Astragalus glycyphyllos and friends

Astragalus glycyphyllos is a great nitrogen-fixing groundover but not a perfect one. It regrows from a crown each spring, leaving a wide open space around it until June. In mid-summer the reverse happens, with a open space at the crown.

To resolve this, I have tried planting several species over the crown. So far best success is anise hyssop (Agastache foeniculum) which is at full size mid-summer (note - this anise hyssop was dead by August 2009). Wild chervil (Anthriscus sylvestris) seems to come up too early, may shade A. glycyphyllos too much in spring.

Anyone have ideas for species to grow in this crown area? My current thinking is to overlap another astragalus, planted about 3' apart, so that each covers the other's crown. I put in a patch of this in spring 2009, will report.

I planted ramps around the outer "donut ring" that does not recieve spring coverage in fall 2008, and this at least seems to be working very well. In spring 2009 the ramps did emerge before astragalus and had some time to grow, but I think they should have been a bit farther from the crown.

We also have violets (species unknown) doing very nicely in another area of the garden growing about the same distance from Astragalus crowns. They are covered and in full shade for most of the year but seem to be spreading all the same. Like ramps this species only needs a little bit of sun in summer to survive, but unlike ramps their leaves persist until fall.

Here they are in April. You can see the whitish dead stalks from the previous year's astragalus coming out of the crown.

Here are the same plants about a month later. The astragalus is about to swamp the ramps. I think the ramps should be a little farther out for best growth, maybe 30-40" from the crown. This astragalus grow about 9' across in 2009. But you can imagine that a ring of ramps all around the astragalus could be a good use of space.

This polyculture works well under taller shrubs, but A. glycyphyllos will sprawl over anything under 18-24".