Leaf loss and gain of A. pycnantha was studied by Maconochie (1975). The plants were in the vicinity of the Para Wirra Reserve, Mt Lofty Range. Peaks of leaf production and loss occurred in the spring (October to December) and followed flowering with long steady states lasting about nine months.
The floral buds of A. pycnantha are produced every month of the year on new shoot growth, Buttrose et al. (1981) and those formed between November and May developed through to flowering. The buds produced between June and October aborted at an early stage. Differences in the rate of floral development caused buds produced months apart to flower at the same time in late winter. The abortion of buds corresponded to the period of pod development thus a competition for metabolites could have been one of the causes of low floral development during this period.
An experiment on the germination of Acacia pycnantha was described by Harding (1940). Treatments consisted only of boiling and/or soaking for different periods. Boiling for 5 seconds was more effective than for 2 minutes and both were better than no treatment. There was no interaction with a period of soaking.
Once upon a time A. pycnantha was the basis of a Wattle bark industry in South Australia, Maiden (1906). The industry is now extinct. Yields and analyses for South Australia are given and the local bark is described as "one of the richest tanning barks in the world".
Ford & Forde (1976) point out that nectar is apparently produced from the petiolar glands only when the plant is flowering and only on petioles close to the flowers. Honeyeaters and silvereyes take the nectar and brush against the flowers and could effect pollination. Insects visiting the glands would not effect pollination but could be a further attraction for the birds. Acacia pollen has been found on the feathers of some birds but not in great amounts. The nectaries do not appear to attract ants to protect against herbivorous insects as they do in the swollen-thorn Acacia of Central America.
In a more extended study of bird pollination and nectar secretion Vanstone & Paton (1988) confirmed the initial observations of Ford & Forde (1976). A. pycnantha secreted extra-floral nectar during the winter and this coincided with flowering rather than the period of herbivore damage to the phyllodes. Phyllodes with axillary floral racemes or those nearer the tips of the branches were more likely to secrete nectar. At least 12 species of birds, including honeyeaters, thornbills and silvereyes, consumed the nectar and in the process brushed against the flowers. When birds were excluded from the flowers by mesh, pod production was significantly reduced although natural rates of pod production were low and variable. Some insects, flies, honeybees and ants, also visited the nectaries but did not contact flower-heads. Pollen harvesting insects were never common on the flowers but do occur. As A. pycnantha is largely self-incompatible it needs pollen transfer to set seeds. Bees however may be less reliable than birds due to reduced activity in cold weather. In these tests less than 1% of individual flowers and 70% of flower-heads failed to produce pods. Almost all phyllodes on the plants studied showed some herbivore damage. About 20% of the phyllodes were classified as highly damaged (i.e. had lost more than 20% of their surface). Little or no damage occurred during the flowering season most being done during late spring and summer during the period of active growth. If nectar flow has a protective function one could expect it to occur during this period.
The three native bees foraging on A. pycnantha flowering in Victoria were studied by Bernhardt & Walker (1984). Most carried Acacia pollen but virtually all also carried pollen from other genera. Bees tended to prefer A. pycnantha to A. myrtifolia whose flowering overlapped. Bees did take secretions from the extra-floral nectaries as well as foraging for pollen. The secretions of A. pycnantha are depauperate in sucrose and are a minor source of bee sugars.
A. pycnantha is a food plant for the larvae of several butterflies including Hypochrysops ignita, Jalmenus icilius, J. lithochroa and Theclinesthes miskini, Fisher (1978).
A. pycnantha is locally established in the Cape Province of South Africa, Ross (1975a). It is also locally established in southern Europe.
The leaves of A. pycnantha have been used to dye wool a golden colour using an alum mordant, Martin (1974).
An account of A. pycnantha as the national floral emblem of Australia is given by Boden (1985). A Wattle Club of Victoria was founded in 1899 to promote Wattle Day. Acacia was introduced to Australian armorial bearings 1908-1912 and the ball of flowers was basic to the design of the Order of Australia honours and awards established in 1975. The unofficial national colours particularly for international sporting events are green and gold based on wattle foliage and flowers. Early stamps showing a wattle were not based on A. pycnantha but probably A. mearnsii or A. decurrens. Wattle has been used on other stamps. Amongst the latter was a set designed by Margaret Stones, botanical artist, one of which featured golden wattle and was issued in 1970.