The notion that lavender does not need or like fertilizer has been proven false. While lavender seems to survive fine without fertilizer or irrigation, it definitely performs better with both. Your plants may look perfectly fine without fertilization but you do not know how much better they could perform with fertilization.
For nitrogen, research has shown an optimal nitrogen rate of around 80-100 kg/ha every year. Based on similar crops, this is probably best split into at least 2 applications over the season with some applied at green-up in the spring and some applied after bloom. In the nitrogen trial, we applied it in three applications in early June, early July and early August. Do not apply nitrogen after about mid-August to ensure plants harden off before winter.
Many lavender growers have questions on how to calculate the amount of fertilizer to apply per plant, especially when dealing with small and irregular areas. The calculation is not too complicated but first you need to determine how much area your lavender occupies.
Determining the Application Area
Most growers have lavender on plastic mulch with grass between the rows. The grass probably doesn’t need the nitrogen, but will definitely steal it from the lavender, since the grass roots will go right under the plastic mulch. At a minimum, you need to consider fertilizing an area that includes a strip of grass on each side of the row. The width to fertilize can be adjusted after a few fertilizer application by looking at how wide a strip of grass is brighter green next to the row.
- If you have drip irrigation, you can fertigate the nitrogen to the plants directly and will only need to consider a small amount of grass on each side of the row (e.g. 15 cm on either side of the row)
- If you are banding the nitrogen as a granule on the grass next to the row, consider a little wider area of grass (e.g. 30 cm on either side of the row)
- If you are broadcasting the fertilizer over the grass, then the entire area needs to be considered in the calculation.
For fertigated and banded application, fertilize the width of row occupied by the plants at full bloom at a minimum, even if that is wider than the grass strips discussed above.
For calculating fertilizer rates, it is best to use the metric system because of the easy conversion between units. You can either do the calculation per plant, per row, or per area; there is no difference in the calculation method. Here is the formula:
Fertilizer to apply in kilograms = ((Length x Width/10,000) * N rate)/(Percent N in Fertilizer/100)
Units: Length and width should be measured in metres. N rate should be in kilograms per hectare. If the answer is a decimal, multiply by a 1,000 to convert kilograms to grams.
Interpretation by Production Method
Per Plant: Length = in-row spacing in metres; Width = width of the row you need to fertilize in metres (as discussed above). Once you convert to grams, weigh the amount you need per plant. Find a scoop that fits that volume of fertilizer and then use that scoop to apply the fertilizer to each plant. That way you do not have to weigh out the amount for every single plant.
Banded: Length = length of row in metres; Width = width of the row you need to fertilize. Once you have the amount per row, if all your rows are the same length than you can find a scoop that fits that volume and use that volume on each row, carefully spreading it evenly on each side of the row.
Broadcast: Use the calculation as is considering the entire length and width of the planted area. Remember when measuring the width and broadcasting the fertilizer to include half a row-width past the plants on each side of the plot or you will only be fertilizing half of each of the outside two rows.
Fertigation: Calculate the area you are fertigating at once. Calculate and weigh the required amount of fertilizer for that section. Dilute the volume in a pail (you may need to inject multiple pails if that amount of fertilizer cannot be dissolved in one pail of water). Inject the fertilizer and once it has all be syphoned in, irrigate for an additional 15 minutes to flush the fertilizer out of the line.
Let’s say you want to split 100 kg/ha of N among three applications per year (33 kg per application).
Here are the crop details for this example:
- 15 rows of lavender with 100 plants in each
- In-row spacing of 24” = 60 cm = 0.6 m in row spacing
- 60 m length of each row
- Rows are 8 ft apart = 2.4 m with grass between the rows.
- Plants are 2 years old and are approximately 2 ft = 0.6 m wide at full bloom.
You plan to band the fertilizer as a granule by hand in the grass on either side of the row. However, since you are banding the fertilizer into the grass, you want to consider around 30 cm of grass on either side of the plastic. The plastic is about 1.5 ft = 45 cm wide. Your total fertilized width would then be 30 cm + 45 cm + 30 cm = 105 cm = 1.05 m wide. You plan to use a slow release fertilizer, so you do not burn the grass under the band. The fertilizer you have chosen is ESN Polymer Coated Urea which is 44-0-0, which means it is 44% nitrogen.
How much fertilizer do you need to apply per row or per plant?
Fertilizer to apply = ((Length x Width/10,000) * N rate)/(Percent N in Fertilizer/100)
Fertilizer to apply = (60 m x 1.05 m/10,000 m2/ha) * 33 kg/ha)/(44/100)
Fertilizer to apply = (0.0063 * 33)/0.44
Fertilizer to apply = 0.4725 kg * 1,000 g/kg = 472.5 g per row
Since you are applying to both sides of the row, 472.5/2 = 236.25 g on each side of the row. Or, since you have 100 plants per row, 236.25/100 = 2.36 g on either side of each plant.
How much fertilizer total will you need to purchase for the year?
15 rows x 0.4725 kg per row x 3 applications per year = 21.26 kg
What if you are fertigating?
If you are fertigating the width of the row could be assumed to be a bit smaller (e.g. around 15 cm of grass on either side of the row). The plastic is 45 cm wide: 45 +15 +15 = 75 cm = 0.75 m wide. You would also be using a greenhouse grade, water soluble fertilizer such as ammonium nitrate (34-0-0). If the drip irrigation system waters all the rows at once:
Fertilizer to apply = ((Length x Width/10,000) * N rate)/(Percent N in Fertilizer/100)
Fertilizer to apply = ((60 x 0.75 x 15 rows/10,000)*33)/(34/100)
Fertilizer to apply = ((675/10,000)*33)/0.34
Fertilizer to apply = 6.55 kg
You would need to dissolve 6.55 kg of the fertilizer in a pail (may take several batches unless you have a large tank) and syphon them into the irrigation system, followed by 15 minutes of irrigation to flush out the lines.
What about new plants?
For new plants, you can fertilize by hand with a water-soluble fertilizer and just apply enough water to wet the area around the plug with a fertilizer solution dilute enough to not burn the plant (i.e. no more than the label recommends for biweekly watering). Later in the summer, increase the amount applied to account for the expanding root zone. Alternatively you can just assume the width of area to fertilize is much smaller (e.g. 30 cm wide strip).
What about phosphorus and potassium?
The requirements of lavender for other nutrients are not fully understood. It is best to manage these nutrients as you would for an average crop. The calculations for these nutrients are exactly the same as for nitrogen, but they are best applied ahead of planting to the whole field according to a soil test. They can also be fertigated in as a water-soluble form if deficiency shows up. For more information on estimating P and K requirements for lavender consult OMAFRA’s Specialty Cropportunities Resource.
What if you are broadcasting or banding an organic N source?
The calculations for determining the amount to apply are no different if you are applying manure, compost or other organic sources. You just need to know the percentage of the nutrient in the organic source. This is best done by sending a sample of the source to an OMAFRA accredited lab for analysis, because different sources of the same manure or compost type can vary greatly in the amount of nutrients they hold. OMAFRA has some resources that provide average nutrient content of a wide range of manure types. Consult OMAFRA Factsheet 13-043 for more information. Keep in mind that these sources release some of the nutrients more slowly and manure applied one year may have carry over value the following year. This needs to be calculated into the following year’s calculations. More information on this can be found in the resource below
For more information on all aspects of fertilization consult OMAFRA Publication 611, Soil Fertility Handbook.