Sourcing and propagating pathogen-free hop plants
The Clean Plant Center Northwest at Washington State University has a hop program which distributes cuttings. In Corvallis, Oregon, the National Clonal Germplasm Repository (NCGR) is dedicated to maintaining a collection of pathogen-free genetic resources for many clonally propagated crops, including hops. If you will be purchasing your hop plants from a greenhouse or commercial plant propagator, make sure to inquire where they source their stock plants, what precautions are taken to exclude pathogens, and what pathogen-testing regimens are completed at the growing facility. Testing services for pathogens affecting hop plants are available through the University of Wisconsin-Madison Plant Disease Diagnostic Clinic, or contact your County Extension Agent.
Hop plants can be propagated from bine cuttings as follows. Before you start, prepare small pots or plug trays with a well-draining moistened potting medium. Have a disinfectant solution on hand to disinfect tools between plants, to avoid spreading pathogens that may be present. Commercial rooting hormone containing 0.1% indole-3-butyric acid (IBA) can be obtained from most garden centers. Dispense a small amount of the powder into a cup or vial and do not return it to the container after use, to avoid contamination. Alternatively, a 10 ppm solution of IBA can be used, ideally with the addition of boric acid to 10 ppm. More concentrated IBA solutions can suppress shoot growth.
Use sharp shears to cut sections of bine with 1-2 nodes, leaving about an inch to either side of the node(s). If leaves are more than 2 inches in diameter, trim them to a smaller size but do not completely remove the leaves. Dip the lower section of the stem (ie the end that would be closer to the root) into the rooting hormone. Insert the stem of the cutting into the potting medium and firm the potting medium around the stem, ensuring that it just covers the node. If the node is not covered, root development is less likely.
Water cuttings regularly and lightly. Re-cover nodes with potting media if exposed. Ventilated plastic domes can be used to increase humidity, but avoid direct sunlight since this can cause temperatures under the dome to spike, stressing or killing cuttings. Alternatively a misting system can be used to maintain moist conditions. Cuttings will do well in a temperature range of 70-75 F, with a photoperiod of 14-16 hours per day. Some varieties are difficult to root when natural daylight is less than 13 hours per day. Warming the potting mix can encourage root growth. Seedling heat mats or cables can be used for this purpose. Some heat mats simply raise the temperature a few degrees above ambient temperatures, but you may also wish to use a thermostat, set at around 78 F, for more precise control.
After 2-4 weeks, cuttings should have developed enough root and shoot growth to allow transplanting to a larger pot. Plants should be hardened off before tranplanting to the field.