Re: Dahlia imperialis cultivars

Just to add to Sean O'hara's comments on tree dahlias;
the double flower forms are generally slower growing
and less tall.  The double white form  blooms
significantly later than the pink, usually one month
later, and tops out at about 12 feet rather than 20. 
Because the bloom is so late and subject to more of
our stronger winter winds and rains, it is less
satisfactory for long bloom.  I have never seen a
double pink form around, so can't say anything about
it.  The single white form is also slower and shorter
than the pink, yet taller than the double in height,
and also seems to start blooming later.  The white
color just doesn't stand out against the sky or
background nearly as much as the pink, but then pink
is hardly ever subtle, is it?

This plant is always best planted where it is
protected from strong wind, although cool foggy
breezes seem to suit it just fine.  The plant stalk
and flowers will always face the sun, so one must
remember that when planting, or consider the flowers a
winter gift to the neighbors.  Sometimes it is not
only the winter rains and wind which can do in the
flowers.  Our east bay wind phenomenon in the fall,
the Diablo winds out of the northeast, can also play
havoc with many east bay garden's large leafed
subtropical plants, but it is far easier to deal with
than other people's hail, gophers, deer, etc.  I had
never noticed this sort of wind in the late fall while
living in San Francisco, and it can be a very
localized event.  If you have lots of pesky squirrels
in your garden, you also might have at least one
squirrel which is attracted to chewing on the leaf
stems, which do resemble a fat juicy stalk of celery,
and no doubt are just as crunchy.

Strybing Arboretum has several other tree dahlia
species in the cloud forest section, all blooming now.
 There are also other very large growing subtropicals
in bloom  in the fall, and the Montanoa species/Tree
Daisies are particularly showy now.  M. grandiflora
has  large sandpapery textured leaves with the scent
of sugar cookies, and is the only species commonly
available in the trade, but other species in the
Strybing garden exist which bloom over a longer period
and are just as showy.  

The cloud forests of southern Mexico have a wealth of
late fall/winter blooming shrubs which have yet to
become popular with the buying public.  The limiting
factor may be due to their tender nature and the risk
of freeze damage, or often rather large sizes; but I
think it really is a matter of their being less
visible to the public, as fewer people go to the
arboretum this time of year.  It has always been one
of my favorite times of year to visit both Strybing,
UC Berkeley and UC Santa Cruz Arboretums, with the
wealth of species putting on a show for the late fall
and winter.  Given that there are so many
meso-american and cloud forest plants which are in
active growth in our winter, it's a shame we don't
grow more of them in locations protected from frosts. 

Deppia splendens is a perfect example of this type of
plant, and I hope the recent introduction efforts of
Pacific Horticulture magazine will popularize this
herbaceous shrub in coastal California.  Another shrub
from Mexico, Illicium mexicanum, also deserves to be
planted more often.  This shrub is also hardy in
coastal Oregon, (Gossler Farms is the only nursery I
know of selling it), and has deep green waxy foliage,
and burgundy red flowers with twisted petals, similar
to Calycanthus, which are quite showy fall through
winter, and often intermittently all year long.  There
is a great example in the meso-american section at UCB
Botanic Garden.

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