Many hop growers are asking this question as the abnormally mild weather persists across much of the USA. For most growers in northern latitudes; probably not much effect will be noted. Average hop vernalization temperature requirements of 6 weeks below 38 degrees Fahrenheit will still be met, resulting in normal burr and cone development later in the season. Expect an earlier emergence of pests and disease that the milder winter failed to kill.
For growers in more southern latitudes the story could be completely different. Effects on the hops in the southern tier could include: lack of proper vernalization, increased pest pressure, and increased prevalence and severity of hop diseases like downy and powdery mildews.
Hop vernalization requirements vary by hop genotype and varieties that require longer chilling periods may not properly reset back to the juvenile spring phase required to grow and produce a normal yield of cones. One of the first signs of incomplete vernalization is uneven spring emergence of new shoots within plants of the same hop variety. Growers in the PNW have noted that this uneven emergence follows through the entire growing season; affecting cone-set, yield, alphas, and maturity dates. The cone-bearing sidearms may not extend properly and only produce cones on the terminal ends. Many affected hop plants showed up to a 6 week delay of burr initiation and a 30 to 50% loss in total potential yield.
What can a hop grower do to compensate? Here is a list of spring crop practices GLH advocates.
- Do a thorough spring cleanup in the hopyard. Remove old bines and plant debris. Get rid of that old pile of bines leftover from the fall harvest.
- Cultivate the hopyard rows early and often. Every time tillage occurs, more field debris containing possible pathogens is incorporated into the soil; where competing organisms can destroy them. Aerated soils create an unfavorable set of conditions for many hop pathogens (nematodes, for example). Double up your field tasks by incorporating nutrients at the same time. Even the simple act of raking over the rows helps reduce pathogen levels.
- Get a complete spring soil test. Include soil pH, alkalinity, organic matter, macro & micro nutrients. If vernalization is a possible issue; don’t add to it by having an additional soil pH or nutrient issue. Check potassium, boron, zinc, and phosphate levels in particular.
- Consider adding springtime slow-release fertilizers to include an incorporated application of sulfur coated urea (SCU). This is a slow release granular fertilizer that will meter the nitrogen to the hops more efficiently in the early spring. (Excess nitrogen in early stages of hop growth can encourage disease outbreaks.) The sulfur coating also has some fungicidal effects and is a required plant nutrient. It’s not a cheap as simple urea; but a lot more of the available nitrogen makes it into the plant.
- Apply an early horticultural grade crop oil application at spring hop emergence and add this useful product to your disease & pest control program. To familiarize yourself with horticultural oils and how to use them; we recommend reading the individual product labels or checking out a website like JMS stylet oil. (Check out the FAQ section in particular). Horticultural oils are all mineral oil-based and some are even OMRI certified for safety.
- Drench disease susceptible hop varieties with a systemic fungicide early and at first shoot emergence. Especially the hop varieties that had disease issues last season. Consider a product like Ridomil Gold because it has broad spectrum activity and works both in the root zone and on emerging growth. It lasts a long time and moves upward in the plant as it grows; providing continuing protection. We recommend this product over the springtime use of soil drenchs of phosphites (Aliette, Phostrol, etc.) because new research shows phosphites can interfere with phosphorus uptake; which is critical for new root development.
- Get control of the weeds early. Excess weeds deplete nutrients and just add to the problem. Always cut, kill, and maim weeds before they set seed. Non-selective glyphosate herbicide can be applied to control overwintered deep-rooted perennial weeds hopyard rows early in spring only if hops are fully dormant and crowns and buds are covered with at least 1 inch of soil.
- Trim early emerging hop plants to even up growth in the row. This trimming or cutting back has to be done early in the growing season because hop plants that are not properly vernalized can delay burr initiation by up to 6 weeks longer than normal. Timing this practice too late means the possibility of almost no cones on some hop varieties.
- A foliar plant growth regulator (PGR) application of gibberellic acid GA3 can mimic the cold requirement. GA3 is a very potent plant hormone and this application has to be done in a very accurate and time sensitive manner to have the intended effect of resetting the hop growth. Do this application wrong and more damage than good can result. Specific instructions include, but are not limited to, foliar application after training when the hops are 5 to 8 feet tall. (5 feet on aroma types and 8 feet on bigger faster growing bittering types). Gibberellic acids should not be applied if burr initiation is expected within the next 3 to 4 weeks. Longer wet-leaf time results in more effectiveness. (Applying GA’s early morning and evenings is more effective.) See product labels for the exact detailed instructions. The PGR product Progibb is labelled for hops and is also OMRI approved. It is strongly recommended to do small trials with gibberellins the first season of use if you have never worked with plant growth regulators (PGRs) before.
Factors Controlling Hop Flowering
University of Northern Iowa