August to October is the time lavender plants recover from the bloom period and put on new growth. August is the best time to fertilize plants and prune. Now that we are getting closer to the end of August, pruning and nitrogen applications should soon wrap up. Nitrogen applied any later than now could lead to too much vegetative growth in the fall and reduce hardiness overwinter.
Pruning can be done into September, especially on younger plants, but pruning too late into the fall could also lead to more injury over winter. Not only will late pruning promote new vegetative shoots that are very tender over winter, but it will also lead to fewer leaves on the plants. A plant that is tightly packed with leaves has more protection from winter winds and often has less injury.
Are your plants looking a little pale and the leaves mottled with silvery-white damage? It could be a sign of garden fleahoppers. Garden fleahoppers seem to be a major issue in plants grown on solid ground cloth. It is possible the extra heat over the plastic promotes more generations of the insect over the season, which allows populations to increase. However, garden fleahoppers can occur anywhere, and are usually in patches in the field.
To confirm garden fleahoppers, put a white piece of paper on the ground next to a lavender plant and give the plant a good shake. If you see tiny, hopping black adults and green nymphs on the paper (Figure 1), then it is likely garden fleahopper. There are no insecticides registered for control of garden fleahoppers in lavender, but applications of insecticidal soaps (e.g. Trounce, Kopa, Opal) applied for aphid control may suppress populations of garden fleahoppers. These would only need to be applied to the affected patches of plants.
Figure 1. Adult (above) and nymph (below) of garden fleahopper. Adults are only about 2 mm long and nymphs smaller than that.
The end of August is the ideal time to take cuttings for propagation. New shoots have been growing for a while and are thick and green at this time of year. Younger plants tend to produce the best shoots for cutting. Ensure you do not collect cuttings that are diseased or from plants that had any symptoms of yellowing due to alfalfa mosaic virus.
This is also a time of year that it is possible to root cuttings outside if you do not have a greenhouse, although success will vary depending on the weather and your setup. Air temperatures are starting to decline, which is ideal for keeping the tops from drying out while they are rooting. The cuttings may still benefit from some bottom heat provided by heated mats or heating cables (e.g. the ones used for eaves troughs). Keep cuttings in bright shade such as under a tree or on the north side of a structure that is free of nearby trees. Cuttings still need light, but full sun will dry out the tops too quickly. They should be out of the wind and will benefit from some misting on warm, sunny afternoons. You will still need a warm, sunny location to grow the rooted plants for a month or so after rooting, such as a cold frame, since it will take 2 to 6 weeks for them to root and then need additional time to get fully established. After that time, it is possible to overwinter the plants in a sheltered and insulated location where temperatures do not get much below freezing. Without a greenhouse, this can still be risky, depending on your setup.
Alternatively, cuttings can be brought to a propagator and they can grow them over the winter in a greenhouse. Since this is the best time to take cuttings, it is also a good time to order your plants for next year if you do not want to bother with collecting cuttings or want cultivars that you do not already possess.