Lavender should be pruned hard every year to keep it dense, bushy, and covered in flowers during Summer.

It's not good to prune Lavender in Winter / late Autumn when it's not in growth, and if you prune it in Summer you will remove all the flowers, so that leaves either Spring or late-Summer / early-Autumn.

Lavender is Typically Hard Pruned After Flowering In the UK

The ideal time to cut back Lavender plants in the UK is after flowering in late Summer: August is probably best, but September is also fine. 

By cutting back your plants right after flowering, the new shoots that follow will have several months until Winter kicks in to develop and toughen up, and the plant as a whole has the longest possible time to prepare for flowering the following year.

Lavender is Typically Hard Pruned In Spring in Warmer Countries

In more Southern parts of Europe where Spring comes earlier, and growing seasons are longer and hotter, it is normal to hard prune Lavender in Spring, when the plants are putting out their first new leaves (around the middle of March in the UK). 

Pruning Lavender this way means you can leave the full head of flower stalks on the plant over Winter, which adds a bit of ornamental value.

  British Pruning Method European Pruning Method
Spring (when first new leaves emerge) Light Trim (cut off the new green leaves only) Hard Prune
Mid-Summer (right after first flush of flowers) Deadhead Deadhead
Late Summer (when colour fades from second flush of flower heads) Hard Prune Either do nothing (leave flower stalks in place over Winter), or trim to deadhead 
     

Can you Hard Prune Lavender in Spring in the UK?

Yes, you can hard prune Lavender in the Spring in the British climate, it won't hurt the plants, but it's not recommended for your regular pruning regime as it will certainly delay flowering and probably reduce the amount of flowers over the season.

British gardeners usually don't hard prune Lavender in the Spring unless there are exceptional circumstances, such as trying to save overgrown older plants (perhaps you inherited them), in order to delay flowering on purpose, or because you didn't get round to pruning them the previous year...

Key Takeaway

The standard regime for Lavender in British gardens is to hard prune them right after they finish flowering, August being the traditional month. 

Hard pruning Lavender in Spring instead is not as good because it gives the plants less time to recover before flowering.

However, Spring pruning is much better than missing a year of pruning, and it's always an option in special situations such as restoring overgrown old plants.

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1949

Lorem ipsum

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut et massa mi. Aliquam in hendrerit urna.

Pellentesque sit amet sapien fringilla, mattis ligula consectetur, ultrices mauris. Maecenas vitae mattis tellus.

1949

Lorem ipsum

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut et massa mi. Aliquam in hendrerit urna.

Pellentesque sit amet sapien fringilla, mattis ligula consectetur, ultrices mauris. Maecenas vitae mattis tellus.

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1949

Lorem ipsum

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut et massa mi. Aliquam in hendrerit urna.

Pellentesque sit amet sapien fringilla, mattis ligula consectetur, ultrices mauris. Maecenas vitae mattis tellus.

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Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ut et massa mi. Aliquam in hendrerit urna. Pellentesque sit amet sapien fringilla, mattis ligula consectetur, ultrices mauris.