By Sean Westerveld, Ginseng and Herbs Specialist, and McKenna Campbell, Summer Research Assistant, OMAFA

There are several cultivars of Lavandula angustifolia (English lavender) that exhibit a second bloom in our climate. By far the most prolific second bloomer so far is ‘Super Blue’. It is not quite as dense as the first bloom, but enough to put on a show. The problem is that the second bloom is often too late in our climate, occurring in mid- to late-October in Simcoe and often killed off by the first frost. Observations over the years suggest that not pruning the plants after the first bloom can promote a staggered second bloom in August and September, but this is often a weak bloom at any one time and the plants would then have to be pruned at some other time of year to keep them in a rounded shape. ‘Super Blue’ has very short stems and is not well suited for bundles or buds. Its best attributes are its dark colour and dense flower clusters for agritourism. Essential oil extraction is the only practical way to extract value from the flowers, but an earlier second bloom could be its most important purpose.

We conducted a small demonstration in the cultivar trial at the Ontario Crops Research Centre – Simcoe to see how handling of the plants during harvest of the first bloom can influence the timing of the second bloom.

Four plants each of ‘Super Blue’ were handled in one of three ways:

1. Just the flower tops harvested on June 28, 2024 as if an oil harvest had occurred, roughly at peak bloom (Figure 1).
2. A full pruning conducted by removing not only the flowers but a third of the green growth on June 28, 2024. This would be equivalent to harvesting for oil and then pruning on the same day (Figure 1).
3. Allowing plants to flower and never harvesting or pruning so that the growing points were never cut off.

Figure 1. Two ‘Super Blue’ L. angustifolia groups pruned by only removing the flower (left) and pruning well into the green growth (right) on June 28, 2024.

The thought was that harvesting but not pruning or not harvesting at all might promote an earlier staggered bloom, but maybe pruning at the same time would promote a more even rebloom. Our only concern was that the rebloom after pruning would bloom to late to be useful.

To our surprise, both unpruned treatments (with or without oil harvest) just sat there for the longest time with only minimal growth of a few shoots. However, the pruned treatment exhibited rapid re-growth of the entire plant (Figure 2). By the end of August, there were many flower buds, enough for a field of plants to start looking good for agritourism. As of September 5, some of them were starting to flower compared to just a few flowers on the other two treatments (Figure 3). That suggests pruned plants will be in full colour for the entire month of September with a fairly full bloom by mid-September. There are many buds that are still green suggesting bloom could last well into October.

Figure 2: Regrowth of two ‘Super Blue’ L. angustifolia groups pruned by only removing the flower (left) and pruning well into the green growth (right) at they appeared on August 20, 2024.

Figure 3. Comparison of ‘Super Blue’ plants on September 5, 2024: Never pruned or harvested (left), harvested and pruned on June 28 (middle), and flowers harvested for oil but never pruned (right).

There are a few notes of caution from this demonstration:

1. This was not a replicated and randomized experiment. Although the treatment was conducted on four plants of each treatment, this does not count as a full scientifically valid experiment . This was also not repeated to know if the results are consistent under different environmental conditions.
2. The main flower harvest in June was about 10 days early this year in this plot, probably meaning that in a normal year the rebloom would begin about 10 days later.
3. The second bloom would only really be good for agritourism and maybe pick-your-own. It is too uneven to use for other purposes.
4. Simcoe has a longer growing season than areas much farther north, so it is unknown if the second bloom would be too delayed in those areas.
5. It is not known how this practice will affect plant health over time or overwintering, since a vigorous second bloom is stressful on the plants. ‘Super Blue’ can have significant winterkill in some years. Row covers over winter are essential on this cultivar.

Next Steps

Now that we know that a harder pruning can promote much faster regrowth than just a flower harvest, it may be possible to promote an even earlier second bloom by sacrificing the first bloom entirely. Maybe pruning the plants way back in mid-June before they get into full bloom can promote an even earlier second bloom. It may sound counterintuitive to sacrifice a bloom, but think of what a late summer and early fall bloom could do for agritourism. It can open up a whole other season for visitors, photography, entertainment, weddings, etc. The point would be to set up areas of the farm that may not be the main attraction in June and July, but that you would open later in the season. Feel free to conduct your own experiments and let me know how they go! The newer cultivar ‘Blue Jeans’ appears to bloom continuously, at least in its second year, so it may also be worth looking at for this purpose.