Irrigation of Lavender
Curtis Swift, Ph.D., Mesa Lavender

In most cases, irrigation needs to be accomplished when plants are first set and again be available for those times when insufficient rainfall occurs.  This may require a permanent or moveable irrigation system.

Irrigation can be done with furrow irrigation, sprinklers, drip tape, drip tubing, or microsprays. Some growers hand water their fields. In all cases the soil needs to be wet to the depth of roots and below. Usually making sure 6 to 8 inches depth is adequate. Shallow or excessive watering results in shallow roots subjecting the plants to dehydration as well as increased insect and disease problems. When soluble salts are a problem additional water will be needed to flush the salts below the root system.

• Furrow Irrigation

  • If using furrow irrigation, the furrows need to be close enough to the plants to wet the root ball. As the plants develop, the furrows can be moved further away from the row of plants. You should gauge the distance required by determining how far water flows laterally from the furrow. Soils differ in this lateral movement of water.
    • Dig a hole where you think the furrow should be and add water. Check to determine how fast and how far the water moves laterally. Adjust the location of the furrow accordingly.
  • In areas where soluble salts are an issue, watering alternate furrows will help flush salts past the roots of the lavender.
    • https://extension.colostate.edu/topic-areas/agriculture/managing-saline-soils-0-503/

• Sprinklers

  • • Impact sprinklers used for irrigation may need to have their nozzles changed out depending on the arc the sprinkler head covers. i.e. 360 degree coverage vs 25% coverage.
    • Ensuring you have head to head coverage will help ensure there are no dry
    • Nozzles and timing of sprinklers should be based on infiltration rate and water necessary to replenish the soil depth.

• Drip irrigation

  • • When using drip emitters, they need to be close enough to each other to account for any movement of the tape or tubing.
    • Running two drip lines, one on either side of the row, is often recommended.
    • Planting at 36 inches and having the tape or tubing with emitters every 36 inches on the surface is problematic even when you pin the tape or tubing down. The shrink and swell of the drip tape/tube will move the emitters away from some of the plants, resulting in poor growth or even plant death.
      • It is common to add 2.5% to the length of the tape/tube to counteract this expansion.
    • Ensure the spacing of emitters is 12 or 18 inches when planting at 36” spacing is more likely to prevent dry spots and dead plants.
    • Placement of the drip line from the plants needs to be based on the lateral movement of water from the emitters as determined with furrow irrigation above.
    • The slope of the land is critical as well as the length of the row when using drip or micro irrigation.
      • I would suggest always using drip irrigation with pressure compensating emitters. A PC emitter will theoretically discharge the same flow regardless of the emitter pressure up to a certain elevation change. Emitter pressure is influenced by changes in elevation and length of the drip line. Use of PC emitters will help ensure the difference in elevation does not cause a difference in water application between the head of the row and the end of the row. Be sure to determine the elevation difference the line of drip tape/tubing will permit. The difference in elevation a system can allow needs to be determined prior to purchasing a system.
      • Redesign of the system may be necessary if the slope is more than the system will tolerate. A 30%+ difference in water application can result when using the wrong drip line on a slope.
        • Growers encountering this problem should consider redesigning the system.
          • Moving the main water delivery line from the bottom of the field to halfway up the field. This will split the system in half ensuring more uniformity in the application of water. The elevation difference will still need to be taken into account when selecting where to place the main line,
        • For new fields the decision of which way to run the rows should be determined based on the slope of the land.
      • The system you purchase must be able to run the length of row you are planting.
        • Drip lines longer than recommended will result in varying application rates from the head to the end of the row. If this is an issue you may need to relocate the main line.
          • Place the main line at the top of the field. The length of the drip lines running down a slope can be much longer than drip lines running up the slope without losing pressure or application rate.
          • Or, split the field as shown in the diagram below.
      • A footer should be placed at the end(s) of the drip lines to connect drip lines. This footer is typically the same diameter as the mainline but not necessarily. This will help ensure any plugging of drip lines will not result in emitters receiving inadequate water.
        • Footers should have a drain plug to facilitate flushing.

