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Growing Rainforest Plants
Peter Radke
There are three secrets to creating a rainforest garden, and no, they are not water, water, water! In fact, water is no more a necessity for a rainforest garden than it is for any other type of garden.
The three secrets are:
- Dig up the garden bed thoroughly to provide as much loose soil as possible;
- Provide a thick layer of organic mulch;
- Protect from dry winds.
Soil Preparation
Rainforest plants, as a general rule, have very timid root systems. Unlike acacias, eucalypts and grevilleas which have robust root systems capable of penetrating hard ground, rainforest plants have timid, shy root systems which tend to stop dead at the first sign of an obstacle. Consequently, rainforest plants planted in hard ground simply will not grow.
Rainforest plants grow best when you garden in the traditional fashion - pretend they're roses or a vege-garden, and plant them in a bed of thoroughly loosened soil. The ideal depth is about 60cm. On a small block of land, this means rolling up your sleeves and getting stuck into the ground with the pick, shovel and elbow grease to physically break up the soil. You haven't finished until your original hard lumpy ground is nice and soft and friable.
If you own acreage, it's well worth while to use a machine - a bob-cat or backhoe, or even a small bulldozer with rippers can prepare a large garden bed in a flash. It's not expensive - for less than one hundred dollars you can prepare a garden bed which would have taken months to prepare by hand, and the resulting plant growth will be spectacular to say the least.
Except on sandy soils, a rotary hoe is not good enough, since it does not dig deep enough, and the rotary action can lead to compaction of the layer immediately beneath the blade. Don't use a rotary hoe unless you've dug up the ground first.
Post-hole diggers are also not a good idea - they dig holes that are too deep and not nearly wide enough, and, except in very light soils, they produce holes with hard sides which roots find very hard to penetrate. Post-hole plants often look lanky and horrible, and they blow over easily, because their roots can't spread out to get a good anchorage. Spend the same amount of money on hiring a dozer, backhoe or bobcat instead.
Mulch
Mulch is essential to grow rainforest plants well. The mulch should be organic, and applied as a thick insulating layer on top of the soil much like a blanket covering a bed.
Unlike many acacias, eucalypts and grevilleas, rainforest plants are very surface rooted. It is therefore essential to keep the surface of the soil cool and moist, otherwise the roots will bake and dry out.
As I said before, rainforest plants are timid - give them an obstacle and they give up very easily. If you allow weeds or grass to grow around the base of rainforest plants, they cannot compete - weeds and grass will severely inhibit the growth of rainforest plants. A thick application of mulch will keep weeds and grass at bay.
Organic Mulch
Thick organic mulch is essential for several reasons:
- It keeps the soil moist by reducing evaporation;
- It controls weed growth;
- It keeps the soil cool;
- It provides a source of recycled nutrients;
- It keeps the soil healthy by maintaining a balanced population of micro-organisms.
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For rainforest plants, it is important that the mulch be organic, since the recycling of nutrients is very important for their growth.
The type of mulch you choose is not important, so long as it is organic. You can use anything, so long as it was a plant once. In north Queensland, bales of mulching hay are popular. In some areas, peanut shell may be available. Many shire councils sell mulched-up garden waste. Wood chip is fine. Newspaper and cardboard are OK, but it is best to shred them first - if you spread them out in sheets they act as a thatched roof and prevent water penetrating the soil beneath.
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Make sure that the mulch you use is clean. If you apply mulch which has dirt mixed in with it, you will defeat your purpose, since weeds will establish in the dirt. And if the mulch contains weed seeds, you've lost the battle before it even starts.
Some types of mulch last longer than others. The short-lasting ones decay and enrich your soil, but will need to be topped up. The long-lasting ones may look good for longer, but your soil stays poorer. In the end, it's a matter of what's available locally at a price you can afford.
Some mulches are quite dusty or contain a lot of fungal spores, so wear a mask while spreading them, and make sure the wind is blowing away from you. Once the mulch is spread and watered down, there is no more risk.
