Myporum voscosum and Acacia accinacea are particularly susceptible to this, although it is not a disease process rather a characteristic of the species. The only way to prevent it is regular pruning during the growing season and not allowing the plant to reach its mature size. Occassionally it is necessary to cut them back to their stumps and allow them to re-grow.

I have been growing Banksias in Vienne (near Lyon) France for 5 or 6 years now. I have some B coccinea seedlings myself, but this species has a reputation of being difficult. In WA, it grows in deep sand dunes near Albany, so use extremely well draining substrate. Secondly, my general experience with other banksia species (I have about 30 different, of which 5 have flowered) is that Banksias, do not like too much lime: I have very hard mains water here (equivalent of 280g of CaCO3 per cubic metre) and my early attempts at Banksia cultivation resulted in extreme chlorosis and death due to the pH rising. I have had much more success since I have taken steps to keep the pH down (I aim for pH 5). I even saved a chlorosed B blechnifolia by dunking it in a weak solution of Nitric Acid!! Even with these precautions some (not all) of my B coccinea seedlings are looking yellow, so I recently gave a dose of chelated iron. Thirdly all WA Banksias detest humidity, so do everything you can to reduce humidity in your greenhouse (fan, etc) and water sparingly especially in winter. Finally Banksias do not like phosphate, so do not use any fertilisers other than extremely low phosphate ones. Banksia canei grows in the Blue Mountains on the other side of Australia and is one of the toughest Banksias as far as cold and frost is concerned, so could be planted outside if you never get much lower than minus 5°C. I have also found that it is also more lime/pH tolerant than most banksias. More generally, as far as frost resistance goes, Australian climates are so different from Western European climates that only information gleaned from those who have tried growing Banksias in Europe is likely to be reliable. I have three 5 year old B canei and they are yet to flower. If you are seriously into growing Banksias and other Australian plants may I suggest you join the (UK) Australasian Plant Society? I would be please supply their address.

Unless you are living in an area of savannah woodland I suspect a good diversity of native shrubs may have occurred on your site pre-clearing. There should be quite a few local natives, especially in your area (which I envy). I live on hard clay loam and have great success with Banksias and Grevilleas, but they're all species from areas with clay. I know quite a lot of the most popular WA plants come from deep sand so would doubtless kark it on clay. My past experience with Eucalyptus caesia (pretty much a shrub where I was in western Sydney at the time) shows it to be tolerant of clay, as is Grevillea biternata. Other WA species I am familiar with or have heard of taking well to clay include: Alygone huegelii Astartea fascicularis Boronia megastigma (well, maybe touch and go) Calothamnus quadrifidus Chorizema cordatum (if given a reasonable humus-rich topsoil) Eremophila - most seem to tolerate clay around here Hakea laurina, petiolaris Melaleuca coccinea (worth a try), diosmifolia, laterita Solanum aviculare (grows like lightning to about 2.5m) Some of the eastern natives (and are not too tree-like) that do well on clay in my experience include: Melaleuca parvistaminea, ericifolia, styphelioides Kunzea ambigua, parvifolia, ericoides Banksia marginata, ericifolia, spinulosa, robur, paludosa Grevillea hyrids of many sorts, aspleniifolia, juniperina Baeckia virgata Acacia species (most, especially feathery-leaved ones) Allocasuarina paludosa Atriplex nummularia Bauera rubioides Callistemon citrinus and cultivars Calytrix teragona Correa alba, reflexa cultivars Crowea all Dodonaea viscosa Prostranthera lasianthos, incisa, saxicola Philotheca myoporoides (used to be Eriostemon myoporoides) Indigofera australis Jacksonia scoparia Leptospermum, lots Myoporum montanum and various others Westringia longifolia, fruticosa Mind you, if you grow on limestone this list may be of sparse use. While most of these species are pretty hardy, some loathe high pH. Also, this list applies mainly to better drained clay (white or yellow to reddish) not soggy stagnant clay (grey). My personal preference would be your own local natives, you have so many real beauties.

I have quite successfully modified the growth habit of a few species. I have an Acacia pycnantha that I have successfully encouraged to grow with a clear trunk for 1.8m metres. Usually they have branches almost down to ground level. It was a simple matter of cutting off the lower most lateral branches right from when I first planted it as tube stock. Now it is about 4-5m tall and has not produced any lateral branches below the 2 metre mark for at least 2 years. It looks something like a mop top plant, though I do not bother shaping the crown. As long as you a vigilent with your pruning I don't really see why you could not encourage a similar growth habit with a Banksia spinulosa. Although it is more likely to be a case of 2-3 main trunks clear of lateral shoots rather than just one.