• • • • An option to a footer at the end of the lines would be flushing each drip line separately.
      • Unless you replace the drip lines yearly or more often, the system must be flushed periodically, and chemicals injected to reduce plugging.
    • Even when using domestic water, fine particles in the water can plug emitters.
    • Even with the appropriate filtration system, the buildup of scum or very fine particulates can be problematic.
    • Chemical injection is often critical in arid regions due to salt building and plugging of the emitters.
      • Chemical injections can also help prevent scum buildup which restricts emitter flow.
    • Chemical injectors can also be used to apply fertilizer, pesticides, and modify the pH.
      • Always check the interaction of the chemical used before injection. This is done with a “jar” test.
        • Put some of the chemical in a jar with the irrigation water used. If any cloudiness occurs, there is a chance the injection of the chemical will cause emitter plugging.
      • The injector for chemical treatment of water for drip system is typically located down steam of the pump but upstream of the filters.
      • The injector must be capable of injecting a constant rate of chemical to prevent clogging.
    • The flushing pressure is typically greater than the recommended flow rate of the system so ensure your system can provide this option.
  • A drain on the footer is necessary to allow flushing unless you make arrangements to flush each line separately.
    • Flushing may be necessary several times during the growing season.
  • Pressure regulators may need to be placed closer to the laterals(s) if distribution uniformity of emitters is a problem.
    • Distribution of uniformity of emitters can be tested by placing catch cans under each emitter for a set amount of time.
    • Catch cans can also help you determine when to flush the system.

• Microsprays

  • Microsprays can have very high flow rates when compared to drip emitters.
  • The barbs and spaghetti tubing used to connect the microsprays to the tubing have smaller diameters resulting in less flow and more friction loss (pressure loss)
    • Vinyl tubes generally have thicker walls and thus experience more friction loss than PE tubing. Vinyl tubes are often used due to their resistance to rodent damage when compared to polyethylene (PE) tubing.
  • Friction loss should be taken into account when considering the total pressure requirement at the hose inlet.
  • Orifices of microsprays can wear and impact water delivery to the plants so need to check occasionally and replaced when needed.
  • Distribution of uniformity can be determined by the use of catch cans. This will help you determine if spray nozzles need to be changed.

Additional Notes:

• Using different microsprays or drip tubing/tape on the same system should be avoided due to the differences in flow rates and delivery application.
• The filtration system needs to be fine enough to help prevent plugging of emitters.
  • Depending on the diameter of the orifice of the emitter, the filter should remove all particulates greater than 1/10 the size of the orifice of the emitter.
• A pressure regulator(s) should be installed to ensure the pressure does not exceed the manufacturer’s recommendations. Keep in mind the pressure regulator should be adjustable to ensure proper flushing.
  • Manufacturer specs provide flushing pressure and flow requirements.
  • Pressure regulators are most efficient when installed just prior to the laterals on the submain.
• The delivery line from the pump or other water source should be sized to ensure adequate flow for the system.
  • Different pipe sizes, length, and internal characteristics have different flow rates due to diameter and friction loss.

Subsoil irrigation or under fabric

Your drip system can be placed at the depth of the planting or under the fabric.

• If placed under the fabric, drip tape and tubing can be chewed on by voles and mice causing problems.
• With subsoil applications, plugging of emitters can occur and plant death resulting without you knowing you have a problem.
• For these reasons it is important to ensure the drip system used has a long life.
  • This includes proper flushing, and chemical applications to reduce emitter plugging.
  • Some drip system manufacturers provide a flagging system you can install at the end of each line so you can see if the line is plugged.

Soluble Salts (EC) affect irrigation scheduling

Electric conductivity is a measure of the soluble salts in the soil and/or in the irrigation water.

• A soil test should provide the E.C. of your soil.  E.C stands for electrical conductivity and tells you the level (content) of soluble salts in the soil.
• Except in rare situations, the salts included in this number will be calcium, potassium, and other salt combinations. Lavandula may tolerant up to about 9 mmhos/cm (dS/m) based on the saturated paste extraction technique (EC_e) depending on the cultivar.  (https://www.uslavender.org/index.php?option=com_content&view=article&id=74:soil-preparation-for-lavender&catid=24:lavender-101&Itemid=138)
• When labs use a different extraction technique (1:1 or 1:3) the EC needs to be converted to EC_e
• Additional water needs to be applied when salts exceed or approach the tolerance level to flush salts below the root system of the plants.
  • This is called the leaching requirement.
  • The LR is typically 1-8% of the water needed to refill the soil profile but may require 30-40% more irrigation water or soil with a high EC_e.

Plant Establishment

In the early stages, prior to the roots being well established, you need to ensure the native soil and root ball are both moist.  There are occasions when native soil covers the top of the root ball preventing the movement of water into the root ball. This is more likely to happen when your soil contains clay.  Some soils will pull moisture out of the root ball so testing the root ball for moisture is critical.

