Answer: Based on your description it is hard to make a diagnosis. Yarrow is a very tough plant with good drought tolerance and may surprise you by coming back from the roots next spring. Scabiosa is also a sturdy plant but it does prefer ample moisture during the growing season in order to keep blooming. Generally, too, it is better to leave the basal foliage on scabiosa for the winter to help protect it a bit. I think I would adopt a wait and see approach, but be prepared to replace them if need be. When you do so, make sure the soil is well prepared, water them in well at planting, mulch around them with several inches of organic mulch such as shredded bark, and then water them as needed to keep the soil moist but not soggy. A weekly deep soaking is preferable to a daily light sprinkling, and the best way to know when to water is to check the soil with your finger -- the rule of thumb is an inch of water a week from the sky or the hose. Finally, removing spent flowers is almost a necessity to keep the scabiosa blooming over a long period, and the yarrow will look better if treated this way, too.
Q&A Library Searching Tips