves of Amaranthus cruentus

Self-seeded Ammi majus in my summer border with the dark leaves of Amaranthus cruentus

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!

Chlorophytum comosum Ocean and Agapanthus line a path in my garden

Chlorophytum comosum Ocean and Agapanthus line a path in my garden

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

Persicaria capitata

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

Syngonium podophyllum

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.

Tradescantia zebrina with Colocasia Black Magic in the garden of Kathryn Hipkin in Sydney

Tradescantia zebrina with Colocasia Black Magic in the garden of Kathryn Hipkin in Sydney

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

Viola riviniana Purpurea

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.

Ctenanthe setosa Grey Star

Ctenanthe setosa Grey Star

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!