One of my astrantias has developed a brown marbling on the leaves. I have several species and the other ones are fine. They are all planted in the same spot.
It sounds like a virus, though I am not certain as to which one. Some varieties of plants, even within the same family, are more susceptible to fungus and disease than others. Other factors, like pests, can also weaken a plant so that it is affected by a fungus while the plants next to it (who are healthier) are fine. I would treat the plant with a fungicide. That should help it recover.
I made a rock garden to look like a rocky river bank of large white quartz and mixed colored rock. It is 2 foot square and 6 inches of drain rock covered with ground cloth and filled with premium potting soil. There is no shade and average temp. is 75 degrees 90 for up to 2 hours in summer. Some snow and lots of rain in winter. This is southwest Oregon, 25 miles from the ocean. Would astrantia, Major Hattie's pin cushion, masterwort be worth planting in this situation?
Actually, astrantia plants normally perform best in moist, shady conditions. You will want to find something suitable for full sun and possibly even drought and heat tolerant. these articles should help with suggestions: https://www.gardeningknowhow.com/ornamental/flowers/fgen/full-sun-plants.htm
https://www.gardeningknowhow.com/special/xeriscape/drought-tolerant-plants-for-nearly-any-landscape.htm
https://www.gardeningknowhow.com/special/xeriscape/drought-tolerant-flowers.htm
Can an Astrantia be started in a pot, then replanted in garden in the spring?
Yes, you can plant Astrantia from seed indoors, then transplant to the garden later. The seeds need to be cold stratified first- in nature this happens outdoors during the winter, but you can achieve it by placing them in the refrigerator for 2 to 3 months. Then they will be ready to plant indoors.
https://www.gardeningknowhow.com/ornamental/flowers/astrantia/astrantia-masterwort-plant.htm
https://www.gardeningknowhow.com/garden-how-to/seeds/seed-stratification.htm