I am not sure about the Wisteria tree, but some trees will sucker readily if their roots or the soil around them is disturbed in any way. Perhaps it may not be a problem if you don't have a garden around it. I am sure someone may know - or the local nursery if they are good.

20m sounds reasonably safe. The Robinias are grafted and the suckering is from the root-stock, which looks similar in leaf but is usually thorny. Mine never flowered for some reason and barely had any leaves so after 7 years I gave it the chop. I have read that root disturbance can cause them to sucker so if you leave it alone and remove any suckers should they appear you may not have any problems.

I've got two in and they are now well over 2 metres probably closer to 3. They are in their 4th year and not even looking like suckering. My neighbor has the white one and is suckers like crazy. I'd do as Alison suggests and leave it alone and only desucker if they come up.

Yes, they do sucker, as alison mentioned, I have one, it is BEAUTIFUL in full flower, would never chop mine down. I had a few suckers few years back, I dug them out and grafted a golden robinia on them. Since than I did not have any more, though my neighbour gets them from time to time and they are from my tree! He does not mind though just pulls them up when they appear.
meggs

I have a purple one and it's never suckered.

I wonder why some do and some don't??

Annie

They are all pretty, one of the best ones i know of is Robinia 'casque rouge', deep pink flowers

When do they flower?

They flower in spring. In W.Australia it is around October. I should think in California it would be April or May.