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Tuesday, June 2, 2015

Air Pruning Will Work in the Hot Desert with Some Precautions

Q. I just read about air pruning of roots in potted plants using either fabric pots or drilling holes in pots and lining with landscape fabric.  Supposedly, air pruning keeps the roots in "check" so that they do not outgrow the pot.  The idea sounds logical, but with our extreme heat and hot winds, could this work in Vegas? I know that pots kept in the sun will overheat and kill roots.  If the pot is large enough, would it only kill the outer roots, similar to the action of air pruning?
I have some large pots and as I get older, it is going to be very hard pulling the plant, cutting the roots and re-potting with fresh compost. Any thoughts would be appreciated.

A. Air pruning is a form of root pruning. There is nothing new about root pruning. Root pruning is used in growing bonsai to help keep the plants small and reduce the amount of top growth. Root pruning is used in nursery production to restrict the size of the roots and increase the survival of field grown trees that will be harvested and sold as bare root, containerized or balled and burlaped.
Machines like these are used in commercial nurseries for root pruning young trees before they are harvested as bare root or transferred into containers. Root pruning concentrates the roots close to the plant so they can be transplanted with a higher percentage of success.
You can use air pruning in our hot desert with high light intensities if you regulate the solar radiation heating the soils inside the containers during summer months.

Container plants should be repotted every 2 to 3 years as you know. You might be able to skip the part about cutting off some of the older roots but if you do not replenish about one third of the soil it will eventually be like growing a plant in sand or hydroponics. You might be able to use compost tea as a soil drench for the plant in the container. That, and the addition of a good fertilizer plus micronutrients, might get you by.
Hopefully good management of plant root systems can help prevent girdling or circling roots, a huge problem in the nursery industry and passed on to the homeowner and not discovered for many years later.

Just as you stated, air pruning is allowing tender roots to come in contact with dry air thus killing them. This is usually a greenhouse technique.

In the nursery trade we used to use copper sulfate to control plant roots. When plant roots in they did a space treated with copper sulfate, they died back from copper poisoning. Copper did not travel back inside the plant but stayed localized where it came in contact with roots. This was sometimes used in what is called pot-in=pot culture or “double potting” where a plant is grown in a container and that container put inside of another container that had a shallow layer of gravel at the bottom to prevent the two of them from lodging.
We used pot-in-pot culture when growing ornamental trees for a research project in the 1990s. The container inside of the container that was buried into the ground helped keep the container soil cool. This reduced the risk of high temperatures in the soil surrounding the roots and root damage.
Growing plants in black nursery containers
in the hot Mojave Desert runs the risk of
root damage due to high soil temperatures.
The outside pot was buried in the ground nearly up to its rim. The outer pot acted as a "sleeve" for the inner pot and helped prevent potted plants from blowing over in the wind. This was a big problem in the nursery trade and required many man-hours to "right" the plant after a wind.  If copper sulfate was not used on the gravel between plants or the inner pot was not twisted every couple of weeks during the growing season, the roots from the plant in the inner pot would grow through the gravel and through the second pot into the ground and the plant would be ruined.

Air pruning is similar but without the harsh chemicals. When plants are grown in black nursery containers in full sun, one half to two thirds of the root system of that plant can be killed during summer months due to high soil temperatures. Surface temperatures of black nursery containers can reach 170° F in direct sunlight in just minutes. High temperatures spread through the container soil on the sides facing the sun. Damage is worse if the soils are dry.

What to do?
           Paint nursery containers white. This helps to lower the surface temperature 6 to 10° F.
           Keep nursery containers shaded during summer months by double potting them, placing a son barrier on the outside of the container on the South and West sides
           Grow nursery plants in partial shade; 30 to 40% shade is best.
           Water nursery plants just prior to the heat of the day so that soils are moist. Moist soils do not gain heat as much or as rapidly as drier soils.

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