- Author: Karrie Reid
I attended an informative conference in October that reported on the latest research results on tree root development in urban soils- the Landscape Below Ground. This was the fourth conference on this subject in the last 25 years. Much of the research seemed to confirm findings I had previously read and which have mostly been incorporated into recommendations and practice, at least in the state of California. However, it is always good to have research repeated and confirmed in a variety of scenarios and species, and there were some new and interesting findings as well. I am going to recap some of both in the next couple of posts.
Pre-plant Tree Inspection
ladder root system
The health of trees initially, especially the structure of their root system, has a significant impact on the survival rate and growth of trees after planting. Fall is an excellent time for planting trees, so pay attention to this if you are installing this season. The most important aspect of the root system (which you should inspect before planting) can be summed up in this phrase: straight roots, some of them near the surface. What trees need, and what you want is a "ladder" of roots, all of them straight or bending less than 45°. You don't want them all going straight down (diving), or the tree cannot stabilize itself in the ground. For all aspects of a healthy nursery tree, see this: Guidelines for Nursery Tree Quality.
At the nursery, a good root system can be accomplished best by one of two methods: potting up to the next size in a timely manner and shaving all roots to the point where they hit the edge at the time of potting up. This eliminates the bends before they set up in the next size. The second method is to use something called an air pot, which has perforations throughout the surface. When roots reach air, they die at the tip rather than continue to grow in a circular manner around the pot. Demand good stock and give your business to nurseries that provide it. The guidelines for growing quality trees can be found here: Quality Tree Production Strategies.
Soil Preparation
Whenever possible, rip or deep till and incorporate up to 25% organic matter to add carbon and air space for optimal root growth (more than 25% is not better). Shallow tillage has only very brief effects on trees, since roots ideally grow down past the initial 12" rather quickly. Roots grow where they can. Sounds obvious, but without oxygen and carbon dioxide exchange space, without water penetration, roots will not grow out into surrounding soil. Minimum root system means minimum canopy. If tilling cannot be done because of existing trees or other infrastructure, you can add some pore space by deep drilling holes 2' deep and 2" wide and filling with small gravel or biochar, which will maintain the space for oxygen and drainage. Additionally, sufficient space for tree roots should be planned in advance. If pavements must be installed over tree roots' eventual growth area, structural soils can significantly enhance the trees' ability to support healthy growth. To read about a project at UC Davis: UCD Parking Lot Structural Soil. For a complete guide to the first structural soil created by Cornell University researchers and to read research updates (scroll halfway down page): Urban Horticulture Institute.
Mulch
Do it. Time and again research confirms the benefits of organic mulches for improvement of soil structure, water infiltration, moisture retention, temperature buffering for root health, and the health of beneficial soil microbes. The results are still out on whether the source of the mulch makes a difference, and research is ongoing to find out if mimicking the native soil litter for a particular species is more beneficial than non-specific mulch from other species.
Next week we'll discuss Fertilizing, Disease, and Tree Stability.
- Author: Karrie Reid
Using a Landscape Coefficient
THE LANDSCAPE COEFFICIENT METHOD basically states that once you've done all your other calculations for irrigation application, such as measuring your system application rate and distribution uniformity, your plant water needs using WUCOLS (http://ucanr.edu/sites/WUCOLS/), and the current plant water needs given your local weather (reference evapotranspiration or ETo {https://cimis.water.ca.gov/}), you then should take into account the density of the planting.
The bottom line here is that even low-water use landscapes will need more water as they mature, and as long as you don't irrigate all year to the July level you can still be responsible and conservative with your water use. If anyone complains, be sure to remind them of the many ecosystem services the landscape provides for these urban heat islands in July!
- Author: Karrie Reid
Tending Your Trees, Part 2
Trees need water. No surprise there. Even after a good rain year in 2016-17, the stress of multiple drought years can still be seen in the larger landscape trees in any Central Valley town or city. Once they have lost roots to repeated dry seasons, the loss to a tree's canopy may not be recoverable, since they don't have sufficient living roots to take advantage of the available water in subsequent years. This may be especially true for the species with a higher water requirement, but even native trees in natural landscapes suffer when droughts drag on an on.
Although most landscape plants can be replaced with relative ease and little expense, trees take years to reach maturity and that represents a much bigger loss of time and money if lost to poor care. For this reason, most horticulturists I talk to recommend catering irrigation to trees during restricted-water years as well as being sure they are watered properly during non-restricted years. So, how do you do that?
To make sure you are preserving your trees' health, scope out their irrigation zone. Most tree roots extend far beyond 
If you get runoff before you've irrigated deeply enough, use the cycle-and-soak method of application until you've reached your target. How often will depend on your soil type. If you have really sandy soil, twice a month in the summer should be sufficient to keep trees healthy; clay soil will only need a monthly deep soak for well-established trees.
