Before converting a long hillside in my backyard to native plants, I excitedly researched and ultimately followed the main stream advice I was given… even though it didn’t feel right. I then believed that you should not have to amend or add to your existing soil when planting natives. Instead, plant species that are suited for your site. So I set about planting natives that were noted for performing well in clay, given they have drainage. In my case, this meant a slope, as water would flow more quickly down hill. I planted clay tolerant varieties of manzanita, ceanothus, philadelphus, rhamnus (coffeeberry), verbena, and redbud. In the end, my results were about 20% loss, 60% struggle, and 20% so-so. Obviously, this wasn’t what I was hoping for.
After watching my plants sit and struggle for about a year and a half I decided to test the drainage theory. I took two coastal buckwheats and two deer grass plants as my subjects. I planted one of each in the existing subsoil and on raised mounds about 6″ high. The results were incredible. The deer grass and buckwheat that were raised, more than doubled in size within the season. The others barely grew at all. Since then, I have been experimenting with various mixes of gravels, soil, compost, and my native clay. I rarely see plants struggle now that I take drainage seriously. I also remain smart about which plants I choose, always choosing plants that can tolerate clay, because ultimately, that is what they are getting. So, here follows my advice for succeeding the first time around with plants that need good drainage.
If your soil drains well, and it has some good topsoil (like maybe you are converting a lawn that was previously doing OK) then you might not need to do much. Especially if you plan to sheet mulch it (layer cardboard, compost, and mulch on top to smother the lawn). You will be providing drainage and enough nutrient to last a long, long time.
Here’s a quick way to check your soil for adequate drainage. Dig a hole about 12″ deep by 12″ wide, fill it with water and see how long it takes to drain out. If it drains completely in 2 hours you don’t have anything to worry about. If it is there for most of the day or longer then you should do something about it. And don’t listen to anyone who tells you what to do without asking you what your soil is like. My situation was a pure subsoil that had been driven over by all things machine, blown clear of its topsoil, and nutritionally depleted by tree and shrub roots. Even weeds had trouble growing there. So….What is your soil like? (here I will be referring to my specific situation, yours may be completely different)
If your soil drains very poorly to poorly, like mine, I recommend you amend it to get good results. First, break it up with a pick axe, an auger, or a rototiller and add compost and pumice. It is important to treat the whole area where you are going to plant and aim for a working depth of 12 – 18″ deep, but sometimes this isn’t possible or practical. The deeper you go the better the results you will get (especially for large shrubs and trees) but I have had good success roughly pick axing down 6″ and mounding above my clay soil with 6″ of a faster draining mix. I recommend to my clients a blend of soil that is 25% lava rock, pumice, or light gravel, 25% compost, and 50% loam or topsoil. This aerated mix will allow oxygen to exist around the root zone which the roots need to respire. Lots of natives have trouble with Phytophthera root rot and amending for drainage in the upper portion of the root zone is a great way to prevent it from getting ahold of your plants. Add a good organic mulch (like wood chips from a tree service) and the organisms will do more even more aeration work for you.
Growing natives in residential yards is a relatively new thing, having these plants commercially available is also relatively new. Houses are built on compacted lots in urban and suburban areas and we don’t have undisturbed, living, native soil. We have sites that have been plowed clear of the topsoil when the foundations are dug, and rode over countless times with bulldozers and tractors. The native vegetation and organisms have been removed, the soil ecosystem is destroyed, oxygen is removed from the soil, and the critical topsoil has been removed. If someone wants to grow native plants, this type of situation should be viewed more like a reclamation effort rather than a weekend gardening project. Compacted, flat clay is one of the worst soils for growing many drought tolerant plants and CA natives. But, if you amend for drainage you will open your options up ten fold with plant selections. If you can’t dig down much it’s still OK. Mound up at least 6 inches with a draining soil and mulch well. Organisms will do the rest, it just takes time. But in the meantime, your plants’ crowns will be in a soil that drains well and that will help considerably.