When applying the mulch, you can either plant your plants first and then spread the mulch around them, or you can spread the mulch first and plant later. Both ways have their pros and cons. If you choose the former, be careful not to trample your plants or compact the soil as you walk around. If you choose the latter, be very careful to keep the mulch clean and free of dirt when you dig the holes to plant your plants.
Mulch should be applied as a clean blanket on top of the soil surface. It should never be dug into the soil - this is for compost, not mulch.
Things such as black plastic are no substitute for organic mulch. Black plastic does not allow the soil to breathe, and it does not allow the recycling of nutrients that is so important. In warm climates, it can cause the soil to overheat. Don't use black plastic. If you really think you need to, don't - make your layer of organic mulch twice as thick instead.
Watering / Humidity
Provided you have a thick layer of organic mulch, you will find that you don't need to water your rainforest garden any more than a normal garden, although it will love any extra water you can give it. Of course, your rainforest plants must be watered for the first month or so until they are established, just like any plant, be it a wattle, grevillea, rose or camellia.
However, rainforest plants as a rule cannot tolerate dry winds. Unlike acacias and eucalypts etc. which have a thick waxy cuticle over their leaves (amongst other adaptations) to prevent excessive moisture loss in dry times, rainforest plants have little protection against evaporation since they have had no need for it in their natural habitat. It is more important to maintain a reasonable humidity level in the air than it is to apply water to the roots.
In drier areas, such as areas which experience an annual rainfall of only 700 - 900 mm, you will need to trap humidity around your plants - say, by planting a shelter belt of local natives, or by creating a courtyard.
Fertiliser
Rainforest plants love to be fertilised - fertiliser brings out the lovely foliage colours that makes rainforest plants such a delight to grow. The rules for fertilising rainforest plants are the same as for ordinary natives - check the N : P : K to make sure the phosphorus is low - less than 3% is best. Water in well, and don't overdo it. Like medicine, a little is good for you, an overdose can easily kill.
Dispelling the Myths
There are a number of myths regarding rainforest plants, that need to be put to rest.
Myth: Rainforest plants need to planted in the shade.
Wrong! Just because the rainforest is shady place to walk in, does not mean that rainforest plants need shade. What it means is that rainforest plants cast shade. In fact, if you walk through the rainforest you will notice that the seedlings on the floor of the forest in the dense shade are not growing - they are sitting in a dormant state waiting for a gap to form in the canopy so that the sunlight can stream in. It is only when they receive full sunlight that they start growing.
If you plant your rainforest plants in the shade, they will grow slowly, and they will become thin and lanky as they struggle upwards to the light.
Rainforest plants can, and should, be planted in the full sun, where they will grow thick and bushy, and flower young. In their natural habitat, most rainforest plants do not flower until their canopy is in the sun. By planting them in the full sun from the start, you will trick them into believing they are already at the top of the canopy.
Of course, what I have said above does not apply to the shade dwelling understorey shrubs of the rainforest - these naturally need the shade.
Myth: Rainforest plants are too large for ordinary gardens.
Wrong! Most rainforest plants grow in the garden to only about a quarter or a third of their height in the forest. By planting them in the full sun from the start, they have no need to grow taller and taller to reach the sunlight, since they think they are at the top of the canopy already. I wonder how many of you have an Ivory Curl or Golden Penda in your gardens? Would you have planted them if you had known that they are rainforest trees from north Queensland, where they commonly grow 20 to 30 metres tall in the forest? In cultivation, however, they are rounded shrubs of only about 5 to 6 metres, and this is true of most rainforest plants, with the exception of plants such as the Kauri Pine and Bunya Pine.
Myth: Rainforest plants should be planted under a canopy of existing trees.
Wrong! Plants planted close to existing trees generally do poorly due to root competition and lack of sunlight. Take the plunge, and plant your rainforest trees in the sun from the start.
Myth: Rare plants are difficult to grow.
