The harder you prune the more rapid regeneration your lavender will undergo.
Those are hard words when you are afraid to cut too much or after watching all this growth happen in one summer to face knocking it all down. But pruning is what you must do for your lavender plants to thrive and live longer.
There are some great videos on YouTube that will show you how to prune, but what does that mean? I have a little plant in my hand ready to go in the ground, now what?
Prune it before you plant it. Just a little off the top, making sure to remove any stems that have developed. This is to help the plants energy to focus on the roots. In this first year you will see some stems and blooms. Cut them, let them grow. You will hear a variety of opinions but most agree prune. So in July, August or September (you have to decide by your plants and this is where the YouTube videos will let you compare size) prune you plants about two inches above the lower woody branches of the plants. You never want to cut into the wood as this retards the growth. Yes, you will be cutting off about 1/3 of the plants, but having tried to grow lavender before and after I knew about pruning I can attest that my pruned plants grew where my non-pruned plant did not.
The following spring your plants might need a trim to shape it if it was blooming late in the fall and the rule of thumb seems to be don't cut the plant too late into the fall. No, I don't have a specific date, as again, it depends on you area. It would be great to see some research on results of pruning in different months, but that is beyond what I can do. So year two you will see more blooms. When you harvest these this is a good time to prune again cutting back about 1/3 of the plant and never cutting into the wood.
Now you have mature plants and this is where pruning can vary. You can prune in the spring or the late summer. One way is to prune when you harvest shaping the plant as you go. Some growers admit to not having enough time to spend on pruning. If you prune in the summer you may need to do a trim in the spring to those ever-bearing varieties that produced stems all the way until frost.
The second thought is when you harvest, not cutting too deep so you don't have all the leaves to contend with the leaves on your stems. Then you will prune in the spring shaping the plant.
How much do you take off when you prune mature plants? Enough to shape it, plus a little. In other words you don't have to take a 1/3 of the plant like you did when they were little. Just remember aggressive pruning extends the plants life and you get better regeneration.
Here in western Colorado we haven't been growing for 10 to 15 years so we can't compare pruning styles yet. Some day we will and will see if the spring pruning made a difference compared to late summer pruning.
What tools do you use to prune? In the first few years hand clippers work great, but when your plants are mature unless you are looking for a good hand workout you want to change tools. Hand shears work and so does a 20-inch electric hedge clipper. So far those who use the electric clippers have not seen damage due to tearing.
Spring: trim the plants as you put in the ground
Late Summer: prune 1/3 - 2 inches above wood and shape plant
Spring: trim left over stems
Late Summer: prune 1/3 and shape plant
Year 3 option 1
Spring: trim left over stems and shape.
Later Summer: prune as you harvest
Year 3 option 2
Spring: prune and shape
Later Summer: harvest