C-35 is a standard root; so your tree will be a standard (full) size. You should check the pH of your soil; and if necessary add some soil sulfur to adjust to 7 or a bit lower. "Hard" water tends to be high pH; if the pH is over 7, you might add a bit of vinegar or acid to the water to lower it. Clay soil is not a problem for citrus; except that the trees grow a bit slower; and you have to take care not to keep it too wet. You can add gypsum, if you wish to improve the drainage and texture of clay.

C-35 is a somewhat dwarfing rootstock. It normally produces a tree about 83% in size of most standard rootstocks. As far as soil adaptation C-35 is well adapted to loam, sandy loam, and sandy soil. May preform poorly on very heavy soils with poor drainage. C-35 also has poor tolerance to salinity and to calcareous soils. Trifoliate Orange is a dwarfing rootstock and is well adapted to clay soils.

So...knowing I have clay soil...and the soil is highly acidic...(gauge reads between 7 and 8)...and I have hard water in my area. Is it even worth planting in the ground? Or should I just put it in a planter bed (15 inches height) and then plant it....OR just put it in a 20 gallon ceramic container ?

FWIW: http://citrusvariety.ucr.edu/citrus/C35citrange.html

1. Mike 90% of citrus roots are in the top 20" of soil, therefore a raised bed works well. A pH of 7 to 8 is not acidic, but rather is basic, but should not be much of a problem. A pH above 8 starts to be a concern. Is your water hard because of sodium or calcium or both?

Mike, if you tell us where you live me may be able to help more, in my experience C35 seems to mostly be used on west coast in the US, and is a rather standard common root stock there, though not perfect for your conditions it is more accepting of clay and calcareus soil than some root stocks that are popular on the gulf coast and Florida.

As to size, expect it to reach about 7 ft tall and 4 ft wide in 2-3 years, probably 6-7 ft wide in 5-6 years, and eventually it may reach into the 15 X 15 foot range after a decade or so,

Silica,

I am not sure what the reasons are for it being hard. I should probably run a test. I am assuming sodium, because I had salt burn and in the process of losing an avocado tree.

Ike,

I live in Orange County, Southern California. As far as size, I plan on pruning it to keep it at a manageable size for myself. I don't I want it to going past 8-10 feet in height.

the tree should be at least 8 ft. away from any fence or wall. You can get away with 4 if you really work at pruning and shaping, but 8 is better. Some insist on 12 ft. I prefer to keep my trees in my own yard and if you plant too close to a fence it will be difficult to do. I agree with Ike on his size forecast.

BTW: C35 does fine on my clay in Danville, but it performs far better in loamier soil as in Napa.

Oranges are generally, regardless of rootstock, much more tolerant of alkaline conditions and hard water than other citrus like mandarins and especially lemons. I have no experience with C35 but if it is reasonably tolerant the tree should do fine. if you intend to water with drip then flushing the root area every month or so will be a good idea.

C35 is a great rootstock, most of my sweet citrus is on it. Your tree should do well in the OC even with hard water, you are in citrus country there. Keeping your tree trimmed to 8’-10’ should not be a problem. Here is a picture of my Cara Cara on C35, it’s been in ground for about 4 or 5 years and it’s planted about 5’ from the wall. The wall behind it is 6’ tall for height reference.

Evan