I think it would work, just a hoop house only maybe smaller. You have nothing to loose to give it a shot.

I've been using a similar hoop house set-up for years over my raised bed with pvc pipe and rebar. My one suggestion is to use short pieces of rebar inserted in the cinderblock holes with the dirt to help keep them locked in place, as an unexpected windstorm could catch the hoop house. Just a little insurance. You can see the hoop house on my blog.

Here is a link that might be useful: Henbogle hoop house

Your idea should work because you are relatively frost free. If you were up North like me in an area where we have a 2 and a half to three foot frostline your block wall would need to buried down below the frost line with a layer of compacted stone as a base, and a trench drain with soil barrier cloth and crushed stone to allow drainage.

The suggestion about using rebar to pin the blocks in place is a must, otherwise the soil and water in the bed, over time, will push the blocks out of alignment. Also there would be the risk of your blocks rolling over if someone was to step on them or hit them with a mower. You might want to think about using the steel curbing used to hold patio blocks in place. It is light weight and bendable. You just dig down a few inches and pin the curbing in place, and then set the blocks next to it. Then you build your bed behind the blocks.

The idea of the hoop house/tunnel is great. Just use a short piece of rebar or pipe small enough to slid into the PVC to give it more "bite" into the soil inside the block cavities.

Good luck with your landscaping project and hoop tunnel.

Remember, you can also steal a little on the season and start a few plants inside the tunnel in the spring and extend your greens a little in the fall.

Throw rocks at me if I am getting this wrong.

Are you thinking about putting a hoophouse over the top of the gladiolus bed and leaving them in the ground?

If so, the hoophouse over the top of the bed will trigger the gladiolus to start growing again.

I say this because you said "since the foliage will be dead by then" If you then put a hoophouse over the bed the dirt will warm up, the gladiolus will think it is spring and break dormancy.

Thank you everyone for your responses & information. Mabel, I liked your hoophouse setup. Thanks for posting the link. Mokevinb, thank you for the information on reinforcing it better.

Chris -- I was under the impression that an unheated hoophouse covered in white plastic in winter was used to help regulate a more constant temperature so the plants in the pots hopefully wouldn't swing back and forth as hard between freezing periods and then our frequent Southern winter warm spells. Is this not correct? Wouldn't the opaque material reduce sunlight transmission and help them stay dormant longer providing it was ventilated during warm spells or do I have this completely backward? Do I need to rethink the hoophouse altogether or simply relocate it off of the glad bed? I'm confused as to how the pros overwinter potted shrubs in the South, or do they simply leave everything to the elements and expect some losses?

Sorry for all the questions. Sincerely, Squirrellypete

Up here Glads are a July/August plant. I have had them freeze out in the winter, so I don't know if the hoop house will cause you to have too much warmth there or not. You might try it, and see what you have happen. Glad bulbs aren't that expensive, so you can always replant, and then figure on doing a fall dig and place them in storage for the winter. Just my two cents, probably not worth even that much.
Kevin