Wrong! Some rare plants are quite hard to grow, but many are surprisingly tough and hardy. Rarity is often more related to habitat isolation than to any intrinsic feature of the plant itself.
Myth: Rainforest plants can only be grown in soil that matches the soil type in the wild.
Wrong! Rainforest plants can be grown in almost any type of soil. Mulch and loose soil are the main requirement; fertilise if necessary.
Myth: Rainforest species from the tropics will not grow in southern Australia.
Wrong! While the climate of north Queensland is hot, inside the rainforest it is really quite temperate compared with outside. Therefore, many rainforest species actually find the more temperate climate of southern Australia very much to their liking. Provided your air is relatively humid, you can grow a surprisingly wide range of tropical rainforest species.
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North Queensland's ivory curl tree thrives in much cooler climates. It features a massed display (left) and the individual flower clusters) (right) may be up to 200 mm long.
Select the thumbnail image or plant names for higher resolution images (61k and 41k).
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Planning your rainforest garden
If you try to create a rainforest garden using only rare or 'collector's' plants, the chances are that it will look awful. A rainforest garden needs a basic structure against which to display the rare and unusual.
To create an attractive rainforest garden that is pleasing to the eye, you need to use a fair proportion of hardy, bushy species that will give the garden its basic structure, bulk and backbone. There are many species that fit this bill, but some are the hardier, tougher syzygiums and acmenas, and plants like Flacourtia, Scolopia and Xanthostemon. Some specific suggestions to start with are: Acmena hemilampra, Acmena smithii, Syzygium australe, S.fibrosum, S.paniculatum, S.tierneyanum, S.luehmannii, S.sayeri, Buckinghamia celsissima, Xanthostemon chrysanthus and Grevillea baileyana.
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Grevillea baileyana is an attractive, long flowering plant which is useful in providing basic structure to the rainforest garden.
Select the thumbnail image or plant name for a higher resolution image (35k). Photo: Keith Townsend
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Pay special attention to the plants that you place on the edge - these are the ones that hit you in the eye every time you look at your garden. On the edge, you should place the plants that have an attractive shape, attractive foliage or spectacular flowers, and the plants that could be smothered if placed in the middle of the garden. Good edge plants are the native gardenias and graptophyllums, Flacourtia sp, Cleistanthus apodus and C.hylandii, Phyllanthus cuscutiflorus, pittosporums, Acmena smithii, Syzygium australe, Syzygium luehmannii, Xanthostemon chrysanthus and Buckinghamia celsissima.
Leggy plants are best in the middle.
Having taken this approach, then you can start placing the more unusual plants that have specific requirements in and amongst this basic backbone.
Rainforest plants have enormous potential in general horticulture in Australia. In the main they are very long-lived and attractive plants, and it is up to members of organisations like the Society for Growing Australian Plants to show how they can and should be used correctly.
Peter Radke is a passionate native plant enthusiast. He has been a member of SGAP for 25 years, and during that time has travelled extensively throughout Australia (and particularly Queensland) observing native plants in the wild and collecting seeds and cuttings for growing in his garden, firstly in Brisbane and later in the tropics.
Peter started his working life as a maths teacher, but his hobby caught up with him 14 years ago when he and his wife Ann established Yuruga Nursery on the Atherton Tablelands to grow north Queensland native plants as a livelihood.
Peter has served on the State Council of the Society for Growing Australian Plants as Excursions Officer and Loan Member Liaison Officer, and has held various positions in his local Tablelands Branch in North
Queensland. He has written numerous articles and books on Australian plants.
Yuruga Nursery has now grown into a large business employing about 60 staff and growing not only natives for gardens, but also tea tree and timber species by the millions for
agricultural projects. Nevertheless, the passion and enthusiasm for native plants continues
and Peter loves nothing more than talking to people and encouraging them to plant native
plants.
This paper was originally presented to the 20th Biennial Seminar of the Association of Societies for Growing Australian Plants, Brisbane, September 1999.
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Australian Plants online - March 2000
Association of Societies for Growing Australian Plants
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