Some trees do suppress the growth of other plants through chemical secretions from their roots. This is called allelopathy. I don't think madrone trees do this but there might be a slight suppression.

It is more likely that the madrone trees are just better at pulling all the moisture out of the soil and shading the understory plants. Even if you are watering the other plants regularly, the soil may also be a bit depleted and the madrone trees will suck up all the water you try to give the understory plants. I have had the same situation with large maple trees and other flowers and shrubs planted nearby.

One combination that has worked well is to plant bulbs under the larger trees. The early bulbs get the first crack at the water in late winter or spring before the trees get active. Then the bulbs dry down during late spring and summer when the trees dominate.

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