Does it harm shrubs to put a layer of flat stones underneath?
Probably not, though you may want to put down some weed barrier first or unwanted plants will soon find their way through. You may have to water more often, and be aware that stones can get quite hot - not all plants appreciate this so keep an eye on your shrubs and remove the stones if necessary. Look out for ant nests, too, and note that, unlike plant and wood-based mulches, stones add no nutrients to the soil.
Is it okay to plant a rhododendron in full sun?
Depends on the particular plant. Many small-leaved varieties will be okay so long as the roots are kept cool with a mulch of compost, leaf matter, pine needles, pea straw or fine bark. Small-leaved rhododendrons are also more tolerant of cold. Flowers are more prolific on plants in sun, though you do need to keep them well watered in the summer months - brown edges on the leaves are usually a sign of stress because of a lack of water.
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