I’ve had two small perennial astilbe for two years. This year the neighbouring hosta leaves are shading the lower part of the astilbe so I removed some of the hosta leaves. I discovered that although the upper unshaded astilbe leaves are greenish-yellow (or yellowish-green); the shaded ones are a distinctly different soft green.
Now I don’t know whether the upper leaves are getting too much light causing them to be yellow-green OR whether the plants are SUPPOSED to be that colour and the lower leaves were not getting enough light.
Please can someone advise me which way to go on this?
Astilbe come in a variety of heights and colours and look well in a part shade to shade, moist garden. Astilbes are notorious for needing consistent moisture to grow to their full size and bloom. That said, there are a number of things that could cause the yellowing of the leaves of your astilbe. The soft green is the more natural colour of the leaves, although most astilbe have dark green leaves. First, as the leaves emerged in the new garden, before the tree leaves came out, days of strong sun could have scorched the leaves. Some crisping of the leaf edges might indicate that. Second, surrounded by mature hosta, the competition for moisture and nutrients are fierce, and the astilbe are losing. The yellowing may be caused by poor nutrition.
Since you had to cut away the hosta leaves to see the astilbe, the solution would be to move the astilbe, rather than disfigure the hosta. Move them to a spot in partial shade, morning sun preferred, where they can grow to their natural size. Add lots of compost to the planting hole, and water well. Keep them consistently moist, mulch around them, and lightly fertilize them with a low nitrogen, organic fertilizer such as a fish emulsion before the end of June.