Summary. I am building one bed per year. So far I have one bed that's about 18 months old, and a new one that I built this past summer. It's too early to judge, but things look encouraging. I think the soil mix I chose is working well, at least some of the bulbs in the first bed survived the summer, and the window screening around the beds is reducing the inflow of weeds (although not as much as I had hoped). I need a few more years to determine how fast the bulbs grow from seed to blooming size. That's the most important remaining test.
The details and more photos are below. I am very open to questions and suggestions. Please don't hesitate to post something in the comment section at the bottom!
Why raised beds? My main motivation was saving time. I have about 900 pots, and I'm adding another 200 or so new Moraea and Gladiolus crosses every year, plus some new species when I can find them. Even if I limit myself to one pot per cross, setting up all of those pots, and finding space for them, is a huge chore (not to mention expensive). Also, the collection is now so big that I can't repot it every three years, and the bulbs dwindle if they don't get repotted on time. If I add more pots I'll just make the situation worse.
I was desperate to find a better way to grow the bulbs. Then I saw photos of Fadjar Marta's rain lily beds in Indonesia, and was inspired (link).
A single one of my raised beds has about the same soil area as 300 pots, so if I do one new bed a year I can raise all of the year's new crosses in it, and gradually move some species over as well.
I'm hoping that the beds will need replanting less often than pots. We'll see.
The structure. I'll describe the second year's bed, since I think it's slightly improved over the first year. The bed is rectangular, 24 feet long and four feet wide (that's about 8m x 1.3m). It sits directly on the ground. The sides are pressure-treated lumber, ten inches high (a 2x4 on top of a 2x6). Quarter-inch hardware cloth was nailed to the bottom, and I also added a layer of landscape fabric under that to keep bindweed from growing up into the bed (a problem with the first year's bed).
There's a roof of shade cloth over the bed, and the sides are enclosed with vinyl window screening (you can buy it in big rolls that are five feet wide). The shade cloth and screen are stapled to the wooden frame at the top.
So far, the screen has been enough to keep rats and squirrels out of the beds. It helps that we have two feral cats.
The soil. I agonized over this. Rocky Mountain gardening guru Bob Nold argued persuasively for pure sand and gravel, but I was too cowardly. So I settled on equal parts sand, pea gravel, and planting mix, delivered by a local supply company. I got a total of three cubic yards of soil (about three cubic meters), enough to fill the bed with half a yard or so left over for other uses.
That's a lot of soil. If you make a bed like this yourself, I recommend renting a Bobcat or other front loader to move the soil. I failed to do that, and had a very nice workout with my wheelbarrow one weekend. All weekend long.
The soil is supplemented with a fairly generous supply of complete fertilizer (about one tablespoon [15 ml] of fertilizer per 2 gallons [7.5 liters] of soil).
I used redwood bender board to divide the bed into rows eight inches wide. That allows for about 300 8" x 7" planting spaces per bed. I did not put bender board between the bulbs in each row. That means they will eventually spread and mingle, but since I'm dealing mostly with hybrids I don't really mind. For species, I alternate genera in a row so I can tell them apart when dormant.
Since the window screening seems to be keeping rodents out, I have not bothered putting plastic chicken wire over the soil. But that was my original plan.
The results. So far, so good. I planted mostly seeds in the first year's bed, and the seedlings looked happy. I am waiting anxiously to see if they return strongly this year. A few of them are up already. I did put a few blooming-size bulbs in the bed, and sure enough they bloomed.
The second year's bed is just starting to sprout.
I had hoped that the window screen would keep weed seeds out of the beds, and so far that's a partial success. The number of weeds inside the bed is a lot lower than outside, but there are more weeds than I hoped. Tiny airborne seeds (like those from dandelions) seem to slip through the screen easily, and I bet they are falling through the shade cloth on the top as well. Larger seeds, like the sharp "stickers" from wild oats, poke partway through the screen and gradually work their way in. The screening has helped, but I'm going to have to do a lot more weeding than I wanted to.