Selecting the right sized lavender plant can make a significant different to the cost of your planting, the speed of establishment and the final structure.
Buy plants in small pots (P9) for window boxes and other plantings into cramped spaces - they tend to be only a year old, but they do better in small containers. If planting outside, do not do so before the end of May and do not expect a solid hedge to form as quickly as with larger plants.
Look for plants in medium-sized pots for lavender borders, hedges and edges. Two-year-old plants in 1.5 litre pots are ideal for this. You get more root and more flower in the first year, and they do not look lost planted at one plant every 12" (30cms). By the end of the first summer, they will all have joined up, and you will have a solid lavender hedge.
If you want lavender as a feature, then buy larger plants in 3 litre pots. We suggest you avoid any in pot sizes above 3 litres - they are expensive and the results can be "uneven". If these large plants are on a special offer discount, they may also have outgrown their pot and so are too pot bound to establish well elsewhere.