Is it all of the crapes? What about the crapes in your neighborhood and community? Are you aware of salt water intrusion into your water source, either municipal or a well? Have you been in a drought in your area?

Other landscape plants are more sensitive than crape myrtles....have you observed damage to other plants?

That is what sodium damage looks like but I wouldn't exclude other possible causes, either. Find out about the water quality first and foremost.

Gypsum is very successful at grabbing sodium ions from the soil solution but would be an unneeded expense unless water and soil tests indicate a problem.

I also want to add that I lived in the Lowcountry of SC for much of my adult life, working as a professional horticulturist. I never saw issues with the crape myrtles, even during severe drought years when salt water intrusion was a huge problem.

Thanks for the response! Yes, it has affected all of my crapes(multi trunk). They are all the Natchez(white flowering) kind. The largest one has all leaves turned entirely brown and the leaves never got very large this year. None of my citrus trees(lime, lemon, mandarin, as well as my sour cherry tree, and mango tree) have been affected. The Crapes all leafed out a bit later than expected(maybe because we had a fairly wet socal winter), but there's been no rain since May, so all city water. We have some tomatoes which might also be a victim of the water I think. How much gypsum would you recommend for an 8ft tall crape myrtle? One thing to note is I got these from a nursery that was getting rid of them from being in 15 gallon container too long last year. It is pretty strange though since out of all trees and plants the Crape's are doing poorly.

Haven't seen any other crapes in my neighborhood having this problem, so concerned it might also be something else. Crapes do surprisingly well in Southern California.

"One thing to note is I got these from a nursery that was getting rid of them from being in 15 galloon container too long last year."

This is a very telling comment :-) What did you do to the rootball before planting? If the rootball was not teased apart properly before planting, as much container soil removed as possible and the roots well hydrated as well, it could be that the crapes have not put out any new lateral root growth and/or the rootball has remained dry or even repelled water, despite your efforts.

As far as timeline- Trees were acquired last July, planted(had a gardener help with this but maybe the part about the rootball wasn't done properly), leaves started flowering but then started turning fall color by September(which seems early for LA), fell off when going dormant by end of November. New leaves emerged in very late march(but no new branches), and started having this issue pretty much right after there was no natural rain in May. The largest tree the leaves were tiny before turning completely brown. Since I've had the trees barely any new branch growth as well. Is there anything to help the trees if there has not been lateral root growth?

I agree that the condition of the root ball might be the center of the whole problem.

By the way, gypsum is never applied according to what a plant might need but how much per square foot of soil. It is a soil remedy not a plant fix. Again, unless you know that the water or soil is sodic, using gypsum is a waste of time and money.

Is there anything that can be done regarding the rootball situation at this point? I understand it's basically amending the soil, but wasn't sure how much to apply around the tree. I know the soil here is on the alkaline side 7.6 PH.

If one of the trees is looking like this, is it still worth trying to revive it?

The trees are in the lawn. This one pictured above, the grass looks super dried out this year. I'm not even sure why since I've been watering the same amount as last summer when it was hotter. I started wondering if something is up with my water supply, or if something is wrong with the soil, or grass. There's a Strawberry Tree nearby that is doing just fine.

Strawberry Tree (Arbutus Marina) below

I guess I can try and dig around and see what it looks like.

Will say the sun seems to be bleaching things worse this year overall.