Evan Elford, New Crop Development Specialist, OMAF and MRA

We are receiving reports of frost injury occurring this past weekend on hops across the province. Damage ranges from no/slight damage in the south-west, to severe damage/plant collapse in central Ontario.  Figure 1 shows some symptoms of light frost injury on young leaves in the south west part of the province.  Leaves and leaders will grow out of this damage, although residual necrosis (browning) will be visible once the leaves fully expand.  This residual necrosis may look similar to hop downy mildew damage so make sure to document any frost injury in your scouting notes for future reference.

Figure 1: Minor frost injury on young leaves of hops in south western Ontario, 2013.

Figures 2 and 3 show more severe damage to a primary leader and the resulting double leader which will grow from the node below the affected area.

Figure 2: Severe frost damage on primary leader; Figure 3: Double leaders growing after physical damage to primary leader.

Early in the season, industry practice is to remove bines that have lost their primary leader (either from frost or physical injury) due to potentially lower yields.  However, little is known about the impact severe frost injury can have on hop yields and I have yet to locate a field experiment documenting yield comparisons of primary leaders to double leaders.

After discussing this weekend’s frost episode with other extension colleagues in Canada and the USA, it appears there is little documentation on severe frost injury on hops and the resulting management strategies for the situation.  Here are some options to consider if your hop yard was affected by severe frost injury:

For plants not yet strung on the trellis:

• Allow new shoots to regrow from the crown and train these in the coming weeks as normal.

For plants already strung on the trellis:

• For fast growing/vigorous varieties, consider re-training new shoots from the crown if you think you will have shoots longer than 20-30 cm long before June 1^st.
• For slow growing varieties, consider leaving the currently trained bines and allow the double leader to sprout from the node below the frost injury.
• To hedge your bets, you could consider leaving the bines with the double leaders and train one more shoot from ground level on the string.

To add to our collective understanding of growing hops in Ontario, consider re-training half of your plants and leaving half with double leaders to see if you observe a difference in plant growth and yield at the end of the season.

Thank you to colleagues at CREDETAO, UVM and UMN for their input.