Lavender in your garden.

angustifolia | Downderry Nursery
Spacing
For informal plantings we recommend 45cm-90cm (18in-36in) between plants, depending on their eventual size.
Planting in groups of three is very effective.
For hedging, lavenders to 60cm (24in) and rosemaries may be planted 40-45cm (15-18in) apart.
For a formal lavender hedge use one type – the effect is stunning!
Any of the angustifolia and x intermedia lavenders make a fine hedge as do all upright rosemaries.

Alan Titchmarsh's tips on growing lavender in your garden | Garden | Life & Style | Daily Express
Lavender is part of the genus Lavandula with about 39 species many found in the Mediterranean and warm climates.

Which lavender?

Lavandula angustifolia “Hidcote” – dark-purple flowers, grey foliage; bushy plants ideal for dwarf edging; height 18-20in.
Lavandula angustifolia “Rosea” – probably the best pink lavender; height 24in.
Lavandula angustifolia “Blue Cushion” – compact, good for containers; purple/ blue flowers; height 18in.
Lavandula x angustifolia “Sawyers” – chunky plants with silver foliage and purple flowers; height 28in.
Lavandula x intermedia “Walberton’s Silver Edge” – one of the few variegated lavenders, each grey leaf is outlined in cream, with soft flowers; height 18-24in.
Lavandula viridis – green lavender with lime-green flowers and grass-green, needle-like leaves; has an unusual resinous scent; height 18-24in.
Lavandula “Fathead” – compact with fat, round, maroon flowers with pink ears; height 18in.
Lavandula stoechas subsp. stoechas (French lavender) – bushy plants with purple flowers; foliage has a hint of camphor; height 20in.
Lavandula stoechas subsp. pedunculata (Spanish lavender) – tall, upright plants with slightly camphorous leaves and mauve flowers; height 30in.
Lavandula pinnatan – non-hardy species; keep in conservatories in winter and on patios in summer; height 18in.
A Gardener's Guide to Lavender

Lavender plants are great investments for borders!
IF its pruned at least once a year beginning its first year. Without pruning they tend to become woody in the center and sprawl.
An Important Point
Never cut into or cut off any of the older wood of a lavender plant unless you’re sure it’s dead.
To tell a dead branch you can scrape the stem and if you see any green it’s alive.

Cutting old wood will kill the plant. (Unfortunately, I’ve done it.)
Always cut only into the new growth – called soft wood.

Pruning a First Year Plant
The best time to prune a first year plant is in the spring just after the flower blossoms on the stalks start to open.
Holding the branches of a year old plant together in one hand cut the stalks all the way to 2 inches above where the soft wood starts. (The soft wood is where the new growth is.) From the top of the stalks to 2 inches above where the soft wood (new growth) starts will probably be approximately 1/3 of the plant.

Save Those Flower Stalks
Put them in a vase WITHOUT water and enjoy them in the house.
I like to put them with bed linens to make them smell wonderful.

Pruning the Second Year
Your lavender bush will double in size by year two. Prune in the spring after flowering and prune approximately 1/3 of the plant. This will take off the flower stalks and about 1 or 2 inches of soft wood (new growth).
Pruning the Third Year
By its third year the bush will have more than have doubled last year’s size and you should have a good size bush.
You can prune in the spring after flowering or in the fall.
If you prune in the fall do it well before danger of a hard freeze, otherwise your lavender could suffer damage.

For Longer Bloom Time
Once my bushes reach 3 years I like to dead head the stalks for longer bloom time.
Just keep in mind that dead heading is not pruning.

Final Words
If you don’t have lavender in your borders or garden I urge you to get several.
Or if you’re patient — start them from seed via the wintersown method.
The flowers smell wonderful and bring lots of pollinators.
It’s one of the most enjoyable plants you could have – especially if you start pruning its the first year and once every year of its long life.

I definitely belong in the twice-a-year camp.
I presume that, like mine, your 'Hidcote' lavender has just about finished flowering, although it may (tantalisingly but rather annoyingly) still be carrying just a few flowers.
I always cut my earlyish flowering lavender such as this one twice, once in August – even though this means cutting off the stragglers – and again in late February.
The first cut is surprisingly hard.
I take the shears and trim it right back, removing the old flowers and their stalks, and another 2-3in of foliage so that for a while the bushes look like grey hedgehogs – just the lowest few pairs of tiny emerging whiskery grey shoots in evidence.
The bushes recover during the next few weeks and become pleasing hummocks for winter.
Then in February (or March if the weather is revolting), I give the bushes a light trim and away they go.
I find two cuts a year keeps them compact and dumpy, which for a lavender hedge is particularly important.

What to grow instead of lavender?
If you have no success with Lavenders because of the growing conditions, but want that type of effect, the next best substitute is Nepeta (Cat Mint)