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http://www.wikihow.com/Grow-Lavender

<font size="2" face="Arial, Helvetica, sans-serif">Pruning is essential       to maintaining a young, healthy and vigorous lavender bush. The secret to       pruning is to start when plants are young and still in pots, by pinching       out the new growth to encourage lateral branching and by cutting off flower       buds in the first year to ensure a larger bush and abundant spikes for the       following season.</font>

<font size="2" face="Arial, Helvetica, sans-serif"> L. stoechas, L. angustifolia       and L. x intermedia may be pruned back by 1/3 to ½. This helps to       keep bushes young and healthy. If cut any lower, the stems will die. Lavenders       that have not been kept well pruned have a tendency to become woody in the       center of the bush.</font>

<font size="2" face="Arial, Helvetica, sans-serif"> Plants should be cut       back at least once a year during their life span. If done in the fall it       should be done well before danger of a hard freeze. Weedeaters and hedge       trimmers work well for pruning. Plants can also be pruned after flowering       in the spring or early summer. </font>

<font size="2" face="Arial, Helvetica, sans-serif">Make sure there are       green leaves to be seen when cutting back. If the plant is cut too far back,       it may not survive.</font>

<font size="2" face="Arial, Helvetica, sans-serif"> Plants that have not       been pruned in the beginning of their growing career may not survive heavy       pruning. If bushes have reached 3 years of age and have never been pruned,       then pruning at this stage may not achieve anything and it may be better       to replace the bush. If, however, there is young growth just above the woody       part, start with lighter pruning to encourage lower leaf growth. Then continue       each year to prune a little heavier. Cut as close to the woody part without       cutting into the wood. Do not cut too far back into the old wood or the       lavender will die. This technique may force new growth further down the       stem and eventually allow the bush to be reshaped. However the success of       this treatment will depend on the type of lavender as some cultivars are       more prone to woodiness than others.</font>

<font size="2" face="Arial, Helvetica, sans-serif"> Lavenders such as dentate       dentata and dentate candicans generally require little pruning unless they       have been grown as a hedge or have become too large for their situation.       The best time to prune in these circumstances is summer. </font>

<font size="2" face="Arial, Helvetica, sans-serif">IMPORTANT:     Pruning a lavender bush to the point where it has no foliage will most likely     kill it. You can try pruning in increments. In spring, cut back by 1/3 to     stimulate new growth. After new foliage has grown in, cut back by 1/3 again     to stimulate new growth at base of the plant. If new growth does come, prune     back to just above the new growth. Never prune out old wood unless it is completely     dead.</font>

<font size="2" face="Arial, Helvetica, sans-serif">From : http://www.lavenderenchantment.com/Growing/pruning.htm</font>

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