I have never cut back any of my lavenders because they have such a hard time making it here in south Texas because of the combined high heat and high humidity.

I have read that after blooming you can prune them back about a third, but it isn't recommended to cut them all the way down. I don't know if your lavenders will come back or not since you live in Kentucky where it gets much colder than here in zone 9. I suggest that you heavily mulch over what remains and see if they will come back in the spring.

You may have committed a 'woops'. Most lavender can be deadheaded, then pruned when new growth begins in Spring, being careful not to cut too far back in old wood where it may be reluctant to sprout new growth. Spanish lavender is longer blooming, and requires slightly different care -

"Pruning Lavandula stoeches (Spanish Lavender) can be a challenge in that in some regions it never stops blooming. One recommendation would be to prune it at least once a season (Early Summer is best), removing stray growth and shaping the plant back to a manageable size. Avoid fall pruning of Spanish lavender and leave plenty of leaf material. "

The issue could be somewhat moot, as Sapnish lavender is not as cold hardy as English lavender or the lavandin hybrids and it may not survive a Kentucky winter. But generally, pruning a lavender back into the woody portion or behind all foliar growth is too severe a treatment (especially before winter) and the plant is unlikely to respond. Lavenders are really evergreen shrubs and few evergreens will respond well to that type of radical pruning.

The habit of Spanish lavender is a bit different from most other forms also. It tends to be far more of a sprawler than a tidy mounding shrub and that complicates pruning as well. Most resources will recommend pruning in spring as new growth emerges although a light trimming at the end of the primary bloom season will refresh the plant and encourage a new flush. Avoid any heavy pruning in fall or winter.

According to the pink bunny's ears lavender in the front yard: if the wood isn't too old then there can be some 'back budding' - new growth on bare wood. As you have fairly new plants you may be in luck.

Your main challenge will be to keep any new shoots protected from late frosts - and excess damp. Excess damp tends to turn the foliage into black mush.

When you cut back use the colour of the stems for guidance. There are the newest, still green ones. When you snip back the dead flower heads go down into a pair of leaf tufts to get more dormant buds to start.

To keep some semblance of symmetrical shape (ha! I wish!!) cut back below the green where the wood is a warm brown, possibly with a touch of yellow - and there are still leaf tufts along the stems. Go to a healthy pair facing the direction you need.

Just in case - take some cuttings in spring and nurture them through to the following spring. They'll often come away in sharp sand or vermiculite. Ensure plenty of air otherwise they'll happily rot before your eyes.

For mulching - I'd choose pebbles. Nice rounded ones if they suit your decor. They keep the moisture in the ground, suppress the weeds, and reflect heat up into the branches to ripen them up for winter. Keep them back from the stem, though, by at least an inch. Thick enough to hide the soil and thwart the weed seeds.

Here in Los Angeles with 2 new French lavender plants planted by the landscaper. When I deadhead the old blooms, do I do each one as it dies, or do I wait for all the flowers to die before I do that? Help! Don't want to hurt the plant.