By Lauren Lewis
In California, where pretty much all gardening is done with water requirements front of mind, the wise approach is to aim for a garden that is dominated by perennial plants- those that live and thrive for many years, rather than a single season or year. (A lot of edibles would be the exception to this trend, but we’ll set those aside.) The reason is that perennials typically need less water as they grow and get established; their roots grow deep and wide and can reach more underground water than those of a very young perennial or an annual. In essence, our gardens aim to mimic a late successional plant community, and to do that successfully, we have to think about the soil.
Ecosystem succession is the process by which an ecosystem changes, after a disturbance, from being dominated by fast-growing, prolifically-reproducing species (among plants this means weeds) to being dominated by longer-lived species that use reproductive strategies of quality over quantity. And the soil — specifically the mycorrhizal fungi in soil that forms a symbiotic relationship with plant roots — supports this process. Recent research has shown that late successional prairie plants (i.e. perennials), grow better in more fungal soil, and also that presence of mycorrhizal fungi in the soil can suppress weed growth. In other words, mycorrhizae have the potential to accelerate the shift toward a perennial plant community in two ways: by suppressing annuals and by promoting perennials.
What this means in the garden is: protect and promote the mycorrhizae. A common approach when creating a new garden is to test the soil for nutrients, salt, minerals, etc, and then amend the soil to fix whatever problems are identified. But this approach can actually wreak havoc on the mycorrhizae and the soil structure. So instead, more-or-less accept your soil for what it is, and choose your plants based on your soil. If you plant perennials that you think have the best chance of thriving in your particular soil and location, the presence of the mycorrhizae will further support those plants.