Lavandin cultivars are in full bloom across the province, while bloom of angustifolia cultivars is starting to wind down. Now is a good time to harvest angustifolias for essential oil. Bundle and bud harvest of lavandins should be winding down, while this would still be a good time to harvest fresh bundles from these cultivars.
With plants in full bloom, now is a good time to compare the size and shape of different plants and determine if some might be mislabelled or off-types. This is especially important if you intend to take cuttings from your plants later in the summer for propagation. Flag any abnormal plants so the cuttings are not mixed up later on. Most of the time when there are abnormal plants it is due to a mix-up at the propagation stage. Occasionally though, a mutant will develop with unique characteristics. Sometimes these can be a source for a new cultivar.
Once harvest is complete, the plant will transition to vegetative growth. This is a good time to fertilize with nitrogen (N) to encourage vigorous new growth. Based on our lavender nitrogen research trial, it appears that around 120 kg/ha per year provides the peak yield. This rate should be adjusted based on the size of the plants. Growers also need to decide whether to split the application in multiple applications or apply it all at once. Splitting the application is ideal, as it provides a steadier supply of nitrogen to the plant over the season. If you are splitting the application, begin fertilization when the plants green up in the spring and continue to around mid-August. After bloom is a critical time to have nitrogen available to the crop, so if applying all of the fertilizer at once, this is probably the best time to apply it.
Calculating Fertilizer Amounts
Calculating the amount of fertilizer to apply to reach the goal of 120 kg/ha over the season is relatively straight forward if you know the size of your plants, your row spacing and the N-P-K numbers on your fertilizer bag. The N-P-K numbers on your fertilizer bag represent the percentage of the fertilizer that is nitrogen, phosphorus and potassium. You will need to determine what percentage of the yearly amount you want to apply now. For example, if your fertilizer is being split in three applications during the season, then only 1/3 of the 120 kg/ha N would be applied now.
Consider the width of the plant at full bloom as the fertilization zone if you are banding the fertilizer near the plants or applying through the drip irrigation lines. For new plants, consider them a minimum of 60 cm (2 ft) wide. If you are broadcasting the fertilizer over the whole area, or the plants touch each other between the rows at full bloom, consider the plant width to be equal to the row spacing. This calculation is much easier to conduct in metric, so convert your row spacing and plant width to metric before you begin (feet x 3.28 = metres; or, inches x 39.4 = metres).
Here is the calculation using metres:
Part 1: (Row length x number of rows x plant width at bloom) ÷ 10,000 = hectares to be fertilized
Part 2: (Hectares to be fertilized x fertilizer amount in kilograms) ÷ (%N ÷ 100) = amount of fertilizer product to apply to the lavender rows or area.
Here is an example of the use of the calculations:
Kelly has 15 rows that are 90 metres long each. The rows are spaced 180 cm (1.8 m) apart and the plants are 60 cm (0.6 m) wide at full bloom. He splits his application in three equal amounts of 40 kg/ha, with one of these applications after flowering (e.g. now). He is going to apply calcium ammonium nitrate, which shows 27-0-0 on the bag (= 27% nitrogen, 0% phosphorus and 0% potassium).
90 metres x 15 rows x 0.6 metres plant width = 810 metres2 ÷ 10,000 metres2/hectare = 0.081 hectares
0.081 hectares x 40 kg fertilizer per hectare ÷ (27 ÷ 100) = 12 kg of calcium ammonium nitrate product required to fertilize the lavender area.
The same calculation is used for organic sources of nutrients. Commercial bagged products often have the same N-P-K numbers on them. For other sources (e.g. compost, manure) it is best to get a nutrient test done on them first at an accredited lab to determine the percentage of different nutrients in the product.