Feeding Lavender is one of those gardening tasks that is rarely necessary and therefore needs to be done with care. It’s really about what not to do.
Lavenders, like all hot-climate herbs, are used to surviving in poor soils and generally don’t need feeding. But, they won’t say no to an appropriate, well-measured boost.
GET THE SOIL RIGHT FIRST
Lavenders are happy in poor, friable and free-draining soil. If they have this and a sunny spot, they pretty much look after themselves. Over-feeding, or feeding incorrectly is more dangerous than under-feeding or not feeding at all.
With the exception of truly acidic soils, Lavenders really aren’t that fussy. Therefore, if your soil drops below a pH of 5, add lime. Cheap – and accurate – pH testing kits can be found at Garden Centres and on-line.
FEEDING LAVENDER – POTASH
The one addition that can make a difference is a sprinkling of Potash in the spring. This will encourage more prolific flowering and deepen flower colour.
FEEDING LAVENDER – NITROGEN
Nitrogen is an essential plant nutrient, primarily promoting foliage growth. Too much, therefore, will produce leggy, ungainly plants, especially in Lavenders.
All branded fertilisers have a NPK (Nitrogen/Phosphorus/ Potassium) rating expressed as three figures e.g. 10-10-10. From a Lavender’s perspective, first avoid any fertiliser that lacks either Phosphorus or Potassium, regardless of how low the Nitrogen content is as both Phosphorous and Potassium counter the effect of Nitrogen. So, the following (and similar) formulations would all be unsuitable:
4-0-8; 7-0-14: 13-0-39; 14-0-10; 20-0-5; 26-0-26
And because Nitrogen Deficiency is a common problem elsewhere in the garden there are fertilisers available that are just Nitrogen – all particularly unsuitable for Lavenders (8-0-0 etc.).
Next, best avoid those formulations that have a high/disproportionate Nitrogen content e.g. 22-5-11; 24-5-8; 28-5-5 or even 20-10-10
Instead choose a more balanced fertiliser e.g. 3-3-10; 4-12-12; 6-5-10; 8-6-13; 8-12-8; 9-7-7; 12-5-40; 12-6-9 etc…
You should also avoid fertilisers with urea. Many (dry) fertiliser manufacturers are replacing ammonium nitrate with Urea which has an extremely high Nitrogen content and “is the most readily volatilized of the dry nitrogen materials.” In other words, plants can absorb Nitrogen from Urea faster than from any other source.
Seaweed and coffee grounds – both have a high Nitrogen content.
It is increasingly fashionable to mix-and-match flowering plants and/or shrubs with vegetables, especially the more attractive varieties. There’s no reason why you shouldn’t plant Lavenders among, say, lettuces or brassicas but avoid nitrogen fixing legumes – peas, beans etc.