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Purple loosestrife likes moist soil and is even at home in soggy, poorly drained areas. It needs generous watering when first planted and during the droughty days of summer. Run a sprinkler or drip system for 20 minutes to a half hour every 5 to 7 days when rainfall is sparse. Soak the soil down several inches. A layer of mulch will help keep soil moist (see below). For information on products see the file on Choosing Watering Equipment
When purple loosestrife seedlings are first set out in the spring, sprinkle a teaspoon or so of an all-purpose granular fertilizer on the soil around the plant stems for rain or watering to soak in. Do not allow the fertilizer to touch the leaves or branches of the plant. Every season as the clump of purple loosestrife grows, repeat this process, increasing the amount of fertilizer slightly each year. Beware of overdoing it. Too much fertilizer harms plant roots, and stimulates excessive growth, making plants more vulnerable to disease and pest problems. These are self-reliant plants and do not need heavy feeding. For more information see the file for Fertilizers
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A 2 or 3 inch layer of some organic material spread on the soil over the roots of purple loosestrife offers many benefits. Chopped leaves, shredded bark, wood chips or something similar will discourage weeds, and help the soil retain moisture by reducing water evaporation and run off. For more information see the file on Using Mulch
They do benefit from routine grooming such as pinching off the dead flower stems before they go to seed. This will improve the appearance of the plant and will encourage it to develop subsidiary branches off the main stems which will produce even more flowers.
Although the various kinds of purple loosestrife grow fairly tall, they rarely need staking.
The easiest way to acquire more purple loosestrife plants is to divide the ones you have. After 3 or 4 years each plant will have spread into an overlarge clump. Sometimes the interior of the clump begins to decline. In the spring, revitalize and multiply the plants by gently digging up each clump so that the roots are exposed. Pull apart chunks of the larger plant, taking care to keep the roots and soil intact. Throw away those chunks that seem old and weak. Replant the other smaller chunks in other areas of the yard as described above.