Answer: Planting directly under a tree can be risky, as you've discovered. Not only might you damage surface roots in the process, but it's likely you'll water frequently to benefit the smaller plants, which can be detrimental to the tree.
If your dogwood was healthy prior to the addition of the bulbs, it will probably regain its health, given time.
If leaves are turning black and hanging on the tree, you may be dealing with a disease called Botrytis. In rainy seasons this fungus causes fading of the white flower bracts and rots the leaves onto which the bracts drop. A fungicide sprayed in the spring will protect newly emerging leaves and flowers.
Remove all the affected leaves and destroy them. Be sure to rake any fallen leaves and remove them from the garden. You'll need to decide whether to prune the damaged branches out now or wait until spring. Pruning usually signals new plant growth and, this late in the season, could produce new shoots that won't have time to harden off before frost. This will result in even more damage to an already compromised tree and will require additional pruning in the spring. On the other hand, if your tree has botrytis, the fungus can remain all winter, ready to infect healthy plant parts as soon as the weather warms. If it were my tree I'd use a fungicide in the spring, then prune out dead branches, disinfecting the pruners between cuts. (Use 1 part bleach to 9 parts water.) When you're finished pruning and disinfecting, be sure to clean your pruners well and wipe them down with oil so they won't rust!
Hope your tree makes it!
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