I have several purple robed locusts. They are very beautiful with dappled shade. Their branch structure is more spare than other locusts. They have multiple suckers coming up which are black locusts, more dense with white flowers. I need the shade so I'm letting the suckers grow. The suckers have long sharp thorns.

Have 4 mimosas. They are late to leaf out. Their leaves stay late in Autumn. Flowers are nice and fluffy. Light shade. Squirrels use these trees to access the roof, fun to watch them scurry.

The locusts are more drought tolerant. Both trees like heat and sun, and the mimosa wants a lot of water.

The locusts will get taller than the mimosas.

I planted these trees on a raised bed patio for dappled shade, and because I need trees there that will enjoy the full blasting sun over the roof. The raised bed patio is a good size and is flagstoned (hand-sized gaps between stones) and I needed a tree with very aggressive strong roots to bind all the dirt in the patio together -- the purple robed locust has very aggressive ropey roots that travel far and has been the perfect tree to bind the patio. There's creeklets alongside the bottom of the patio raised bed so once those locust roots hit the creeklets it can slurp up all the water it desires.

Some folks do not like mimosas but I do, very pretty in summer, birds flock to them. Have seen old ones make a lovely umbrella canopy.

Also have a shademaster locust, nice rich green, and several neighbors have very beautiful golden locusts.

Silk tree and Purple Robe locust can be expected to grow double that size, 25' is small for a tree. Like most deciduous trees, neither lend themselves to being tipped back to control size. A silk tree at the Vancouver (WA) Community Library was 46' tall with an average crown spread of 75' in 1990. Another, at the Hulda Klager Lilac Garden, Woodland, WA was found to be 49' tall with an average crown spread of 47' in 1993. Both are in the same climate as Portland.

Common here comparatively recently Purple Robe is already over 33' tall in at least one Seattle park. "Said to grow 35'-50' tall" (Jacobson, NORTH AMERICAN LANDSCAPE TREES, 1996, Ten Speed Press, Berkeley)

If you have time and inclination to hoof around the Hoyt Arboretum something might catch your interest there.

Here is a link that might be useful: Trees that merit planting in Seattle

75' crown spread! Hooboy, that would cover the entire front stretch of my property, LOL

That must be some sight to behold.

Bboy your posts are very informative and interesting. Thanks for the scientific factual input you give.

bboy, I have been misled! My favorite Silk Tree was not supposed to get that big. It's only been resident at our home for 4 years and it is about 12 feet tall with a very wide canopy. It is an absolutely lovely tree -- I love the flowers, and the leaves that fold themselves up at night time. It is just leafing out now. It is messy though, dropping flowers and leaflets all summer. Mine is underplanted with epimedium and when stuff drops I just ruffle the epimedium leaves and sweep the stuff under the carpet. It is very well established dispite my lack of attention.

In keeping with what the two examples I mentioned had done by the 1990s Sunset WESTERN GARDEN BOOK (2007, Sunset Publishing Corporation, Menlo Park) describes silk tree as producing "Rapid growth to 40 ft., with a canopy spreading to as much as twice the tree's height". However, they do also state that this species "Can be cut back to make a 10-20-ft-tall umbrella." This would surely be something on the order of pollarding or stooling rather than heading back the branches a short distance each season.

According to Sunset Purple Robe "resembles 'Idahoensis'" which is "A tree of moderately fast growth to a shapely 40 ft. tall, 30 ft. wide".

>>bboy, I have been misled! My favorite Silk Tree was not supposed to get that big.

Depending on what kind of microclimate you are in, it could remain much smaller. I've observed that in not-sheltered gardens or in gardens with diminished summer heat accumulation, the maximum size after 20-30 years can be about 20x20'. Of course south of the Puget Sound area and away from the coast, it's going to be hot enough in summer for the tree to get big.

There is a very mature silk tree in the Wallingford area planted in a narrow parking strip that must come close to Bboy's record tree in size. The canopy easily spreads fully across the street and the height very close to 40', if not more. It is a pretty impressive spectacle when in its midsummer bloom but it also has a very nice winter silhouette.

That's probably this one:

"Bagley Ave N, south of N 39th St: a street-tree, W side, 41' x 6'8" x 67' wide"

--Jacobson, TREES OF SEATTLE - SECOND EDITION (2006)

(The tallest local specimen he lists (with measurements) is a crowded one at Kubota Garden Park 55' tall)

Yep, that's it! About a block north of my nephew's house.