Hi Wobur,

I saw your post, but guessed nobody really knew the answer to your very specific query. My very amateur take on it would be:

Eucalyptus apparently need to be planted young to enable them to develop stabilising roots. Don't know how young is young. You would need to check this info, can't remember where I read it.

They are pretty tough and have had to endure starvation and drought in my garden without dying.

They grow really fast and quickly become a nuisance if not dealt with by severe pruning.

If it's really important to have a tree of a certain height why not get one that only grows to that height?

They can be cut back more or less to the ground and respond by producing a shock of young foliage.

My hunch is that if you hacked it off at ten feet or so in spring and fed it well then you would get a lollipop of juvenile foliage by autumn. Don't know if it would put up with this every year without complaint and it would probably look pretty dreadful until the new shoots were fairly well advanced. Depends how much that bothers you. If you didn't fancy the sawn-off look you could prune it back to a neat lollipop framework in late winter. It might start sending out shoots all along the lower trunk and these would have to be removed during spring and summer to maintain a "standard" look.

I certainly don't water my potted E gunni in the winter, so I doubt if yours would need it either.

They are easily & cheaply grown from seed so can easily be replaced if things go wrong.

I don't think it is going to be viable long-term kept at a height of 15" with mainly juvenile foliage. I think you will get fed up with pruning and ultimately it will be hard to keep it a good shape and with young foliage. I think that 5 years down the line you may want to remove it, anyway.

So it would not matter much if it was staked loosely or rigidly at this point.

All in all, I wouldn't worry much about rights and wrongs, just do what seems sensible.

It depends a bit on your attitude to gardening. I'm a bit gung-ho and rely on massed planting for effect rather than single perfect specimens. I don't mind things being temporary, either. However, your sister may be more of a perfectionist and like to have things "just so".

Like nerd, I also thought that probably nobody had tried exactly what you want to do. I heve never grown E gunii but have read that it grows into a nuisance very quickly for some people. I believe that there are several problems with them 1. They grow very fast 2. They break off easily 3. They blow over 4. They need pruning if you want the perfoliate juvenile leaves.My father had one which he has now cut down as it blew over frequently. A neighbour has one which was allowed to grow tall and then hacked off leaving a very ugly mop of folialge at the top of a stick. It has also fallen over and now grows lopsided. I would also consider the appropriateness of a gum tree as a specimen in a Cotswold lawn but that is a matter of taste. I realise that this doesn't help since you have already got one. Do as nerd says - try your ideas out and see what happens. Gardening is not governed by immutable laws. There are lots of ways of doing things. ps Nothing really needs watering in an English winter unless there is a drought and it's newly planted. Even in the summer if something needs watering consider whether it's appropriate for your garden. Only pots and newly planted things should need watering. pps. Have you thought of coppicing it and having a multistemmed shrub rather than a tree?

They can be chopped as little or as much as you want each year and will sprout several feet of pale juvenile foliage every year. This can be done as a tree form or down to ground level as a shrub. Obviously with that growth rate they can quickly get out of hand so be prepared for the maintenance.

An established Eucalyptus shouldn't need watering. But give extra water during hot dry weather in the first year. Note that Eucalyptus do use a lot of water but they survive dry conditions by having an extensive efficient root system, and this may dry out the rest of your garden.

I have 3 Eucalyptus trees (1 snowgum 2 gunnii)in a very small garden. We get lots of rain in Scotland which probably make the trees grow really quickly. Hard to stay on top of pruning - but not impossible and worth the effort.
No 1 gunnii was only a twig when planted some 14 years ago.Its now almost tall as the house (yes even with regular pruning).I regularly chop and saw bits off with no ill effects at any time of the year. They are very tough even despite my sporadic pruning and having been partially uprooted during severe storms.
The snow gum is slightly slower to grow in my experience in comparison to Gunnii and has more attractive bark. I had a long search before I got it as garden centres seem to prefer gunni.

The eucalyptus flowers were a pleasant surprise as well as the buds that have provided me with endless possibilities for my art classes providing year long leaf and pod availability and strong distinctive scent for sensory classes too.

Eucalyptus Gunnii...easy peasy! I never feed it, water it or cosset it. I only cut it.
We moved here over three years ago. There is a youngish specimen here with a girth of around 6 inches. It had already been coppiced at 5ft, that is, its tip had been pruned earlier in its life. This encouraged three stems to grow which had then been pruned to leave three stubs atop of the trunk. Each of these stubs had sent out shoots which attained a length of around 8ft over the next couple of years or so. I cut these back so they became the new stubs. I have now completed the cycle as these stubs are now shooting - but also at various points down to ground level. All the shoots I don't want below the original coppicing I just rub off while young. It might sound a bit dramatic or complicated but it's not and there have never been any problems. Pruning back every year or so will give you young foliage on a regular basis which sounds quite pleasant in the breeze.