I have been instructed by Denise Horchner of the Perennial Poppies Garden Club to write about my Phillip Island Hibiscus, Hibiscus insularis. This Australian species could be described as the ultimate hibiscus. As far as I’m aware, I’m the only person who uses this critically endangered wildflower as a flowering hedge. Planted to welcome visitors and to shelter my front garden from desiccating wind, birds and people love its blooms. I make jam and a soothing tea from a species that has become my signature plant.
Do you have a signature plant? Something special that says to other people that this is your garden? For me, Hibiscus insularis is a very special plant and the hedge I planted in very uncommon. I bought mine as tubestock from the Growing Friends of the Royal Botanic Gardens, Sydney back in 2003.
When I planted my hedge in Brisbane it was believed that the low point for this species from tiny Phillip Island, threatened by feral animals (now eliminated) and soil erosion (now stabilised) was when its population had been reduced to seven individual plants in the 1970’s. I planted 21 cuttings so I could say I had three times the wild population growing in my hedge. Now DNA analysis tells us the wild population is derived from just two genetically distinct individuals, so I have ten times the wild population sheltering my garden from wind.
Like many other species used for hedging, Hibiscus insularis flowers annually. In warm temperate Sydney, it produces a modest number of flowers recurrently from October until May. In the humid, sub-coastal subtropics of Bayside Brisbane, Hibiscus insularis flowering is restricted to a single mass blooming during the winter-spring period.
Whenever you prune a hedge of a species that flowers once a year, if you prune after flowering and you time this right, it will have enough time to produce flower buds for next season. Learning when to prune is species specific, but it guarantees a flowering hedge. By pruning once a year, you create an informal hedge. If a formal, crisp, neat hedge is the aim, it may not flower.
Choose the right species, and an informal hedge can be extremely rewarding. The best example I have ever seen was a hedge of Berberis x stenophylla at Kew Gardens. About forty metres long, it used to separate the Aquatic Garden from the Iris Collection and the Jodrell Laboratory. In full bloom, it was spectacular and scented and you can make jam from the fruit.
In Australia, lillypilly (Syzygium spp.) can make a fabulous informal hedge. You get flowers, fruit, colourful regrowth and you can make lillypilly jelly simply by learning to prune them annually after harvesting the fruit. A lillypilly hedge of famously psyllid-proof Syzygium luehmannii had been my original intention here in Wynnum until I decided that Bellis, my experimental new home and garden, ought to instead have a hedge of my signature species.
Wild Hibiscus insularis are said to flower when they reach thirty years old, but planted as tubestock (mine was planted in May 2004), they start flowering within a year or so. Flowers open a creamy yellow, darkening to a soft pink before (mostly) dropping off. This leaves a coronet-shaped calyx which often drips nectar well after the flowers have finished. I can’t think of any evolutionary benefit this might offer a pollinator, it’s too late for pollination. Viable seed are produced in limited quantity.
Sydney’s humid, sub-coastal warm temperate climate suits this hibiscus. There, finger length cuttings of firm, semi-ripe wood were propagated on a heated bench under mist during summer.
To be accurate, Hibiscus insularis is a national plant rather than an Australian plant. The Norfolk Island group of which Phillip Island is a part, is over 1,000 kilometres east of our east coast. While those islands are geologically and biologically unique, there’s an overlap between some of the wildflowers bridging the ‘them and us’ of Australian botany and gardening. But those islands are remote and have been isolated long enough to evolve some of their own unique plant and animal species including Streblorrhiza speciosa, a now extinct shrub. The most familiar extant endemic is the Norfolk Island Pine which is grown worldwide.
Feral animals had eaten away a lot of the groundcovers that had prevented tiny Phillip Island from losing topsoil in stormwater run off into the ocean. Feral animals have now gone and the handful of surviving adult hibiscus have started regenerating by seed. To help the species recovery plan, the Royal Botanic Gardens, Sydney donated seed and cuttings from adult plants growing in their collections.
