Pruning is one of the most important practices for a successful lavender business. It affects the health of the plants, the longevity of a field, the ease of harvest, the yield of flowers and the appearance of the plants. The available literature on lavender pruning can often be confusing and contradictory, especially when it comes to when in the year a grower should prune. The confusion comes when literature from different growing areas are compared. In milder climates, growers can prune virtually any time of year because of the lack of a harsh winter. In Ontario, pruning at the wrong time of year can be very damaging to the plants. Pruning techniques also vary depending on the age of the plant and type of lavender being grown.
Based on research conducted over the past few years in Ontario, and trial and error, the optimal time to prune in Ontario is becoming clearer. Aggressive pruning too early or late in the growing season can result in substantial winter damage, poor flower yield or plant death. Too little pruning and the plant can become open and woody in the centre. The weight of the flowers can pull these loose branches to the side, making harvest difficult. These plants are also more prone to winter damage as heavy snows snap the woody branches and leave the stems exposed to disease.
For newly planted plants of Lavandula angustifolia (English lavenders) or Lavandula x intermedia (Lavandins) it is best to remove any developing flowers and prune off any dominant shoots. If one or two shoots are allowed to develop, the plant could become very tall, with minimal lateral growth. Pruning off these shoots 5 to 15 cm above the soil, depending on the size of the plant, will encourage side branches and a more compact plant. When the new growth has grown back about a month later, prune again to remove about 1/3 of the new growth. Plants can be pruned several times during the growing season, with the last pruning occurring no later than September 1. Pruning too late in the season could result in vigourous new growth that is not hardened off properly before winter.
The timing of pruning for older plants is much more important than for first year plants. In the spring as soon as the plants begin to show green colour, any dead material should be removed. The plants can be lightly pruning at this time to remove any damaged branches or to lightly shape plants affected by winter kill. Aggressive pruning in the spring is not recommended because the developing flower buds will be present on the new growth. In addition, the new growth after pruning appears to be more susceptible to spring freezes, and damage can be increased by early pruning.
The best time to do a hard pruning of older plants is after bloom (early to mid-August). If done after plants are completely done blooming (e.g. after oil harvest), new shoots will already have developed on the plants, and pruning these will encourage more side shoots. Harvesting will remove the flower stems, but will not act as a pruning. Any dead flower stems remaining on the plants can allow a point of entry for fungi and bacteria to damage the plant as they decompose, and these must be removed shortly after harvest. Plants should be pruned vigourously in the second year, and less and less each year as the plants mature. When you look at a cross-section of a mature plant, you will notice woody growth at the bottom, semi-woody (semi-hardwood) growth in the middle that is partially green and still has older leaves attached to it, and new vegetative (green) growth. As a general rule, the top 1/3 to ½ of the green growth should be removed. Since older plants will have less new growth each year, this general rule will result in less pruned off each year. The same rule applies to both L. angustifolia and lavandin cultivars, despite the differences in growth between these types. Pruning into the woody growth can kill the plant. Plants should never be pruned enough to remove most or all of the green leaves. If leaves have been lost to Septoria leaf spot, plants should not be pruned as vigorously or they may not recover. The plant should be pruned into a flattened dome shape (height slightly less than half the width of the plant). If the plant is too vertical, flowers that emerge from the side of the plant the following year will be more horizontal and more prone to falling onto the ground after a rainfall. A well pruned plant will look like a solid dome and will not open up when brushed lightly by hand.
Depending on plant growth, it may be possible to prune lightly again in late August to keep plants compact and improve shape. A second pruning will only be necessary on fast growing plants such as lavandins in the second or third year of growth.
Once a plant has become woody and open in the centre, it can be difficult to get the plant back into the proper shape. For these plants, a harder pruning can promote growth of new shoots around the base. After bloom, these plants can be pruned back ½ of the new growth. Green leaves must remain on the plant or the plant will likely die. Once new shoots emerge from the base, the woody branches can be pruned back even further. While these may eventually die, the new shoots can replace them. This may only be successful on plants that are younger than 5 years old. Another possible technique is to remove one in every four woody stems each year, leaving the centre of the plants more open to sunlight, which will encourage new growth from the base. Eventually, younger branches will replace the older ones.
Fall pruning is recommended in other regions, but is more risky with our colder winters. Minimal research has been conducted on pruning in Ontario and the risks associated with fall pruning are unknown. It may be possible to prune successfully in the fall if the plants have good protection over the winter. However, growers wishing to experiment with fall pruning should test it on a few plants first to ensure there are no issues. As a general rule for all perennial crops, new shoots should not be encouraged prior to the winter, because these are more susceptible to winter damage. Pruning in the middle of September when temperatures are still warm could encourage new growth. Pruning when plants have already gone dormant in late-October or November may be less damaging because plants will not send out new shoots at this time of year.
There is still a lot to be learned about pruning in Ontario. Different growers may be successful with different methods and at different times of year depending on their cultivars and growing conditions. Some growers may need to prune at a certain time of year when they have available labour in their production cycle. Growers should stick with what works for them, and when attempting a new technique, they are encouraged to trial it on a few plants and not risk damaging their entire acreage.