Hello Olga. Please post pictures so that we could see what the trees look like.

The leaves are nice and green, no signs of bugs (nursery confirmed no bugs). These were both planted by professionals in Jan of this year and have been handling it very well, that is until the over watering. The soil was moist an inch down and then someone in my family accidentally watered the trees this weekend and the orange, which was doing okey went bald, confirming that it's a water issue. I removed all the mulch and planning to dig some holes on the sides of the root ball, not touching it...just to help with evaporation. The branches are green under the bark.

This is before this weekend's watering (mandarin), orange was almost normal:

This is today after the weekend's water dose:

Orange:

Mandarin:

Thanks SO MUCH in advance for your help!

We are on a hill, limestone type soil, sandy not clay like btw

The soil was moist right under the mulch before the weekend and I've dug up a half foot trench around the tree, not touching the rootball. The soil is moist throughout and it's very sandy, so doesn't stick and roll into a ball in my hand. I'm also thinking, as a potential cause, it could be the temperature changes. They have been fluctuating a lot for months. This weekend was 90F during the day and then it dropped to 45F at night yesterday and again low 80s today. I've been reading temperature swings can cause leaf dropping too. Not quite sure what to do.

Olga, where are you located? Do I understand that you dug a trench encircling the tree? You may have damaged feeder roots... or it may just be the temperature changes / moisture changes... citrus likes consistent moisture. Have they done this before - or are you new to the property?

BTW: sandy soil and on a hill and in the ground = very VERY doubtful that too much water is the issue.

What was the pH of the water. Any chance that the water was output from a water softener?

I'm in California in the Santa Cruz Mountains, new to the property. The water was directly from a well, so no water softener. Since the trees were planted in January of this year, it's easy to see where the rootball was so I dug a trench a bit further from it, making sure not to touch the trees. It's been a very wet winter here, a lot of moisture. I agree that the sandy soil on a hill is doubtful to result in water damage. The leaves dropped while green, not yellow and throughout the tree, not just at the bottom or one side. If it is temperature / weather conditions, what can be done to help the trees through the stress?

It may be the temperature fluctuations, you said they are 20 year old trees planted this January? where were they before? Its only been 4-5 months they probably aren't adapted to their new conditions yet. Between the new water regime, soil, temperature conditions it may take them a while to adjust if they ever do. I am not familiar with transplanting mature trees but I feel like I've heard its hard for them to adjust.

They were in a nursery before, in-ground. Few companies transplant these, mostly they deal with huge trees for commercial buildings. They transplanted 6 more large trees from my old backyard at the same time and they are doing wonderful (all non-citrus).

Perhaps the only soil that citrus will not support is calcareous; if you have that soil, you will have to suffer your citrus to be in containers.

I know folks in near Santa Cruz that are successfully growing citrus... so assuming the soil is acceptable, I think the leaf drop experienced is environment related. The roots have not had time to grow out / deep and this past month shifted from cool rainy weather to hot spikes and dry. if there is a chance that moisture at the rootball dropped while/it was hot, then I suspect that was the cause.

I think if you give them good, consistent moisture and feeding, they will settle and establish just fine.

Would B1 or B2 help the trees settle in a bit better or would that be an overkill? I'm told it's just like a hormone, not like fertilizer for the trees.

Santa Cruz is not a citrus area; and it is likely because of the soil.; and the OP did say limestone soil.

Olga, I am a big fan of liquid feeding after transplanting... I believe that it is an effective way to get nutrition to the tree while its roots are still establishing in the new location. I do suggest that you alternate with a high N recipe (weak solution, and only after already watering the tree). [I have alternated between miracle grow and Master Nursery Liquid Gold. ]

I also believe that the fact that your tree has some leaves after transplanting months ago is a good sign. I think the roots are going to take much more time to settle in. In my transplant escapades, this process has taken from 1 to 12 months and longer with some rootstocks (i.e. Carrizo has taken up to 2 years to get "traction").

John: I cannot speak as to the soil or its make-up and acceptability for citrus. But, as an example of success, a business associate of mine bought a property in Santa Cruz many years ago with a reasonably large yard... there were and still are 15 mature Meyer Lemon trees kicking out an abundance of fruit.

Good luck Olga, please do update us as to progress - George

Could be just settling in...I realize it can take a while. Thanks for the suggestions - I'll try it out