PH will be lowered in the soil. irrigation will be provided.

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what do you mean pH will be lowered ...

most trees dont need irrigation.. after proper watering after transplanting.. to get fully established ... irrigation can be used for such if the zone is dedicated to water trees properly ... but again.... once established the should not need ongoing irrigation .. i mention this is you are designing a zone just for trees ... or thinking about adapting a zone that has plants with wildly divergent water needs as compared to a tree ...

ken

ken?

most trees dont need irrigation.. after proper watering after transplanting.. to get fully established .

Asian or Eastern American Acers certainly do need irrigation in dry-Mediterranean climate southern California. As would all of the various commonly planted "summer monsoon" "summer rainfall peak" subtropical climate plants like Hymenosporum flavum. Although Acers as a whole are tough and I can imagine some of them subsisting in shaded swales of cool, coastal microclimates in southern California...that likely doesn't apply here.

I guess what the OP is trying to say is they will not be watered with straight-from-the-tap LA Basin water, which is very alkaline. Or some special soil preparation will buffer that alkalinity. In that case, I suppose any of those could be grown. (and many of them are certainly grown without special water or soil adjustments, I would think, in the western US more generally) IIRC we've seen pictures of eastern US Acers planted in SoCal. That being said, I would wonder if more northern selections of something like sugar maple would languish due to insufficient winter chilling. That would be the issue, not soil pH.

Here is a discussion about A. palmatum, but applicable here too: https://www.houzz.com/discussions/2020244/maple-in-coastal-southern-california

good enough.. i surely dont know the vagaries of CA gardening ... and if his list includes your list.. id roll with your info ...

but the point i was trying to make .. apparently not successfully .. lol ... was that your irrigation plan has to take into account what plants are on or in a zone ...

to water a tree properly... you need to water deeply and infrequently ... to get water to the feeder roots what suck it up ... but if you water in a manner of just enough to moisten the top few inches.. which many perennials and annuals can enjoy ... you may not be watering properly for a tree ... and especially for a transplant of any depth ..... if that makes any more sense than my prior attempt ...

we just dont know how and if OP understands irrigation ... and more specifically.. tree irrigation ....

ken

Whats the best way to acidify soil?

Depending on what your current soil pH is, acidifying further may be unnecessary. Most maple species will adapt to a rather broad range of pH, including those that tend towards alkalinity. A. truncatum is likely most adaptable to a high pH.....8.4.

It is also important to understand that acidifying the soil is not a once and done situation. Soils will return to their indigenous pH over time so repeated attempts to acidify will be required. And that is not easily accomplished wrt to established or existing plantings. If the soil pH is too far off from the preferred range, it is far better to select a plant that will tolerate the existing conditions.

Agricultural sulfur is the best method for acidifying soil, although it takes time to affect a change and will need to be reapplied on a routine basis to maintain the desired level.

As david noted, I'd be more concerned about the pH of the irrigation water, however that is relatively easy to alter.

Soil is relatively clayey

It depends upon why you are choosing Acers and where you are located in the OC. If you want the maples for autumn color, and you do not live in an area that produces sufficient chill to reliably produce it, there are better trees to grow.

not autumn color necessarily. but after a bit of research acer x freemanii seems like the best bet

what is the best way to plant a tree in clay soils like in the picture above?

Not expressing a particularly strong opinion here, but take a look at descriptions and tolerances of a species like the Caddo Maple - Acer saccharum - from Oklahoma and other provenances far removed from north and eastern North America. This line of plants is much well suited to drier and higher pH soil conditions, but may be a bit difficult to track down in California.

Thats what i was looking at today, there are some flashfire maples for sale online, but they are quite expensive

I got a mixed bunch of maples when orchard supply hardware closed. I bought all the 1 gallon trees which they didn't water anymore and marked down 75 percent. My experience has been that they do need extra water, they prefer raised beds in a clay soil which mine is. They did very well in pots even with our alkaline water near Disneyland.

I use part sifted soil, part peat moss, part sand, part pumice, part coarse perlite, good handfuls of charcoal (not ashes) from my fireplace and I mulched with nitrolyzed fir and redwood compost. A few times a year, I flood the soil to flush out excess salts.

Every year a heatwave will burn all the leaf tips in late summer or autumn so that there is basically no fall show but the little trees are still very pretty spring and summer. I love my trees and think they are worth the extra effort. If you spend the time to train them, they can be magnificent, even in winter when they have no leaves. I would try for morning sun as a preferred location. I tried late day sun but even then with warm air Temps, delicate leaves can burn. Mid day sun is too hot unless you have a lath or other plants making shade.

Wherever camellias do well is your best bet. I'd get a reasonably priced 5 gallon camellia. Try it for a year in the spot and then decide.

If this is a new home, compaction of the soil may be a problem. You must have good well draining soil or a raised bed. If you cannot dig down two feet easily with a hand shovel you will need to fix that before considering these trees.

What species if maple are they?