I would make it at least 18", better 2 ft. Citrus get 85-90% of their food and water from feeder roots in the first 18" of soil.
More important, before planting, do a drainage test; fill the hole with water and see how long it takes to drain out. If the water doesn't drain out, you are essentially creating a giant pot without a drain hole and your citrus roots will quickly drown.

Make certain the soil immediately beneath the existing rootball IS NOT disturbed.
If it is, the tree will sink. More than you will estimate.

I also believe the depth of the hole should be at least 18 - 20 inches deep. As your soil is so difficult to dig, you might consider just building a 1-foot raised bed on top of the hole. Citrus are a shallow rooted tree, with 80 - 85 percent of the root system developing only in the top 20" of soil.

I have the same problem like you have. I dig 12" hole and filled it up with water and it took 2 days to drain out the water. I think I will continue dig some more hoping the soil will soften and drain out the water.

If it's any comfort, we have about 20 citrus here and there on our property. We dig holes 3' deep and amend the soil. We hit boulders a lot when we dig. Bummer! So we move the hole until we can get a clean dig.

I will say, water does dissipate in an hour. Period. An hour. The soil here is well drained. If yours isn't, then you have to dig really deep and put some rocks in there for drainage.

Clay soil sucks. Where there is a will, there is a way!

Good luck!

Suzi

Hi everyone. Thanks for your replies.

Johnmerr, Randy, Suzi, after I dug the first 10 inches, I used water to make the soil softer so I can continue my digging. I poured a 5 gal-bucket of water to the hole and it took around 24 hr to drain out the water. So, this is not good! I will take more time and dig them at least 2 ft deep.
Silica, thanks for your good idea. I will do raised beds for my trees.

i agree that you should think about doing a raised bed. Youll have better luck controlling the moisture.

Mike

Do not amend your soil. This can cause "bathtubbing" as well as sinking, as Jean has already mentioned. The current line of thought with planting trees (any tree), is to plant in your native soil. TOP with amendments, but do not amend the hole. You say your soil is compacted. What is actually more important to know is whether or not you're on clay, sandy loam, loam, sandy, DG, etc. You don't mention where you live (not in your "Zone" GW sig line as mine is, above), so it's hard to give you definitive recommendations. If your soil is clay, then it is actually better to plant your trees on mounds, to improve drainage. Draining in 24 hours is not actually all that bad, but again, it indicates to me that you may be on clay. So, planting in a raised, mound manner would be better for your trees so they don't end up sitting in a giant pool of water. It isn't really how deep the hole is. It is more importantly where the top of the soil in the pot is, in relation to the top of your soil around the hole. You don't want the tree to be planted any deeper than the top of the soil in the pot to the top of the soil in the ground, and for clay soils, you would want your tree to be above the top of your soil level on the ground. So, don't worry about how deep your hole is. Worry about what soil type you have where the tree is being planted, and where the top of the tree's potted soil depth will be in relation to your soil type, and the level of the soil at the hole site. And, clay is actually great soil to plant citrus trees in, as clay is very rich in minerals. Just need to work out the drainage issue. Raised beds or raised mounds either way, will provide you proper drainage if you're on clay.

Patty S.

Patty, I live in Redlands (San Bernardino County). I think my soil has more amount of clay than sand and silt. There are some citrus farms in my area so I think citrus grow well here.
I read some website about holes for fruit trees before I dug my holes, and most of them mention "don't amend your soil", so I planned to dig them about 15 inches deep. But my soil is so compacted and I wonder if I have to dig deeper.
Thanks for you input Patty.

Nguy

Trees naturally un-compact soil, Nguy. In fact, I read a study just a little while ago about how well trees actually do in compacted soil. I would do some research, contact your local MG group. If you're on clay (which is very possible in your area, I used to live "over the hill" from you in east Moreno Valley), so planting on a bit of a raised mound would be ideal. Or, if you want to go to the expense and effort, raised beds. TONS of citrus farms in your area, Redlands was one of the first places in S. California that citrus was commercially planted. They do very well in clay soil, thrive in fact. If you actually stop off and walk through some of those old orchards, you'll see the trees are planted in berm rows, with the irrigation troughs running between the raised row of berms, lower than the trees. So, plant in a raised fashion. Don't worry about the compaction, the tap roots will work their way down. Feeder roots stay within in the top 18 to 24", and will do just fine as well. Top with some nice amendments, and you should be just fine.

Patty S.

Nguy - Redlands (like Glendora where I live)
used to be one of the major citrus producing areas. I'm
sure the soil is amenable to citrus. At least I hope so,
I just planted my first 3 citrus yesterday.

Hi Patty & Qaguy. Sorry for my late reply since my Wi-Fi was down for the last 2 weeks. You are correct, there are a lot of citrus farms in Redlands. There are some of them within 1/2 mile from my house.
Patty, I actually stopped off at one of these citrus orchard before I dug my holes. I was wondered how deep the holes in these farms. You are right, the citrus trees in these farms are planting in berm rows, with the irrigation troughs.
I dug my holes around 2 ft now then I will do raised mounds and top with some nice amendments as your advice.
Thank you all.
Nguy

There you go :-)

Patty S.