"Dave Poole" wrote
The natural range of Albizia julibrissin is inland-continental, from
northern Turkey in the west, right through Iran and as far east as
China. Typically, temperatures are searingly hot in summer, but it is
bitterly cold and dry in winter. I'm not aware of it being a native
of Korea but the medicinal qualities of the bark, roots and leaves are
highly valued by sino-asiatic races, causing it to be planted well
outside of its range. Therefore it has naturalised in many places.

It is extremely cold tolerant when dormant given the right growing
conditions, but also requires high summer heat to thrive. Despite its
delicate, tropical appearance, it withstands drought remarkably well.
Full sun at all times is essential if the wood is to ripen
sufficiently for flowering.

Every good flowering specimen that I've seen has been on dry,
moderately alkaline soils with no additional watering or fertilising.
In fact neutral to slightly acidic, humus rich soils encourage
infection by fusarium disease. This is a soil-borne fungus that
enters via the roots and blocks the sap-conducting vessels, causing
the leaves to yellow and die suddenly. Young plants are especially
prone - often losing upper branches and in some case the entire plant
above ground can be killed. As happens with wilt disease in Clematis,
there is a possibility of regeneration from the rootstock, but the
infection often recurs.

In common with many plants native to regions where the climate is
typically continental, growth only resumes when temperatures rise
sharply. As a result, Albizia will often be the last of the trees to
leaf-out in the UK. This is beneficial in terms of avoiding damage by
sudden, late frosts, but of course at our latitudes it also leads to a
short growing season. If the ensuing summer is 'indifferent',
flowering will be poor or non-existent.

The best spot to grow Albizia julibrissin is close to a sunny, south
facing wall, where it is naturally drier and benefits from reflected
heat. Unless the soil is especially impoverished, I would not apply
any fertilisers, humus or mulch. Nor would I give additional watering
once the plant has become fully established - regardless of the
weather. I'm almost tempted to suggest that neglect is the best
treatment if planted in the right spot!

Anthony Stokes wrote:

A greenhouse environment may also be unsuited as peak daily temperature
over
40C seem to cause rapid death to most of the above ground growth.

It is the relative humidity and accompanying turgid atmosphere that
encourages this - peak temps of 40C or more in 'open air' are very
much to its liking. The temptation to keep it evenly moist in such
heat (admittedly a seeming necessity for pot grown specimens) is
likely to encourage fusarium, which causes the sudden die-back in
mid-growth.

There seem to be many of these trees available in local garden centres (
typically at 5 - 10 feet high, in pots ) ~ so presumably someone somewhere
is having success with them in England ?

I wonder how many of these are being sold with the correct cultural
advice.

Thanks again Dave, I now know the very spot.
A friend has a beautiful one in their garden about an hour from Toulouse,
very hot summers (daily about 30°C) and dry but cold winters (down to -12°C
last winter). Flowers beautifully and self seeds about (a couple of which
and some seeds are on their way to me later in the year I've just been told)

--
Regards
Bob
In Runnymede, 17 miles West of London