The second version of your asparagus question on sustainability.SE specifically asks about reducing water requirements:
I have been looking into applying this with asparagus, to reduce water usage involved in growing this food, but have been unable to find recommendations for plants to plant alongside (and given the spacing between plants, this leads to a lot of exposed soil). I would like to grow asparagus with as little external water input as possible. What can I plant next to asparagus to help cut down on the water loss, weeding requirements, and fertilizer requirements? I.e. how can I grow asparagus sustainably?
If your aim is reduce water usage, then the thing that immediately comes to mind is a thick organic mulch for both suppressing weeds and retaining moisture. Some (free!) mulches that would be suitable for a permaculture garden:
- autumn leaves
- don't just chop & drop your cover crops that are grown elsewhere, drag some over to the asparagus
- arborist wood chips
(Note that I do not have an asparagus guild, but I will lay out how I would go about assembling one. Conventional wisdom is that you want to keep all that soil around your asparagus bare, and while I find a couple of mentions of asparagus in permaculture guilds, I didn't see any lists of plants.)
In the table for Asparagus on p. 182 of Ed Smith's "The Vegetable Gardener's Bible", it mentions basil, calendula, parsley, and tomato as good companions for asparagus. The text, however, mentions that while tomatoes are commonly recommended as companions (see wikipedia for example), both asparagus and tomatoes are fairly hungry plants, and you don't the tomatoes to steal nutrients from your asparagus.
So using that principle, you wouldn't want to plant anything near it that is going to use up a lot of nutrients.
Another hurdle to finding good plants to go along with asparagus is that you want something that will actually grow beside the asparagus. Basil is another suggestion in the list above, but basil likes full sun and the asparagus can get quite tall, casting shade on the basil. So basil is questionable, but since it's fairly easy to grow, and it's just an annual, I think it would be worth trying as an experiment.
In Barbara Damrosch's "The Garden Primer" on p. 270, she suggests that during the first year of growth, the asparagus won't grow too high and you could plant bush beans between the rows. This seems like a good choice: nitrogen-fixing beans would help the soil, and a dense planting of beans will shade the soil quite well. In later years, she suggests early crops like radish or lettuce. Radish seems like a good choice -- light nutrient usage, and they're out of the ground before the asparagus gets very tall. Lettuce seems like it will use more nutrients, but if you aren't using it as the only companion and therefore planting a ton of them then it sounds like a reasonable guild member. (For that matter, other "baby" salad greens might be reasonable as long as you harvest them while they're small and don't plant so densely that they suck up all of your nitrogen and other nutrients: kale, spinach, chard.) If you end up with substantial plantings that you're harvesting, definitely consider a generous top-dressing of compost in the spring; this is challenging if you've got a couple inches of wood chips on the ground, so plan accordingly.
Note that I'm assuming that asparagus is the apex of your guild -- it wants full sun, so I don't think you want anything to overshadow it. With that said, I suppose it would be possible to plant something taller on the north side; perhaps a small fruit tree that won't spread so far as to overshadow the bed but would provide a wind break and some extra shade for the soil on the north side. A deciduous tree would also provide leaves that you can use for mulch (i.e. providing multiple yields & stacking functions). The risk here, of course, is that a tree is rather permanent -- the other plants suggested are more temporary and thus very suitable as experimental guild members. Although now that I write that, I suppose a tree has a several decade long life span but the asparagus bed will only be around for about 15 years so you could replace the asparagus with something else if it didn't provide good results. (A "successful" experiment as long as you learn something, right? Be sure to share what you learn!)
Finally, at the bottom layer you want a low-growing, nitrogen-fixing ground cover. I'm a big fan of white clover for this in other settings. The bees love it when it flowers, it tolerates light traffic (so you can plant it in lightly-used paths), a thick planting it the soil and crowds out weeds, and if it starts to get too tall you can mow it and leave the clippings in place.
Note that I see rye mentioned as a winter cover crop in conventional asparagus growing, but it seems like a terrible idea in a permaculture guild around perennials like asparagus. I love rye in my annual garden because it is very hardy, scavenges a bunch of nitrogen that might otherwise be lost, and can put on a good flush of growth in the spring; however it is hard to kill, often requiring multiple passes with the rototiller (the linked article mentions glyphosate) -- not possible with the asparagus.
If you were going to use an annual winter cover in your asparagus, you'd want something that is guaranteed to winter-kill. If you do this, make sure you leave the residue in place to compost back into the soil -- you don't want to be removing nutrients from the asparagus bed by way of cover crops!
- mulch with straw, leaves, chips, etc.
- experiment with basil
- experiment with quick spring crops like radish or baby salad greens
- use a ground cover like white clover
- if you're feeling adventurous, try a fruit tree on the north side of the bed