Dear Master Gardener: I didn’t cut my hydrangeas back last fall. Should I cut back my Lime Light and Annabelle Hydrangeas now?

Answer: Leaving the flower heads on your hydrangeas to provide winter interest in your landscape is a great idea! And, now is a good time to prune them back, unless they are less than 2 years old. In that case, snip off the old flowers, but wait a few growing seasons to let the bush get better established. Lime Light has cone-shaped flower heads and is a panicle hydrangea (Hydrangea paniculata). The stems should be pruned to just above a fat bud. If your Lime Light is old and large and you want to reduce the size of it, you can do so by removing several of the largest stems near ground level. Annabelle has round flower heads and is a Hydrangea arborescens. You can prune these shrubs down to the ground, or not at all if you want a larger shrub. Flower buds for both these types grow on new wood (this season’s growth).

Related: Ask the Master Gardener: Lilies add beauty and fragrance to any garden

Dear Master Gardener: My husband keeps mowing over the roots of our river birch trees. I know we should make a flower bed under them, but I don’t know what to plant. The city recently limbed them way up so there is more sun reaching the root area. Any ideas?

Answer: Sometimes landscape trees grow roots on top of the surface of the lawn. Mowing on the roots often causes wounds which can allow diseases to enter the tree. In Japanese gardens trees are treated as sculptures, so one option is to leave the roots exposed. First, decide how close to the tree the lawn should be allowed to grow, then edge the lawn, and remove any grass and weeds around the big roots. Add a 2-inch layer of wood chips to cover the soil, but don’t cover the exposed roots. Never cover the roots of a tree with more than two inches of soil or mulch or you may smother the roots and kill them.

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Another option is to plant a ground cover. They tolerate a wide range of soil conditions, block weeds, and create a visually appealing carpet under the tree. Creeping thyme, moss phlox, wild strawberry, and sedum are low-growing and do well in sunny areas. If you want to add some height, color, and attract pollinators, garden phlox (Phlox paniculata) produce bright pink-purple flowers and bloom for a long time (late July through September). Phlox paniculata (the native species) is a butterfly and hummingbird magnet, but avoid its many cultivars because pollinators never, or very rarely, visit them. Birds, bees, and butterflies have evolved with native plants and usually prefer them over those bred by humans (most cultivars). Joe-Pye weed and New England aster are good companions and also attract pollinators.

Related: Ask the Master Gardener: Primulas provide a blast of spring color both indoors and outdoors

Dear Master Gardener: Should I trellis my raspberry bushes?

Answer: Raspberries are a delicious fruit crop to grow in the home or community garden. They are easy to maintain, especially if properly trellised. The goal is to keep the canes and fruit off the ground and leave enough walking space for you to harvest fruit and maintain the plants. According to the University of Minnesota, a raspberry trellis should include sturdy support posts and several levels of wire or twine strung between the posts on either side of the plants. If you use wire you will also need earth anchors behind the end posts, and gripples (wire joiners) or strainers. Both serve to secure the wire and keep it tight. The advantage of using wire is that it will not loosen or wear out over time in the way that twine does. Twine takes less time and fewer supplies to install, but it does not work as well as wire.

Related: Ask the Master Gardener: Get a head start by planting calla lilies in pots

Dear Master Gardener: A friend in another state has a pagoda dogwood and it’s quite beautiful. Can we grow them here in Minnesota?

Answer: Not only do they grow well here they are native to Minnesota! The pagoda dogwood is a beautiful tree with distinctive horizontal branches which curve upward at the ends. The tree produces clusters of creamy white flowers in late May or early June, followed by clusters of dark blue fruit in late summer. In autumn the foliage turns a lovely yellow to burgundy-red. If you want to attract songbirds to your garden this is a fabulous addition because they love the berries (not edible for humans). It will grow in full sun if located in a protected site and given enough moisture, but prefers protection from the hot afternoon sun. In nature it is an understory tree. The mature size of a pagoda dogwood is 20 feet by 20 feet.

Related: Ask the Master Gardener: Air plants growing in popularity among houseplant enthusiasts

April Gardening Tips

  • Lawns are still quite spongy and wet. Hold off on raking and walking on your lawns until they have a chance to dry out or you could damage the grass plants and compact the soil. As soon as the soil is no longer soft and muddy, sow grass seed to fill bare spots or where a little thickening up of the lawn is needed. Avoid seed mixes dominated by perennial ryegrass because this grass is not winter hardy. Better grasses for Minnesota lawns are fine fescues, tall fescues, and Kentucky bluegrasses.

  • Start seeds for tomatoes. They need six to eight weeks indoors under lights or in a bright, sunny window before they are large enough to transplant into the garden.

  • Start seeds for annual flowers such as marigold, cleome, dianthus, nicotiana, sweet alyssum, and salvia.

  • Plant pansies, violas, and johnny jump-ups outdoors at the end of this month or beginning of May to enjoy them as long as possible. They thrive in cool weather and typically don’t hold up once summer temperatures start to rise. They will even withstand light frosts, although cold temperatures could temporarily turn the foliage a purplish color. Keep dead-heading the faded flowers to promote more blooms.

Pansies covered by snow.  Photo by Jennifer Knutson
Pansies covered by snow. Photo by Jennifer Knutson

  • As soon as the soil is workable plant seeds for cool-season crops such as radishes, peas, cabbage, Brussels sprouts, kale, leaf lettuce, and other greens.

  • Don’t worry if we get another snowfall (after all this is Minnesota!) and your daffodils or other early blooming bulbs are up and bent over. As the snow melts, they usually re-orient themselves and begin to grow more upright again on their own. The snow is not cold enough to damage the plants. In fact, it actually protects the foliage from colder air temperature.

  • When choosing an Easter lily, look for a healthy, heavily budded, dense, straight growing plant. Pass up any with yellowing leaves or leaves with brown tips. When you get home remove the pollen-bearing stamens from the flowers because the pollen stains clothing and linens.

  • Begin to remove mulch from tender roses, bulbs, and flowering perennials in slow stages as it thaws and dries. But be ready to protect plants if we get a really late hard frost.

Related: Ask the Master Gardner: 3 options for those in search of striking, exotic house plants

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at umnmastergardener@gmail.com and I will answer you in the column if space allows.
University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.