Lavender likes full sun. An hour or two of shade is ok but too much will cause the plants to be leggy. Bloom poorly and more susceptible to fungal disease.
Lavender needs a very well drained, gritty, alkaline soil (ph 6.5-7.5 or even higher). They do not tolerate heavy, wet, or poorly drained soils. To improve drainage, plant on raised mounds or beds to drain away excess water. Avoid planting at the bottom of a slope or in a low depression that may collect run-off water.
Use pea gravel, decomposed granite or oyster shells to deflect water that may splash up on the base of the plant. Hardwood mulches (like pine bark) may be helpful during extreme droughty times and may reduce fungal disease problems but too much mulch around the base of the plant can hold in too much moisture during wet periods. Mulch in between plants in a row but avoid having the bark too close to the base of the plant stalk.
Lavender wants to be dry and needs little water once established. During the first year, if there is no rainfall, water regularly (once every week or two) when the soil is dry down to the root zone. A drip irrigation system is helpful to control the amount of water applied and is a more even method. Place emitters 4"–6" away from the stem. Avoid overhead sprinkler systems, this causes the plant to "splay" open and it promotes fungal disease. Once established, water during times of drought, especially prior to blooming, to help promote flowering and improve oil yield.
A light feeding of a well balanced fertilizer the first year is helpful to get the plant off to a good start. Once established, side dress in early spring or spray with compost tea for organic treatment.
Give ample room around each plant for good air circulation. High humidity, heat and moisture can cause fungal root, crown & stem diseases. If you suspect disease, send a sample off immediately to Texas A&M plant pathology lab for diagnosis and recommended treatment. Plant shield and actinovate or a specially formulated compost tea, may be applied every 3-4 months and are best used as a preventative.
Trim plants back lightly just after blooming. Cut back by 1/3 to 1/2 in early spring to promote new, vigorous growth and encourage blooms. Your plant will be healthier and live longer with yearly pruning practices.