Frost and Winter Plant Protection

Up until this week, the cold November nights foreshadowed the wintery cold coming soon to a landscape near you. This week’s foot of accumulation really drove that point home! Protecting vulnerable plants from the chills of the fall and winter is a yearly part of putting the garden to bed for the season.

There are two groups of plants needing cold protection: newly planted plants, and vulnerable established plants.

The question is often raised how late can someone plant woody and herbaceous species into the fall. The answer of course is, “as long as the ground is not frozen”. Newly established plants vary in their tolerance to frozen ground. A layer of wood chips or loose leaf litter, held down via a netting or mulch, insulates the soil enough to allow further root growth and enhanced establishment. The protective layer then moderates the effect ambient air temperature has on soil temperature. The key is to occasionally to check how plant tissue is fairing under the protective layer. Mulch does a great job buffering temperatures, but also retains moisture during a season when saprophytic fungi are hard at work. Check for any signs of rot. We are planting a lot (and I mean a lot) of ground cover currently in the Japanese Garden, and it will need a thin layer of protection.

There are a few vulnerable plants elsewhere in the botanic garden needing a layer of protection each fall. Vitex and tea roses benefit from a mound of wood chips, for example. Of course, all our potted material growing in the nursery is placed close together and mulched quite heavily on the tops and sides. This includes the extra tulip bulbs we pot up and overwinter outside. In early spring, these tulips are uncovered and placed in large display pots as part of our spring display. A few weeks of cold, and the tulips begin to grow under all those chips!

I would say not all mulches, nor the amount applied, are created equal. Woody plants and more moisture tolerant herbaceous perennials benefit from thicker layers. Plants that do not like wet feet, such as the thyme being planted copiously in the Japanese Garden, tolerate only a light, airy mulch layer. An alternative to mulching vulnerable herbaceous plants is leaving dead stems throughout the winter as an extra layer. You want enough cover to provide adequate insulation, yet airy enough to allow for good air and moisture drainage.

For those plants not planted yet in the garden, garden staff have to choose different options. Tender perennials over winter inside our small hoop house. Hardy shrubs and perennials simply get a large load of wood chips piled on and around. As for the extra thyme we need for the Island Garden, but did not plant due to a little snowfall, it ended up in a minimally heated storage unit and will hibernate this winter.

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Winter is coming whether you like it or not; you might as well be prepared.

Josh Steffen, Horticulture and Facilities Manager