|BANKSIAS OF TASMANIA
Banksia marginata variability by Jonathon Esling and David Marrison
Banksia marginata pink form Photo: David Marrison
As with Grevillea, which is represented only by Grevillea australis in Tasmania, the island state has been a bit short changed when it comes to species diversity in the Banksia genus. While Tasmania had at least two other species in the past (Banksia integrifolia and Banksia kingii) we now have just two species, Banksia serrata and Banksia marginata. Banksia serrata is listed as rare in the state and has a fairly limited distribution, being confined to the Rocky Cape National Park on the north coast and Flinders Island in Bass Strait; in contrast, B. marginata has a widespread distribution. The paucity of species diversity in the Grevillea and Banksia genera is compensated at least in part by the huge variability in form observed in both G. australis and B. marginata. There are at least seven distinct forms of G. australis in Tasmania and until a recent revision they each had the status of ‘variety’. The focus of this article, however, is to highlight the range of forms of the hardy and highly adaptable B. marginata.
Cape Pillar vista Photo: David Marrison
In Tasmania, Banksia marginata occupies an amazingly wide diversity of habitats, being located in coastal heaths through to subalpine areas, and correspondingly varies greatly in growth habit, leaf size and shape. These variations occur between habitats but also within individual stands. Plants range from totally prostrate forms - as found in the wind blown north coast at Petal Point and exposed sea cliffs at Cape Pillar on the southeast coast - to small trees of 12m or more in sheltered positions. Leaves are usually arranged alternately, attached by a short 1-3mm petiole. Leaf shape seems to vary from oblong to oblanceolate, lanceolate or linear. Margins can be entire or serrate, while tips are either blunt or finish with a sharp point or a hooked tip. Flower colour is a fairly consistent yellowy golden lemon, although green and pink flowers have been observed. Flower size ranges from just a few centimetres up to 20cm height, depending on exposure and other environmental conditions. Some forms are lignotuberous while other forms can reshoot from epicormic buds. Suckering has been observed on plants where root disturbance or other forms of trauma have occurred.
Banksia marginata is distributed throughout the state from an altitude of just above sea level in the coastal zone through to 1200m in the Mt Field National Park. This adaptable species occurs in wet sclerophyll, dry sclerophyll, subalpine woodland, buttongrass moorland, coastal and heathland communities. Given its widespread distribution, plants also grow in a variety of soil types including mudstone, dolerite, basalt, sandstone, and granite amongst others.
Many of the native nurseries in Tasmania were quick to take advantage of variation in Banksia marginata and brought several forms into cultivation. These include ‘Ridgeway Spread’, ‘Coastal Spread’, ‘Coles Bay Compact’, ‘Superbush’ and ‘Petal Point Dwarf’. The following descriptions focus on the above mentioned forms, although there are several others in cultivation at the Royal Tasmanian Botanic Gardens which include selections from South Arm and Southport. ‘Mini Marg’ is a dwarf form from Tasmania’s northeast coast which seems to feature more in interstate, rather than Tasmanian nurseries.
|Examples of forms commonly cultivated
Photos: Jonathon Esling
‘Coastal Spread’ Royal Tasmanian Botanical Gardens
A slow growing, low spreading form 1.5m high and 2m wide with crowded foliage along the stems. Flowers yellow/lemon 50-80 mm high. Leaves are oblanceolate and generally do not exceed 15 mm length and 4 mm width. Originates from Tasmania’s east coast.
‘Coles Bay Compact’ Royal Tasmanian Botanical Gardens
A low spreading form 2m high and wide. Leaves 20-30 mm long and 5-10 mm wide, tipped with a series of sharp points. Yellow flowers in Autumn are 60-90 mm high. Originates from Coles Bay on Tasmania’s east coast.
‘Petal Point Dwarf’ Royal Tasmanian Botanical Gardens
A compact bush to 1m high and wide. Flowers yellow/lemon 50-90 mm high. Leaves oblanceolate with entire margins, 10 mm wide and 20-35 mm long. This form originates from Petal Point on Tasmanian’s north coast.
‘Ridgeway Spread’ Island Bonsai
This low spreading form often takes on a sculptural habit, with a height of 2.5m and width of up to 3m. Flowers are yellow/lemon 50-80mm high. Leaves are linear 5-10 mm wide and 10-80 mm long with entire margins. The original plant came from Ridgway, a suburb on the foothills of Mt Wellington, 300m above sea level.
‘Superbush’ Island Bonsai
This form takes on a more formal habit with tight upright growth and long linear foliage. Height between 2-3.5m, width of around 1.5m and flowers yellow/lemon 50-90mm. Leaves are linear 20-60 mm long and 3-5mm wide. This form has proved to be more difficult than some other forms to propagate. Original specimen was found south of Hobart.
Variation in foliage of above mentioned forms with three additional miscellaneous seed grown examples. From left. ‘Ridgeway Spread’, miscellaneous seed grown, ‘Superbush’, miscellaneous seed grown, ‘Petal Point Dwarf’, ‘Coles Bay Compact’, miscellaneous seed grown, ‘Coastal Spread’.
Examples of additional observed forms Photos: David Marrison
Cape Pillar on Tasmania’s southeast coast is home to some stunning forms including totally prostrate, miniature and pink flowering forms. Propagation trials are currently underway to see if seed collected from these forms remains true to type.
Prostrate form at Cape Pillar
Semi-prostrate form at Cape Pillar
The subalpine/alpine areas of Lake Rhona and the Denison Range in South West Tasmania have some very low growing, bushy forms that would be suitable for cultivation in frost prone areas.
Miniature form at Lake Rhona
Bushy, low growing form on the Denison Range
|The Southern Forests area south of Hobart has
forms with linear leaves, making them resemble Banksia spinulosa
from a distance. Several of these forms had great horticultural potential
but were lost during logging operations before cutting material could be collected.
One of the narrow leaved forms in Southern Forests.
Several subalpine forms observed in the Western Lakes-Central Plateau area also have horticultural potential, growing up to 1.5m with flowers up to 20cm nicely displayed on the tops of branches. Unfortunately these forms have so far proven to be difficult to propagate. Lagoon of Islands has some large and very busy forms with small leaves not unlike the ‘Coastal Spread’ form.
There are many other locations in Tasmania that contain interesting forms including: