One of the big questions lavender growers face annually is when to take their row covers off. Row covers have proven effective at protecting sensitive cultivars over the winter, but it can be difficult to know when to put them on and take them off again. If you take them off too early, a late winter or early spring cold snap can still cause a lot of damage. On the other hand, if you take them off too late, the higher sun angle around this time can cause a greenhouse effect under the cover and this can cause plants to advance too fast and become more sensitive to normal spring frosts in April and May. This is especially true if there is an extended sunny period with above normal temperatures.
Complicating the decision is the thickness of the cover. Some growers are using a much thicker cover that the sun cannot easily penetrate such as Hybertex covers. The risk of a greenhouse effect is much less with these covers. However, these covers come with the risk of keeping plants damp for longer periods. This is not a big issue when plants are cold, but as temperatures warm, the risk of fungal growth is higher under thick covers.
Historically, once we get past mid-March, the risk of those really damaging -20C temperatures is much lower, especially if the forecast does not show any major cold snap. There is a risk of snow and cold in early April, but if plants are still somewhat dormant, they can handle normal April cold snaps. The current forecast is for temperatures to be well above normal next week with an extended sunny period. To keep plants more dormant during this period, it is probably best to remove covers over the next few days, especially for the thinner covers. Keep in mind though that weather is very unpredictable and there is no way to know what April and May may bring. Row cover removal is entirely a grower decision based on conditions in their area. If possible and practical for your operation, you may want to keep those covers handy in case you want to protect plants again from an unusual freeze later this spring.
