Where should I plant these Herbaceous Perrenials?

I tend to go to the RHS for information. Here is their guide to Lavender.

How to grow lavender / RHS Gardening

 

Ooops! There goes that.

I presume it needs 20-30cm depths as per the instructions to plant on a mound (if heavy soil).

Pity, as the soil type I have available is perfect.

 

Then just plant them on the ground. The ridge is only for heavy, waterlogged clay soil.