A Hortau crew was out at Cal Poly in San Luis Obispo a few days ago for an installation and it was hard to miss the dozen or so stone fruit trees with pink blossoms already emerging on the bare branches, giving their tell-tale signs that spring is just around the corner.
Although the coastal influence on weather in the vicinity of our San Luis Obispo, Calif., headquarters tends to keep temperatures a little warmer this time of year, it’s a sure bet that bloom will be underway for growers in the Central Valley and the rest of the country before too long. If you’re a grower who tends to hold off on irrigation after harvest, now is the time to fill up or top-off the soil profile in preparation for bloom.
Even with a good amount of rain — which, if you’re in California or more than 60 percent of the Continental U.S. for that matter, that hasn’t been the case this winter — chances are only the top few inches of the profile is wet enough to facilitate a substantial root flush.
There are several advantages to having a full soil profile and keeping it moist before, during and after bloom.
Trees and their structures are susceptible to frost both above and below the surface. But as buds begin to swell prior to bloom, most notably due to rising temperatures, they lose the ability to tolerate cold temperatures and a frost — even before bloom — which can severely harm or kill the developing buds.
Typically, anything below 28 degrees for more than 30 minutes can impair developing buds and cause damage upwards of 50 percent or more in crops like almonds if proactive measures aren’t taken.
While we mentioned briefly in our last post that water gives off heat as it freezes and acts as an insulator to delicate structures like feeder roots, it can also help to absorb heat from the sun during the day, which can help increase temperatures in the orchards into the evening hours.
When soil becomes wet, it darkens. Much like wearing a dark colored t-shirt on a hot day, the dark soil can absorb more of the sun’s heat (assuming your ground cover or cover crop is below 2-3 inches in height). As the trees head into full bloom this warming effect from the dark soil has several benefits.
The first, and most obvious, is protection from extremely cold temperatures. Radiant heat trapped in the soil releases via evaporation and convection, helping to warm the temperatures in the orchards by a few degrees. That little bit of extra heat can also facilitate better pollinator activity during daytime hours.
“Water holds a lot of heat and it makes the soil darker, which makes it absorb more light,” says Ben Smith, Hortau Director of Grower Support. “If you keep it moist during bloom, it’s going to stay warmer and that will encourage bee movement and help protect against frost.”
Another factor to consider in keeping the soil moist is root development. All the growth activity that is happening above ground is also happening below the surface as well. Just as buds are swelling and gearing up to break on the branches, root flush is happening below the surface simultaneously, both of which rely on energy stored in the tree prior to dormancy (see “It’s Important to Limit Stress During and After Harvest”). During root flush, the single-cell root hairs are emerging and will become the foundation of water and nutrient uptake for this year’s crop
If soils are too dry, it will inhibit the growth of these new structures which will reduce the amount of nutrients and water the crop will receive once photosynthesis starts firing on all cylinders.
There is a common misconception in saying that that roots will reach and try to find the water. While it is true that roots will seek out water, they won’t do so in dry soil. They’ll search in moist soil, but as soon as they hit a dry layer they stop.
With that in mind, consider a soil profile and its moisture content where only the top 12 inches of soil is moist. You’ll certainly get root flush in the top 12 inches, but you would essentially be limiting your active root zone to that depth, until moisture gets to the deeper levels to create optimal conditions for new root growth.
Conversely, if the soil is too wet it can become anoxic, which can also kill roots and could potentially open the door to diseases such as phytophthora, which can develop in as little as 24 hours of soils becoming saturated, driving home the importance of keeping soil moisture in the comfort zone.
The goal in irrigating this time of year is to minimize stress to the crop as much as possible by creating conditions favorable for breaking dormancy. And, since the crop’s water demands aren’t too extensive right now, it comes down to keeping soil water availability in a range that steers clear of excessively wet or dry.
When the profile is full, you will get more root flush initially right at the beginning of the season, which allows the trees to utilize the full root zone for water and nutrient uptake when they start pulling water after leaves harden off.
This Central Valley almond grower has kept his soil tension levels in the comfort zone through dormancy. Since December 1, 2017, he has only needed to irrigate one time and got a little help from a rain event the second week in January.
“It’s a really easy time to irrigate, but it’s a critical time that gets missed because we come through winter dry and we start the season dry,” says Smith, who has already seen buds beginning to swell in almond orchards in his region. “If growers don’t have their soil wet now, they need to do it now.”
Irrigation frequency for permanent crops heading into and through bloom will be low, as the plants’ water needs will be low. Once the soil is lubed up, growers will only need to top off as needed. This could be as little as every two to three weeks depending on soil type, evaporation rates and the crop’s water demands, which will increase as the season progresses.
This graph illustrates the increasing water demands from the same block of almonds from the graph above heading into the season last year. The grower’s soil was adequately hydrated prior to and into bloom, which allowed him to irrigate only when needed once leaves hardened off and the trees began transpiring.
Growers using Hortau should aim to keep tension levels in the blue band of the soil tension graph to ensure adequate moisture availability for the limited water use by the crop. With off-set placement of tensiometers at multiple depths, growers can see both lateral and vertical movement of water in the profile so they can know with certainty that their soils are hydrated heading into the new season and beyond.
To learn more about irrigation management with Hortau, set up a consultation with a representative in your area.