Hibiscus insularis has both small leaves and short internodes on its branches and these two qualities are what you should look for if you decide to create your own signature hedge. When I was planning and designing the ‘Rare & Threatened Plants Garden’ for Sydney Gardens, I decided to use Hibiscus insularis as a formal, non-flowering clipped hedge to separate sections of that garden. I also used hedges of Hibiscus insularis to enclose the pavilion in their Rose Garden. Large-leaved plants look permanently tatty if the leaves are slashed by pruning equipment. Long internodes mean a plant will create an open hedge with internal gaps. Hibiscus ‘Snow Queen’ is one of the exceptions, but many hibiscus don’t make attractive clipped hedges.
When my hedge blooms, they drip sweet nectar and birds and bees find them irresistible. I use the flowers to make a syrupy tea, useful for alleviating a sore throat. I add honey and lime juice for flavour and colour. The petals can be dried for future use.
In the wild this plants makes a largish, spreading shrub with beautifully layered branches and neatly serried rows of tooth-edged, little leaves. Pruned as a shrub they reach 3 or so metres high and 4-5 metres wide. Allowed to grow as a tree they’re often multi-trunked with a broad arching canopy, reaching to 4 or so metres high and 4-5 metres wide. Storms can break tree branches when in flower.
My hedge was propagated from one plant, which in turn happens to have been propagated from the type specimen. This term is applied to the original plant from which the species was formally described. I donated flowering samples to a botanical illustrator and now the painting is part of the collection held by the National Herbarium of NSW.
Left to fend for themselves, Hibiscus insularis won’t really thrive in Brisbane: our heatwaves are becoming too hot, causing their leaves sometimes curl and turn yellow. I have also had five frosts in my allegedly frost-free climate, and while they may just be technical frosts at -0.5C, they cause some bud drop and tips of the foliage turn purple. But when autumn rains arrive, my hedge rapidly greens up and sets loads of flower buds in readiness for another annual show time.
The only pest to visit my hedge is cotton harlequin bug, aka hibiscus beetle, Tectocoris diophthalmus. These are easily picked off and drowned in soapy water. Learn to identify their egg masses and all you need do is squash them.
At planting time you have your one and only chance to dig and enrich the soil with compost. I planted my hedge in a ridge of soil 20cm high. While this is normally used to improve drainage around the base of the plants, in this instance the ridge holds back any stormwater that attempts to rush out of my garden. So far, the hibiscus are happy and I am happy not to have lost any valuable rainwater from my property.
In a warm climate, it is wise to water a hedge well the day before pruning – especially during drought – because they lose moisture as a result of being cut. It’s also wise to renew mulches after pruning. Why? Removing foliage admits more sunlight which heats up and dries out soil. Just use a layer 3-5cm deep and don’t push mulch against the base of each plant, in wet weather it may encourage attack by fungi. To keep my signature hedge unique, I use pandanus nuts as mulch and I have underplanted it with a row of Tillandsia.
Branches suddenly exposed to strong sunlight may also scald. I learned patience and prune my hedge during cloudy weather, ideally following useful rain.
I feed my hedge once a year after pruning with pelletised, certified organic poultry manure. Once every month they get a foliar feed with liquid seaweed. It’s ironic that this is the one plant in an otherwise rain-fed front garden that has to be watered weekly during prolonged drought. But that is a reminder that quite a few of our more commonly grown plants will need life-support as our emerging new climate intensifies.
As I said to Denise, this is the irony. My front garden hasn’t been watered since 2005. The only plant that must be watered is an Australian plant, but the justification is that it’s also one of the most beautiful of our critically endangered wildflowers.
Sydney Gardens once had nine adult Hibiscus insularis. In 2000, I recorded a story ‘Hedge on the Edge’ with Damian Brown of ‘Gardening Australia’ (when it was produced from Tasmania) and another story with Sammy Lucas of Channel Ten’s ‘Totally Wild’ (1999) with the largest and best specimen which used to grow in Lawn 29. It and other specimens have since been culled for reasons unknown, which proves the point that rare plants are not always safe even in a botanic garden.
Director, Seed Savers’ Network Inc.
Revised 16th February 2020