Answer: I think the problem is more with the planting area than with the plant. I'm assuming that the plastic is layered beneath the rock. Plant roots need room to spread and the plastic would block their path. It also doesn't allow for proper drainage. You didn't mention what the depth of this area is, but most shrubs would need at least two feet for root growth. Smaller perennials and flowers would need about one foot.
If you are using the plastic to control weeds, the rock should do that. You are correct that the white rock would both trap and reflect heat, so that can be tough on plants if they don't thrive in sun and heat. (It can also extend the growing season for plants that are frost sensitive.)
I'd suggest removing the plastic, digging up the soil to at least a depth of one foot, improving it with compost, and then planting something. Choose plants whose mature size will fit in your 2 x 5 area. (Hydrangea would outgrow that.) Otherwise, you have to constantly cut them back if they spill over onto walkways, decks, etc. This isn't healthy for the plant long-term. Finally, you might want to use the white rock as a mulch for weed supression, but then cover it with bark chips, compost, or something similar so that heat retention and light reflection isn't a problem. I hope this info helps!
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