Based on observations in previous years, early August appears to be the best time of year to prune lavender in Ontario. In a normal year, 30-50% of the new green growth should be removed to ensure a tight and rounded plant. Pruning encourages more lateral branches and more blooms the following year. Pruning will also ensure next year’s flowers remain upright, rather than opening up and collapsing under the weight of the flowers. This will also ensure even flower heights that are easier to harvest. With all of the winter and frost damage to plants this year, knowing when and how much to prune is much more difficult.
The majority of plants this year have over half of the new growth coming from the base of the plant or lower down the branches. If the plants were heavily pruned in the spring to give this growth plenty of light, then the new growth should be thick and vigorous. Pruning off 30% of this new growth now will not be an issue because there should be plenty of leaves remaining after pruning. If the plants were not pruned in the spring, the new growth may be elongated with wide spacing between the leaves. Pruning these plants aggressively now would probably remove most of the leaves and prevent a rapid recovery. These plants cannot be pruned as aggressively. It is probably best to just prune off the tips of any branches that are sticking out above the canopy. This will encourage more branching without damaging the plant.
Due to the weaker plants this year, Septoria leaf spot and insect pests like garden fleahopper have been causing more damage. The damage is more severe inside the canopy of the plant. Flowering weakens the plant further, leading to even more disease. If the inside of the canopy is heavily diseased, it is best to delay pruning until there is more vigorous new growth. Pruning can occur anytime in August, but should not be delayed beyond that point. If disease remains significant by the end of August, then only a very light pruning of the taller shoots should occur, no matter how the plants were handled in the spring. As a general rule, the plants should always have some healthy green leaves remaining after pruning to feed the plant until new growth emerges.