Propagation of hop plants from nodal cuttings was trialed in two different hydroponic systems: deep water and nutrient film technique. Deep water systems consist of a nutrient reservoir with a floating support for cuttings; circulation of the nutrient solution can be achieved with a submerged pump or using airstones, which we chose to use. Nutrient film technique (NFT) systems pump the nutrient solution through a shallow substrate to create a “film” of nutrients within which the cuttings are placed. A schematic and photograph for each system is shown in Figure 1, and a guide to construction, assembly and maintenance of systems is available in pdf format: Small-Scale Hydroponic Hop Propagation Protocol.

Methods

We constructed 4 units for each system, each able to accommodate at least 12 cuttings. The nutrient solution used was based on a hydroponic fertilizer formulation for cannabis, a crop which is closely related to hops: http://www.centennialseeds.com/2012/05/04/a-hydroponic-fertilizer-formulation-for-cannabis-using-peters-20-10-20/ (last accessed 10/10/2020). Reverse osmosis (RO) water was used to prepare the nutrient solution. A 1000-fold concentrate for this formulation was prepared as follows: 504 g Peters 20-10-20 “Peat-Lite Special” (ICL Fertilizers) and 243 g magnesium sulfate (Epsom salts) was dissolved in 1000 ml of RO water. To prepare the nutrient solution, 30 liters of RO water was added to each system reservoir, plus 30 ml of the 1000-fold Peters/Epsom concentrate and 9.7 ml of “N+ 14% Liquid Calcium” (Jay-Mar, Inc). A paint mixer on an electric drill was used to thoroughly mix the nutrient solution. The pH was adjusted to 6.0 with concentrated phosphoric acid, and the electrical conductivity (EC) was checked to ensure it was in the range of 1200-2000 microSiemens/cm. System pH and EC was monitored every 2-3 days, and adjusted with phosphoric acid (pH) or 1000-fold Peters/Epsom concentrate and N+ 14% Calcium. Rarely, pH may need to be lowered, and a one molar solution of potassium hydroxide can be used for this.

In each of three trials, propagation of three different hop cultivars were trialed (Table One). Stock plants were well-established greenhouse-raised plants originally obtained from the National Clean Plant Network or the USDA National Clonal Germplasm Repository, and were confirmed to be free of common hop pathogens. Each experimental unit consisted of 4 single-node cuttings, with 4 replicates assigned to each system (a total of 32 cuttings per variety). Experimental units (sets of 4 cuttings per variety) were randomized within the systems. Cuttings were staged in damp paper towels before being placed in the systems, and care was taken to maintain the appropriate root-shoot orientation. If leaf diameter exceeded 2 inches, leaves were trimmed to a smaller size. Cuttings were rated for shoot growth (number of new nodes) and root growth (scale from 0-5, Figure 2) three weeks after planting. Between trials, systems were decontaminated with 10% bleach and rinsed with tap water.

Table One: Start and end dates and cultivars for trials 1, 2 and 3.

Results

Cutting survival was 100% in almost all cases. In trial 1, 2 of 16 cuttings for cultivar Tahoma failed to grow in the NFT system, and in trial 2, 1 of 16 cuttings for cultivar Centennial failed to grow in the deep water system.

For the majority of cuttings, shoot growth was in the range of 2-5 new nodes produced in the 3 weeks of the trial. In general, varieties performed similarly, but significant differences in shoot growth were seen in trial 1, where shoot growth of Nugget and Tahoma was greater than for Magnum in the deep water systems, and shoot growth of Nugget was greater than for both Magnum and Tahoma in the NFT systems (Figure 3).

Overall, root growth was similar across varieties in both hydroponic systems (Figure 4). The only significant difference noted was more vigorous root growth for Nugget compared to Magnum and Tahoma in the NFT systems (trial 1).

The two hydroponic systems did not show strong differences in propagation success and cutting vigor. Root vigor tended to be higher in the deep water systems, and shoot vigor tended to be higher in the NFT systems compared to the deep water systems, but this was not a significant difference.

Readings for pH and EC throughout the trials are shown in Figure 5. pH readings tracked closely for the NFT and deep water systems. EC tended to increase, and was generally higher for the NFT systems. The increases in EC were most likely due to water loss through evaporation, which would be expected to be higher for the NFT systems.

Conclusions

Since the deep water and nutrient film technique hydroponic systems performed relatively similarly, the choice of system comes down to price and simplicity. The deep water systems are less expensive, at a total cost of $213 for four systems, vs $281 for four NFT systems, and also considerably simpler to construct (see Construction, Assembly and Maintenance Guide linked above). Both systems can be planted at much higher density than used in these trials. We recommend use of the deep water systems for efficient and cost-effective propagation of hop plants.