Hello and thank you for contacting us about pruning lavender. It would be helpful to know whether yours is an English Lavender or not since the pruning is somewhat different. Here are the important points to remember:

English lavender 
Blooms on new growth – pruning early or late in the season is beneficial.
Start pruning young plants in the second year after planting and repeat annually.

If you like a neat, tidy plant: 
1. Lightly prune new growth in early spring to gently shape and control plant size before budding.
2. Do a midseason cleanup after the first bloom/harvest to encourage a potential second bloom. Shape the plant and remove any dead growth.
3. Make deeper cuts during the final pruning in the fall after the final round of blooms – remove green plant growth by 1/3 to 1/2 (depending on variety) to shape into a tidy mound for winter. Leave at least two nodes of the plant foliage above the woody stems. (See attachment.)

Never prune into the woody stems of lavender.
New growth is generated only from the green plant material left above the stem after pruning.
The woody stems at the base of the plant will not generate new growth.

Pruning Spanish & French Lavender
These are the least hardy of the lavenders and need a more gentle touch. Do not cut them back hard as with the other lavenders – it can kill them.
Give them a trim after the first flush of flowers fades. Deadhead for the rest of the season. Shape foliage into a mounded form in late summer.