Answer: Loosestrife is a moisture-loving plant but it is unlikely that one plant could outcompete established shrubs to that extent; loosestrife will also survive quite happily under normal garden conditions and does not require a wet soil. A full underplanting of lythrum however could conceivably cause a problem -- the root system is huge. You should also note that loosestrife is illegal in many states because of its ability to spread quickly by seed and displace many native plants in the wild, especially in wet areas; for this same reason many gardeners refuse to plant it.
Lilacs are generally very sturdy and undemanding plants needing only full sun and a moderately fertile soil to bloom well. Possibly the most common reason for bloom failure is improper pruning. Lilac bloooms on old wood so it should be pruned only immediately after it blooms.
Mountain Laurel on the other hand is a somewhat delicate plant in that it requires an acid, humusy, moist yet well drained soil. It is shallow rooted and will not tolerate either poor drainage or drought; it also resents wind.
Unfortunately, based on your description I can't be more specific as to what could have caused the problems with your shrubs. You might wish to check with your County Extension (494-3003) and see if they can help you troubleshoot also.
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