Can you imagine a detail for, something as basic as the planting hole? Oh yes, specs for size and shape are already in the book and you most likely have real nice sticky-backs in your file right now! Let’s look at this rather ordinary garden task of preparing the planting hole as it relates to Xeriscape.

Since the beginning of time, (life did begin in a garden, you know), gardeners everywhere have developed and held on to their “own” methods of planting. Through the years, both success and failure brought us today’s common adages: “Twice as wide and half again as deep,” “One-third amendment to two-thirds native backfill,” “Loosen the root ball,” “Don’t disturb the root ball,” “Leaves up – roots down” – and on and on. In Xeriscape planting holes, the main concern is the relationship between soil, water and plant.

Size and Shape

Round holes with rough edges seem to do just fine. Avoid square bottoms and smooth sides as this creates a “bathtub” effect, especially in heavy soils. If an auger is used, it’s not a bad idea to come through and scarify the edges with a shovel or bar so the roots don’t just coil as if they were in another container. A little room around and under the root ball, (twice as wide and half again as deep) is nice, for the amended backfill or for simply removing rocks and; breaking up the soil around the new plant. It’s hard to water and maintain a plant that just barely fits into the planting hole.

To Amend or Not To Ammend

Whatever works best for common ornamental plants, works best for Xeriscape plants as well. Most garden soils are heavier (denser) than the light, loose, fast draining mix in which container plants are grown. While it is true that the roots must eventually explore native soil, they will be reluctant to do so if there is not a transition zone between the light mix of the root ball and the surrounding soil. Small Xeriscape plants (4", 1 gallon), properly chosen for the site, will require little or no amendment and a smaller transition zone than larger plants (5 gallons, 15 gallons), which may require 25 to 50 percent amendment. The mix for most boxed specimens usually contains some soil, and may closely match the native material at the site, so the blending will depend on the nature of the rootball mix and the native soil. Generally speaking, larger plants need more amendment.

Soil Mixture

A precious resource, planting holes can make or break the early establishment of the plant as it relates to soil moisture. Dig into an “old” (not freshly graded, cut, filled, compacted, or otherwise made “new”) soil sometime, even in the middle of summer. Notice the moisture, that is, cool darker color as you dig deeper? This is capillary water, attached to each soil particle as a thin film, bonded by molecular attraction. Now, realize that this “hidden” moisture, in many cases, may not be available to plants because of its strong bond to the soil particles. However, it’s value in the soil is seen when the first rain of the year or the water from our sprinkler irrigation follows this capillary film down through the tiny channels, into the pore spaces of the soil, where it does become available to the plant.

What’s all this have to do with the planting hole? Well, sometimes, in mass plantings, we see all the planting holes augered or dug long before the plants are installed. This causes the loss of precious soil moisture due to evaporation during the dry season. Not good. But, if we allow rainfall or sprinkler water to pre-moisten the hole and adjacent backfill by carefully planning the early digging, or irrigating the area and all the empty holes...very good. Of course, it’s easy enough to just dig the hole and install the plant all in the same action, too!

Decisions

Does all this make the rather mundane idea of holes sound so “high-tech” as to warrant a couple more pages of detailed plans and specs? We can’t even measure (let alone draw) capillary water. No, let’s not make it more complicated. Instead, let’s use a little “Hort-sense.” Perhaps the diagram below, first produced by the Municipal Water District of Orange County in the Xeriscape brochure (@ 1985) will be helpful in outlining the process. That initial establishment period is the most critical to the Xeriscape plant, and it all starts in the planting hole.

Mike Evans is co-owner of the Tree of Life Nursery, wholesale growers of California native, plants in San Juan Capistrano. He has served on the Xeriscape Committee in Califomia since its foundation.