As I mentioned in my last update, normally I would have taken the row covers off my lavender at the beginning of March because of the warm temperatures and sunny weather at the time. Instead, I put a datalogger under the cover to record air temperatures. As I have always said, thinner row covers like the Agryl P-40 cover used in my plot turn into a greenhouse in the spring with the higher sun angle. However, I never had any data to back it up.

The best way to determine the potential impact of warmer temperatures under the cover on growth of the lavender is to do a Growing Degree Day (GDD) calculation. This calculation involves determining the average temperature for each day (Maximum temp + Minimum temp/2) and then subtracting a base temperature below which we would not expect any growth. The most common base temperature used for perennial plants is 5°C, and this is what I used for my calculations. The values from each day are then summed to come up with the cumulative GDD over a period of time. In theory, if the model is perfect, plants will be at the same stage from one year to the next when they reach the same cumulative GDD. For example, let’s say flower buds become visible at 300 GDD in a cool spring on June 1. If the next year, 300 GDD accumulate by May 15, then we would expect bloom buds to become visible around that time.

There are a few problems with my comparison of temperatures under the cover and ambient air temperatures:

1. The lavender plot is over a solid sheet of black ground cloth and the ambient air is collected over grass. Temperatures over the black plastic are probably a degree or two warmer on sunny days even without the cover, depending on wind speeds.
2. I intended to collect temperatures at around 30 cm above the cover, but my sensor and the solar shield it was in fell off the stake they were on at some point. This probably means more temperature extremes on sunny days and clear nights.
3. The temperatures under the cover had dramatic swings, sometimes going up to 26°C during the day and -7°C at night. Under the GDD calculation the cold night temperature would drag down the average and few GDD would accumulate. However, at those warm daytime temperatures, plants would definitely be developing and that wouldn’t go into reverse at night. To account for this, I had to adjust nighttime temperatures below freezing up to 0°C which may affect the accuracy of the calculation.
4. In early April, temperatures under the cover were way too hot and plants probably did not develop at those temperatures, which means GDD calculations may overestimate the progress of the plants.

With all that said, the differences between ambient air temperature collected near the ground over grass and under the row cover were dramatic. Maximum temperatures were on average 11 degrees warmer under the cover and average temperatures were 5°C warmer under the cover. Between February 26 when I put the datalogger under the cover and April 9 when I removed the cover, 269 GDD accumulated under the cover compared to 66 GDD over grass, a difference of 203 GDD. Based on normal air temperatures for April and May in this area, in an average year it would take around 40-45 days to accumulate 203 GDD. That means the lavender under the cover could be at the same stage on April 9 as it normally would be around May 20. Given the problems with this test, it is probably closer to May 10 to 15.

Incidentally, the plants look fantastic. They are green and vigorous with no signs of winterkill. They do look like they normally would on May 15. While that may sound like a good thing, that means they are also much more prone to freeze damage. Plants at that stage of development may only be hardy to -5°C, compared to probably -10°C or -15°C in early April. Freezes below -5°C are relatively normal for this time of year and can easily occur for the entire month of April. If I had taken the covers off in early March, they would still be hardy at those temperatures. Plus, it was never cold enough after that period to cause any damage to the lavender if they were uncovered.

As I mentioned, this is only a rough comparison and only applies to the Agryl P-40 cover that I used. I plan to do a more thorough comparison next winter and spring including use of Hybertex Pro, the thicker cover some growers are using. One last thing to consider is the incredible temperatures observed on April 9. The high for the day under the cover was 40.4°C and the low was -2.4°C. That cannot be good for any plant.

As a result of this experiment, my recommendation stands: it is best to remove thinner row covers when most of the snow is gone and before the first sunny and mild stretch of the spring. In my area that is normally around March 10. I would normally look at the forecast at that time and just ensure that the risk of temperatures below -15°C occurring again is low. There may still be cold periods and snow after that time, but the plants should remain dormant and hardier and able to withstand those temperatures.