Last year I abandoned planting annuals in the bottom two levels, and opted for the tough Autumn Ferns (Dryopteris erythrosora) that have done well in other parts of the garden. Then the Summer From Hell arrived, and coupled with a fallen tree that allowed additional late day sunshine to reach the bed, the ferns were toast (though the oak sprouts were still plentiful...grumble).
Thus I decided to go with more drought and heat tolerant specimens - ones that can still take some scattered shade for much of the day. The first step was to dig out all the existing soil so it could be replaced with one more suited for xeriscape specimens.
This first step quickly turned into more work than anticipated. The soil was so ridden with roots and oak sprouts that a regular shovel couldn't be used (it kept getting tangled and caught so that all the captured soil would spill out or the blade couldn't be lifted). So I ended up using a trowel and my hands - a scoop at a time, the wheelbarrow loads rolled out.
The roots and sprouts were snipped and pulled until bedrock was reached. Of course, there were also massive roots that simply had to be detoured around.
And irrigation pipes and television cables to be dodged and carefully left unbroken and uncut.
Then, in hopes of eliminating (or at least re-directing) the oak sprout return, the bottom was lined with weed barrier cloth (there should be plenty of soil above the fabric for drought-tolerant plants - I hope!). Weaving this under the existing rock walls required some careful juggling.
Then the beds were refilled with two pickup truck loads of Thunder Dirt from Geo Growers.
Next, the excess cloth was trimmed away from the rock borders.
And finally, plants that had been patiently waiting for over a month were placed into the beds. The middle tier received a Twin-flowered Agave (Agave geminiflora) and three Calamar Squid Agaves (Agave bracteosa 'Calamar'). I hope to eventually add some Soap Aloes (Aloe maculata) between them. The lower tier received three Agave 'Blue Glow'. Plans still include several 'Cherry Coke' Dyckias and, as time goes on, succulents will likely be added to help fill some of the empty spaces.
Most of the plants should eventually double in size. So this is the new revised Three Tiers Bed.