This may be an old observation but I'm struck by what I'm thinking. I purchased two old and ornate large concrete planters. It took two pretty strong people (fine--they were men) to hoist them into my car and then out onto my front patio even AFTER I emptied them of dirt to stick my own mix in there.

I planted them earlier this week and this morning I was looking out the front window and realizing I had planted them too deep into the planter. I'm used to regular plants that grow taller and so left the dirt a few inches below the rim of the planters (which are shaped like squat urns and have fluted tops (I will post a photo later). The semps are invisible for the most part although a robust Little Missy sedum plant is showing itself.

Do I need to bring the dirt level right up to the top of the planter to plant semps and sedums? And THEN add the grit layer to protect them more?

This happened with me last year, Rita, with the first trough I planted in. When I plant future ones, I will fill it higher with my soil mix and tamp it down and add more since it will settle a bit. I remember watching a video of a person making a semp planter and they even suggested it being slightly mounded which makes a lot of sense.

Another idea you might consider with large pots is to not fill them completely with soil. I've seen others here suggest putting plastic milk cartons in them first and then adding soil to the top since you don't need 12"+ inches of soil for semps and sedum. Makes the planters much lighter and more movable. I want to do some large galvanized tubs and this is going to be my plan..

JesseInCT said: Another idea you might consider with large pots is to not fill them completely with soil. I've seen others here suggest putting plastic milk cartons in them first and then adding soil to the top since you don't need 12"+ inches of soil for semps and sedum. Makes the planters much lighter and more movable.

Those cement planters are going NOWHERE because they are probably more than 120 pounds a piece. I made them heavier still by adding back the sharp black rocks the previous owners had in the bottom! Milk jugs sound much easier and a way to keep plastic out of the landfill!

Just how deep does the soil need to be? I put probably three times as much soil in those planters than is probably necessary.

I also completely destroyed a planter trying to drill a hole in it before successfully drilling three others. My first reaction was dismay followed closely by, "hey--crevice planter!".

I'll let someone more experienced than me tell us how deep the soil should be for semps and sedum. I would imagine not too deep; I have semps that are happily thriving stuck between small nooks in my retaining wall. I'm far from an expert on these plants though so I will wait for Lynn or one of the others to chime in your question.

Soil depth can very according to what the soil is composed of and how often you will be watering it. So, it can be anywhere from say 2" and up to the depth of my raised beds which are about 16" deep.
There are semps planted in tiny pinch pots/hypertufa with maybe an inch of soil. Of course these will need more attention to watering in the dry season. I find that soil is wasted for semps in big deep pot, so really like the idea of adding light weight items to the pot before adding soil. Crumpled bubble wrap, milk jugs, etc, all work well.
This is also a time to think about making your potting mix fast draining. If you are using large containers and filling them with potting soil, they make retain way to much moisture long term. There just won't be enough roots to take up the moisture down deeper in the big pots.
Maybe some one can add more to this?

I can't add much more to the advice of keeping the soil mix fast draining by adding some sort of perlite, pumice, chx grit, larger grit sand/gravel, crushed rock, etc.
I use 2 and four inch pots for planting of my semp cultivars but when I use larger pots for doing container plantings, I keep the soil depth more shallow by placing, at bottom of pot, plastic containers as close as possible to each other (sort of like fitting puzzle pieces close together) to avoid large air pockets where the soil may sink thru. I fill any air pockets with smaller plastic containers or large piced pumice/ gravel/rock. Earlier I used to pack Styrofoam peanuts into these store bought net bags that oranges/tangerines come in and put these tied bags at the bottom of large pots. After filling the pot's bottom, I put window screening or weed barrier cloth between the plastic layer and soil layer to help prevent soil from falling further down the pot.

You do have to be careful with the packing peanuts, some of them disintegrate on contact with water.