Cold temperatures and hops…

This spring we have seen unseasonably warm temperatures in March leading to early sprouting in hops across the province. As we continue to see variable temperatures through April some questions are beginning to arise about the susceptibility of hops to cold damage.

Most hop cultivars are generally considered to be cold hardy once established and the plants can sustain moderate frosts. Young tender shoots or newly planted rhizomes, however, may be affected by excessive freeze thaw cycles. The plant will send up new shoots if early ones are frozen off at a cost of additional energy from the crown. An application of mulch or straw in the autumn can be used as a layer of insulation to prevent the ground from warming up too quickly in the spring and to prevent damage from excessive freeze thaw cycles. Leaving this layer of mulch on too late into the spring, however, could result in delayed emergence of the shoots and delay crop development. If straw or mulch has been removed from the plants and there is potential for a heavy frost, plants can be re-covered for a few days to prevent freezing. Re-covering plants with straw or mulch may provide a 1-2oC temperature buffer.

The photo below, submitted by a cooperating hop grower in Ontario, shows the desiccation of rhizomes and shoots around a hop crown when a layer of mulch had been removed this spring exposing the new rhizomes and shoots to cold temperatures. In this case, too much mulch was removed and the exposed shoots will not recover; however, the main crown appears unaffected. In the long-term, the dieback of the outer shoots will probably not affect the final crop since only 2-4 of the strongest bines (typically selected from the centre of the crown) will be trellised and the remaining shoots pruned back for the rest of the season.


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