The warm weather over the past week has put us about 10 days ahead of normal based on Growing Degree Days in the south (Figure 1).

Figure 1. A lavender plant on May 3 in Simcoe showing significant green-up and new growth.

Further north, plants may be even more ahead of normal due to the lack of snow cover this winter, which in a normal year prevents temperatures from warming up as quickly in areas downwind of the Great Lakes. That means that plants will be as advanced as they normally are around May 13 or later. This also means that various activities in lavender will likely be earlier than normal even if temperatures the rest of the spring end up being near normal. This includes:

Four Lined Plant Bug Emergence

Four lined plant bug nymphs typically emerge in mid- to late-May, depending on the area of Ontario. If that is 10 days earlier this year, emergence may begin within the next week in the south.  There are very few options for controlling four lined plant bugs in lavender in Canada. The only options available are insecticidal soaps that are registered for aphid control and may suppress populations of four lined plant bug at the same time. To have any chance of success, insecticidal soaps should be applied when the nymphs are small and less mobile. This is probably within the first week of hatching.

If you had a problem with four lined plant bug last year, it is likely it will be as bad or worse this year. To manage them, scouting will need to begin soon. Look for slight browning and bending of the newest leaves on the plant at the tip of the shoots. The nymphs are initially bright red and 1-2 mm long and will hide as you approach a plant. When they are found, it is best to spray immediately and then again a week later, in case new eggs continue to hatch after the first application. For more information (incl. photos) on four lined plant bug, visit the four lined plant bug profile in the Lavender module of Ontario CropIPM here.

Have your sprayer and product ready for the application. The insecticidal soaps registered for use on lavender are Safer’s Trounce (PCP 24363), Kopa (PCP 31433), and Opal (PCP 28146). These products are commercial products so applicators will need their Grower Pesticide Safety Course Certificate to purchase and use the products or be supervised by someone with the certificate. To learn more about the program or to book a course, visit the Ontario Pesticide Education Program at opep.ca. Some of these products are acceptable for organic production and others are not. If you are certified organic, contact your certifying body to ensure the product is okay to use before application.

Pruning

Growers often do some minimal pruning in the spring to remove dead growth and get plants into a rounded shape. While dead growth can be pruned off at any time of year, cutting off living tissues (beyond a few taller stems) is best done before plants start growing too much in the spring. This is because the flower buds will begin to form shortly after the new growth begins. If this growth is cut off too late, bloom buds will be cut off as well. This could either reduce flower yield or force the plant to produce new buds which requires more energy and may reduce flower or plant vigour. Consider either pruning earlier this year if plants have not begun to produce new green growth or waiting to prune until after bloom.

The exception is if you need to rejuvenate plants. Older plants sometime lack vigour and have a sparse canopy with many dead branches (Figure 2). These plants tend to have a weak bloom with short stems. Sometimes these types of plants benefit from a complete rejuvenation by cutting off all of the branches within 5-10 cm of the ground and letting new growth emerge from the crown. Research has shown that the best time to do this is in the spring. Rejuvenated plants will still flower but yield may be lower and flowering uneven until the following year. Lavandins do not respond as well to this type of pruning. Only rejuvenate lavandins if there is green growth coming up from the crown. Sometimes thinning a lavandin plant for a year by removing 50% of the branches can encourage new growth that will allow for a full rejuvenation the following spring.

Figure 2. A 5-yr old plant in need of rejuvenation. There is already significant new growth at the crown and these new shoots will take over if the woody growth is cut way back.

Bloom

The weather over the next month will play a bigger role in how early the lavender begins to bloom than the temperatures up to this point. But, if temperatures are normal for the rest of the spring, bloom would likely begin around 5 days early this year. Temperatures are forecast to remain above normal for the next 7 days, meaning bloom may begin even more ahead of normal. Keep this in mind when planning agritourism events.