• To increase successful establishment, ensure the root ball of the new plants are scored or teased apart as you plant. If root balls are too compact, roots don’t spread properly. Check out https://www.youtube.com/watch?v=-5gk2yVAQtM
  • Scoring the root ball helps ensure roots grow into the native soil.
  • Neglecting this process can result in roots girdling the plants resulting in its decline and death. (https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/environmental/girdling)
  • Ensure the planting hole is not glazed on the sides or bottom as this will prevent root movement into the native soil.
  • Never add sand or gravel to the bottom of the hole as this will result in a “perched” water table. (see https://en.wikipedia.org/wiki/Water_table)
• Some growers establish their fields by hand watering each individual plant.
• Some growers use a movable sprinkler system during the establishment phase.

Irrigation Frequency

Frequency of irrigation and how much water you apply depends on your soil type, and climatic conditions in your areas. For that reason, no one cannot provide you an accurate recommendation on how often to irrigate or how much to water to apply at your site based on how the water their fields. Just because the grower next door is watering at a specific schedule does not mean the soils in your field are the same. Soils in a field often vary with each having different requirements for irrigation amount and frequency. Growers can fine-tine their irrigation needs if they know what soil textures are in their field.

• Note: A soil moisture meter does not always give an accurate reading especially if you have soluble salts in the soil. I would suggest using your index finger to verify the root ball is receiving sufficient moisture. You should check several plants throughout the field to ensure adequate moisture throughout the field. This should be done on occasion during the year to ensure adequate moisture is applied.

Determine the soil moisture with the soil ball technique

You can use the soil ball technique to fine tune your irrigation as this will approximate the percentage of moisture in your soil. https://www.wcc.nrcs.usda.gov/ftpref/wntsc/waterMgt/irrigation/EstimatingSoilMoisture.pdf

• Once you have reached field capacity as indicated by the soil ball technique, there is no benefit to applying more water and this practice should be avoided. The exception is when soluble salts are an issue.

Keep in mind newly planted lavender will need more frequent irrigation as the roots are still quite shallow and restricted to the root ball area. Checking the moisture content of the root ball with your index finger is helpful in determining how frequently to irrigate. Once roots spread into the surrounding soils your irrigation scheduling will be much easier.

Evapotranspiration

Some land-grant colleges provide local ET data on a daily basis. (See https://coagmet.colostate.edu/ as an example.) If your land-grant college is unable to provide data on the ET at or near your site, there are two great options for you to determine the ET yourself.

• The ET Gauge is a great instrument used to finetune the water requirement on your property. https://www.specmeters.com/soil-and-water/soil-moisture/et-gauge/et-gauge/ This comes as a read alone or has the capability to send ET data to your cell phone or computer.
• Tensiometers can also provide you guidance on timing of irrigation (https://lawr.ucdavis.edu/cooperative-extension/irrigation/drought-tips/field-use-tensiometers) Soluble salts do, however, influence the reading.

Determine soil texture

Soil texture determines the water holding capacity of the soil and when you need to irrigate again taking rainfall into account.

You may already know the texture of your soil, if not Web Soil Survey (usda.gov) provides the best possible update on the soil textures of many fields. Not all areas of the United States have been surveyed but hopefully your fields are included.

• Enter your address and click on view. Select AOI to outline the area you are interested in and click on Soil Map. This will show you the soil texture and any specific properties and qualities of the soil you are working with.
  • While the Web Soil Survey may not be completely accurate it is a great place to start. Keep in mind that as you amend the soil with organic matter you will increase the infiltration rate and water holding capacity of the soil but knowing the texture will be a great starting point.
• If your field is not on Web Soil Survey or if you wish to doublecheck the soil texture, Clemson University has an excellent “jar” test. You can find that at https://hgic.clemson.edu/factsheet/soil-texture-analysis-the-jar-test/

Texture changes with amendments

Amending the native soil can change the infiltration rate and water holding capacity of the soil so you will likely need to make changes to your irrigation as the year(s) progress. As soil settles after planting, these factors may also change.  Double check the infiltration rate and water holding capacity as soil compaction occurs to reassess your irrigation strategy. You may need to do this yearly until the field is established.

Even though your soil may hold 2 inches of water in a foot of depth, not all that water is available for plant uptake. This is where the MAD comes into play. The permanent wilting point is also critical to know. Once the water availability drops to or below this point, the plant will reach its permanent wilting point. This is the point of no return for the plant.