Several daffodils multiplied and choked out my coreopsis between the sidewalk and house. Should I dig up now and replant only one daffodil bulb, and then try dividing/replanting the remaining coreopsis ('Mercury Rising') clump? - D.D. -
The threadleaf coreopsis variety 'Mercury Rising' is one of our favorite new perennials. (Okay, so it’s been around for a decade. We lose track!) We feel for your poor baby, squeezed between concrete foundation, pavement and muscular daffodils. Easy to see how the coreopsis would run out of room to grow.
If a threadleaf coreopsis’ new shoots have nowhere to grow because bulbs beneath are filling and competing for every drop of water and iota of nitrogen, the coreopsis will decline. So step in and divide underlying bulbs to keep the fight fair.
Yes, we would dig up the whole shebang, now, divide to individual daff bulbs and return no more than three good sized single daff bulbs to that spot. Plant them deep, 12" down. Deep, they will spend more of their "dormant" time each year beefing up one bulb and less time making daughter bulbs.
As you backfill the bulb planting hole or holes, add compost in a volume equal to the bulb- and coreopsis debris you removed. It may look like this makes the bed high. It doesn't. The renewed soil will settle and when it does the growing will be better if it is not depressed from loss of organic matter.
The extra bulbs can go wherever you please but do not replant them in this space.
Salvage every piece of coreopsis you can, choosing new, supple rhizomes over drier, woodier bits. Space the pieces to cover however much space you want the coreopsis to cover next year. It will!
You can also rotate crops, try something different in combination with your daffodils. Geranium 'Rozanne' is also long blooming, but larger. Golden feverfew (Tanacetum parthenium 'Aureum') has great foliage and will bloom all summer if you keep it deadheaded. Or you might try something that has attractive foliage in summer before it blooms in fall, such as Sedum cauticola.