Friday, April 6, 2012

Avoid Creating Geometric Shapes in Arid Plants

Arid plants are not supposed to look like boxes, spheres, pyramids, or aircraft carriers. Pruning to create a novelty topiary like one might find at an amusement park is tedious work requiring lots of time and dedication. It is best left to professionals working with compatible plants such as boxwood or myrtle. These plants must be sheared frequently to maintain those exotic shapes.

Most of the plants that we call arid adaptive flower profusely. The main reason to refrain from shearing them is that many of the flowers are inadvertently pruned off in the process. As a result the bloom potential is greatly reduced. Secondly, these plants have not evolved to maintain large amounts of foliage. Shearing them produces lots of foliage thereby reducing their water efficient nature. This includes Texas Ranger (Leucophyllum), Cassia (Senna), Creosote (Larrea), and many others.

Texas Rangers In Unnatural Shapes

Why do landscape crews shear arid adapted plants?  Well, it is mostly because they have been taught to do so by prior landscapers. There was a notion that the more you pruned off and hauled away the better as the property manager could visually see a change in appearance.  Those who know better certainly notice all right. It's also very easy to teach others how to do it. Mindless work that fills hours on the time cards that result in large labor cost passed on to the customer.

In the long run the landscape winds up looking like any other heavily trimmed property, with fewer flowers, a large load of foliage going to the landfill, and a water-thirsty collection of shrubs that must now regrow everything that was pruned off. This doesn't sound very eco-friendly does it?

Pruning desert-adapted shrubs to maintain a natural shape and to improve flowering is easy. First, if the shrub has been routinely sheared to create a geometric form then it should be pruned hard to eliminate all the unnatural top growth. Heavy trimming of this type is called rejuvenation pruning. The shrub is simply cut down to within 4 to 6 inches above ground level. The new growth that emerges may be trained to create a more natural shaped shrub.
Timing is critical to succeed with radical procedures such as this. Rejuvenation pruning should be done during the winter for summer flowering plants (Texas rangers, crapemyrtle) and after the bloom cycle for spring flowering plants (Cassia, brittle bush, autumn sage). Once the new growth fills out you may conduct maintenance pruning in one of two methods: the two-step naturalistic or the one-third per year method.

The two-step naturalistic method was created by the late Eric Johnson from Palm Desert. His method called for the trimming of the heaviest stems each year. These are cut back by one third of their length preferably to a point where an inner branch may then take the lead. Once the largest stems are cut in this manner then all the remaining branches are lightly pruned removing several inches of foliage in a variable manner. This does not equate to shearing and the shape should be informal in appearance. Repeat annually at the appropriate time.
The other method is the one-third per year method. This pruning eliminates the heaviest stems all the way down to ground level. New stems will emerge near this cut replacing the older stems. The process is repeated yearly at the appropriate time. By the end of the third year all of the above-ground stems are less than three years old ensuring that all of the new foliage produces bountiful flowers.
Informal pruning practices such as those described will produce a bounty of flowers, sustainable new growth, and a more water efficient plant. In return the maintenance effort is greatly reduced allowing more time for irrigation management and weed control. Let's not forget that less is hauled to the landfill thereby reducing the burden of disposal. An lastly, most of the work can be done without the use of power tools. Imagine a landscape crew that does not have to use hedge shears and blowers powered by gasoline engines. You could actually hear the birds singing while they work! What a novel concept!
Beautifully Shaped Cassia in Full Bloom

Monday, March 21, 2011

Soil Analyses

Soil Analysis and Fertility

Horticulturists routinely provide soil analyses on behalf of contractors, landscape architects and homeowners. Personally I think the process is invaluable especially for new projects. It allows the client an opportunity to really make a difference in the final product. As they say, “you only get one chance to make a good first impression”. The same could be said for landscape projects: you only get one really good opportunity to alter the soil pH prior to planting. Even with prior planning the impacts are often minute.

Soil tests are also helpful in determining the basis for plant decline, poor growth and vigor, or inferior yields on fruit and nut trees. In fact, many of the disorders that we encounter in the field are related to less than satisfactory soil conditions, and soil testing can assist you during the site examinations. I would venture to say that over 50% of the trouble calls we encounter are soil related.

Of primary importance is the selection of a quality laboratory to perform the analysis. We often are asked to interpret the reports of a variety of labs and the data presented in the reports can be confusing and quite variable. It is critical that the lab work is conducted by a facility that is geared to perform agricultural analyses. Do-it-all labs may not have the expertise to conduct the tests in a meaningful and reliable manner for agronomic purposes. 

The quality of the soil sample is also quite critical. Occasionally we receive requests to pull a single soil sample for a 10 acre parcel. One sample would hardly be representative of the entire site, so decisions must be made as where landscaping will be placed as opposed to the sites for parking or structures. This can help to pinpoint the best locations for sampling. Take enough samples to make the analysis worthwhile.

Soils that are imported or stockpiled on site are often analyzed for suitability. Here too the size and method of the sample collection is the key to the accuracy of the analysis. Imagine pulling one grape off a single vine in a 100 acre field to determine sweetness for the entire crop. It would be similar to taking one fruit jar of soil from a 100 cubic yard pile for submittal to the lab. This is why composite sampling is important to ensure that the makeup of the sample is representative of the stockpile.

Soil fertility analyses can provide a lot of detailed information, such as pH, percent organic matter, nutrient holding capacity, the actual fertility levels for each element requested (such as nitrogen, phosphorus, and potassium). Even undesirable components such as soil salinity and excessive levels of micronutrients (such as boron) can be determined. Both salinity and nutrient toxicity can be more difficult to mitigate than nutrient deficiencies.

If you desire the lab can also run additional tests to determine texture (percentage of sand, silt and clay), bulk density, and water infiltration rate to name a few. These and other test procedures can assist the designer or contractor in the determination of best management practices for the soils prior to and after construction.

Once the lab results are in then you need to analyze the results. This requires a lot of familiarity with soil chemistry. Often the soil lab will provide generic recommendations for modifying the soil. For the inexperienced this can be very helpful. But to really interpret the reports you should consider having someone knowledgeable about the soils for the region go over the numbers. Agronomists and experienced horticulturists will be able to provide you detailed information regarding the cultural practices and products to help remedy the most difficult soils.

It is often said that “prescription without diagnosis is malpractice”, and making determinations about fertility without soil testing is pretty similar. If we do not know what the specific needs are for a particular plant type or soil then any recommendations regarding the addition of nutritional supplements would be made without sound judgment. So, the next time you encounter an odd plant disorder or embark on a new landscape project be sure to include a soil analysis in the package. Clients may not appreciate the science involved, but in the long run it could make a huge difference.