I had a wonderful butterfly garden this past spring, summer, and fall. This was its first year of planting. Should I trim the plants that have died back due to frost? I'm worried about destroying eggs or cocoons.
You have asked a very perceptive question! It is true that by cutting back your butterfly garden plants you may destroy some butterfly pupae and even some eggs. Many insect eggs will survive through the winter on standing, dead plant material, though more probably overwinter on living plants. Many insects, Lepidopterous and otherwise, overwinter in dead vegetation both as pupae and adults. Quite often, dead flower stalks add winter interest and structure in the garden. Many gardeners find plants in this stage of their lives to be quite artistically structured and beautiful.. Moreover, standing, dead plant matter often provides both shelter and food to overwintering songbirds. Here is a nice article on the topic of butterfly overwintering.
There is another side of the issue regarding this topic, though. Standing dead plant material harbors pests - including butterfly predators - as well as the more appreciated insect species. Also, many serious fungal pathogens overwinter in dead plant tissues. Cleaning up the garden in winter will help reduce the incidence of both pests and diseases in the new growing season. In fact, cleaning up debris and leaf litter before the growing season is one of the simplest and most effective pest and disease management strategies the gardener can employ.
Leave whatever dead plant material you wish to leave until late winter. At that time, cut it back and clean it up. Plants that you suspect may be harboring your butterfly friends, you might carefully set aside near the garden until any insects have had a chance to emerge come spring. By cleaning your garden you'll surely lose some butterfly eggs and a few butterfly chrysalides, but many will still survive and repopulate your garden when the weather warms.
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