Wildflower box problems this year

This is my third year growing wildflowers in planters. My planters are 2'x8'x12" high and this year I added two more for a total of five. The three from last year I did a fall seed planting, first time I did this in a planter. There seems to be problems in all boxes.
Baby's breath: In the past years this plant, one of the first to bloom, was merely an accent plant. This year this plant was very aggressive and acted like an invasive weed choking the growth of all other plants in the boxes. In one box I failed to act until the flowers waned and then after pulling the baby's breath I was left with a relatively bare box.
Before pulling: http://www.brandylion.com/gallery/Garden2005/119_1918
After pulling:
http://www.brandylion.com/images/after-pulling.jpg
This box, planted in the spring, has the same problem:
http://www.brandylion.com/gallery/Garden2005/120_2036
There's also a bare spot in that box that I can't figure out. I keep reseeding it but nothing wants to grow there.
Here in Chicago we're suffering from a pretty bad drought requiring me to water everything every day. I water heavily yet all the wildflower planters dry out after one full day in this 90+ degree heat. I'm wondering if perhaps that the severe lack of rainfall in the Spring stunted the other plants allowing an invasive plant like baby's breath to take over. I'd like to figure this out so I don't make the same mistakes next year.
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The plant best suited to the conditions (bb in this case) has succeeded in smothering or stunting weaker competitors, creating a bare patch of ground which it can colonise further. That's what weeds and wildplants do and why people call very successful, persistent wild plants "weeds" when they grow in gardens.You haven't made a mistake in wildflower cultivation, just in wildflower expectation.
In nature, wildflower habitats are never fixed. They are transient, endlessly responding to climate stresses, local insects or animals. Plants compete for light, water, nutrients, pollination. Some weeds are annuals and some long-lived perennials. In whatever wild-seed mix you grow, or different climate conditions, in successive years some plants will disappear and others will grow bigger and stronger. If gardeners mimic the natural controls (fire, flood, tight grazing or insect attack) then the following year sees yet another different set of survivors.
Janet.
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