Now, I have something scandalous to tell you. Trees and lawn don't mix. What?! Horticulturally speaking, their requirements are quite different, but in the real world they are planted together all the time. Tree roots spread deep and wide while lawn roots stay in the top 6-12 inches of soil. Trees almost always require no additional fertilization, but lawn will need a regular regimen of nitrogen, at the very least, to keep a good, dense stand of turf. Whether you are letting the lawn die to replace it, putting it on restricted water, or keeping it healthy, an additional deep-soaking of trees on a monthly basis will ensure that their roots go down deep and their canopy stays healthy. To put together a simple, low-cost irrigation system to supplement tree water, see this link: TRIC: tree ring irrigation contraption .
- Author: Karrie Reid
Tending Your Trees, Part 1
Trees in the urban landscape contribute an enormous number of benefits to the environment and the well-being and even prosperity of the urban inhabitants in the shadow of their canopies1. They take longer to reach maturity than any other component in the landscape, and should be given special care to make sure they reach the stage where they can contribute these benefits. This care should include proper planting, irrigation, and pruning, especially when they are young. Training young trees to have strong structure can have significant impacts on their health and longevity.
Even though the tops of deciduous trees may still be resting, the roots underground are waiting for warmth to send signals that initiate bud break in spring. Studies have shown that cuts made in spring have the greatest amount of wound healing in the first year, making March an excellent time to prune. The overall structure of the tree is still visible, and cuts should have excellent healing potential. If spring pruning flowering trees, wait until after bloom to avoid cutting flower buds. (Autumn pruning has the slowest healing rate, and should used only for trees whose size you are attempting to control more than usual.)
Healthy, strong landscape trees should be trained to have a single, central leader with side branches arranged radially around the main trunk and spaced 18-24" along the trunk. Favor branches that are less than 50% of the diameter of the main trunk that make a 45-degree or greater angle with the trunk, as these will be the strongest. Remove any branches with very narrow angles of connection to trunk.
If you would like to learn more about how landscape trees and shrubs grow and how to prune successfully, take Module 3 of the Green Gardener Qualification Training, "Plants in the California Landscape", which includes an evening class on Trees & Shrubs: Growth, Planting and Pruning", with a half-day Saturday pruning workshop. (Direct registration link here.)
1. Benefits of Urban Trees, 2018. FAO. http://www.fao.org/resources/infographics/infographics-details/en/c/411348/
- Author: Karrie Reid
Roses!
The word "rose" conjures up different images and feelings for different people. As landscape features they have long had a reputation for being garden princesses that require ample water, careful fertilization, and fungicide and insecticide treatment. Additionally, most people think they need to be pruned in very specific ways ("prune to a leaf of 5 leaflets"). But roses come in a large number of classes and species, and hybridization over the years has produced some really remarkably sturdy roses with none of these requirements to bloom and look great over a long season.
In the UC Landscape Plant Irrigation Trials (https://www.facebook.com/UCLPIT/), we have evaluated the water use and performance of 17 different landscape roses in hot, dry Davis. We have discovered that most of them perform beautifully on the low end of moderate water use, or about half of what a cool-season turfgrass, like tall fescue, needs. In our trials, the plants receive no fertilizer, no pesticides of any kind, and no special pruning techniques. During the late winter, we simply chop them down to about knee level! We actually learned this method from the director of the International Rose Test Garden in Portland, Oregon. The low-growing carpet roses get a cut back with electric hedge trimmers.
The secret to their performance on this irrigation level is the establishment year. We plant from bareroot stock in late winter, and water them through the first summer on regular, deep irrigation with a ring of drip irrigation tubing in the root zone of each plant- about once a week to a depth of 18 inches. We also use 2-3 inches of chipped wood mulch. The second summer they are good to go on only monthly irrigation with 14 gallons of water per plant, or about 3". If you are working with sandy soil, you would need to irrigate more frequently with less water.
New Rose Trial
Beginning in 2018, we will be evaluating new rose introductions as part of the American Rose Trials for Sustainability. These trials will be located in Stockton at the Robert J. Cabral Agricultural Center, and will trial 20 new cultivars each year. Unused turf areas will be converted this next week to rose trial grounds, and I will post pictures of the conversion as it progresses. One of the selling features of converting the ornamental, but useless turf sections was the calculated water savings: 3,656 ft2 of turf at 65% distribution uniformity would use 103,000 gallons of water, while 60 roses in the same area on drip will use 6,175 gallons, a 94% savings! Even though this is a trial layout, and not a densely planted landscape, that's still a great selling feature for converting this area to trials.
We look forward to finding more great landscape roses for sustainable California gardens.
