Latin name: Yucca spp. ("YOU-cah")
Common name: Yucca
Originally from: hot and dry parts of North America, Central America, South America, and the West Indies.
Blooms: Not very often, but when they do you'll see a big spike with whitish flowers on it.
Light: Full sun, or partial shade.
Water: Rain is enough.
Where to find in P. Garden: In a row along the wall, as well as several dotted around on the back slope by the compost heap, and one in the large middle bed.
(UPDATE: they all rooted and are growing well!) They're very tough plants, so I expect they'll take up the challenge and I'll be hacking them back with a machete before you know it. John also gave us a full-on rooted Yucca tree (top) which is very impressive and gives the other Yuccas something to aspire to!
We also have several Yucca flaccida "Garland's Gold" which is a yellow and green variegated Yucca with floppy leaves, which has a compact form - it doesn't turn into a tree like many other types. Then we have three Yucca whipplei on the steps. Yuccas are lovely, bright, tough plants - I'd love to get more species of Yucca for the garden, too. Any suggestions?
UPDATE June 2016:
Yuccas are now a staple at PG and PRG due to being impervious to drought, fire, freezing, direct nuclear strike... The Y. guatamalensis cuttings we planted on the cactus wall took off and we have since taken cuttings from them. We added several variegated Yuccas to the gardens as well and they are awesome: Y. aloifolia "Marginata" Yucca "Bright Star" Y. elephantipes "Marginata" Y. gloriosa "Variegata" and a couple others.