Rooting Hormone Application and Moisture Management
This week, Dr. Josh Henry shares some technical training he's developed to help growers better understand Rooting Hormone.
Problem: Without understanding the process of rooting, or the latest best practices for rooting hormone application and moisture management during propagation, you might be working harder than you need to when producing successful liners.
Josh's Tip
Typically applied as a dip or spray, rooting hormones like indole-3-butyric acid (IBA) and 1-naphthaleneacetic acid (NAA) can be used to improve rooting success and uniformity of vegetative cuttings, especially in slow or difficult-to-root species.
How roots form: After a cutting is stuck, auxins play an important role in stimulating root development. Auxin is a naturally occurring plant hormone that is produced in the meristem (shoot tip) and is transported downward. When auxin accumulates at the base of the cutting, it stimulates root initiation (adventitious roots). This is why rooting hormones, which are synthetically derived from auxins like indole-3-butryic acid (IBA) and 1-naphthaleneacetic acid (NAA), can be applied to improve rooting success and uniformity. Roots move water, which is why it makes sense that roots develop from xylem (water transporting vessels), not the callus.
Types of rooting hormones: Rooting hormones are available as powders or liquids and are typically applied as a dip, soak or spray. While the potassium salt formulations of these rooting hormones are water soluble, other formulations must first be dissolved in solvents like alcohol, which can cause phytotoxicity.
The application methods are specified on each product label and depend on the product formulation. The key to effective use of rooting hormones is to apply the active ingredient in a way that contacts the base of the cutting, where root formation is desired. Dips into powder or liquid rooting hormones have long been used in vegetative propagation and while they are effective, they also are labor-intensive and pose risk of spreading disease. Thus, rooting hormone solutions used for dipping should be changed between crops.
Rates and volume: The typical rates used for propagation depend largely on the product formulation, application method and crops being grown. Foliar applications to annuals generally range from 50 to 400 ppm but may be as high as 800 ppm for herbaceous perennials. Higher rates pose the risk of leaf and stem distortion and chlorosis (yellowing). Application volume is an important consideration. While typical spray volumes (2 qt./100 sq. ft.) can be effective, higher volume sprenches (4-8 qt./100 sq. ft.) can help deposit rooting hormone near the base of the cutting, providing higher efficacy. However, sprenches provide more total active ingredient than sprays per unit of area, so application rates should be lower when application volume is increased.
Timing: When using powder or liquid products as a dip, the application is made at the time of sticking. For foliar applications, sprays should be applied within 24-48 hours of sticking. Making foliar applications the day after sticking gives cuttings time to rehydrate, providing lower stress conditions to spray. Turn off the mist for one to two hours after application to allow the rooting hormone to be absorbed by the cutting. If rooting is slow, a second foliar application can be made one to two weeks after sticking.
While rooting hormone is generally beneficial for any plant species, some species yield greater benefit from their application than others, as can be seen in Table 1 from this e-GRO Alert.
Remember, always read and follow the label to ensure products are compatible with your intended application.
For more information, check out these additional resources: