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Ask the Master Gardener: Picking the prime time to prune your trees

Pruning at the correct time is important for the health of your trees.

Man pruning a tree.
Pruning during the late dormant season before spring growth begins is best for most trees.
Contributed / Metro Newspaper Service

Dear Master Gardener: Is spring the best time to prune trees?

Answer: Pruning at the correct time is important for the health of your trees. Pruning during the late dormant season (late winter, before spring growth begins) is best for most trees. Oaks should be pruned by now โ€” they should not be pruned in April, May, or June due to the potential for oak wilt infection. Apple trees, including crab apples, should be pruned any time between February and early April because spring or summer pruning will increase the chance for infections and/or the spread of fire blight. Trees that have free-flowing sap, such as maples, butternut, walnut, birch, ironwood, and blue beech can be pruned when their leaves are fully expanded in late spring or early summer to prevent bleeding. Remember not to remove more than one-fourth of the live foliage.

Although there are a few exceptions, evergreens need little pruning. Different types of evergreens should be pruned according to their varied growth habits. Spruces and firs do not grow continuously, but can be pruned anytime because they have lateral (side) buds that will sprout if the terminal (tip) buds are removed. It's probably best to prune them in late winter, before growth begins. Some spring pruning, however, is not harmful.

Pines only put out a single flush of tip growth each spring and then stop growing. Prune before these "candles" of new needles become mature. Pines do not have lateral buds, so removing terminal buds will take away new growing points for that branch. Eventually, this will leave dead stubs. Pines seldom need pruning, but if you want to promote more dense growth, remove up to two-thirds of the length of newly expanded candles. Don't prune further back than the current year's growth.

Arborvitae, junipers, yews and hemlocks grow continuously throughout the growing season. They can be pruned anytime through the middle of summer. Even though these plants will tolerate heavy shearing, their natural form is usually most desirable, so prune only to correct growth defects.

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Dear Master Gardener: I planted two shrub roses last year and wonder when and if I should prune them.

Answer: Spring is the best time to prune roses. Your goal will be to remove dead, diseased and damaged canes, to increase air circulation, and to improve and enhance the plantโ€™s shape. There is often some winter kill, even on shrub roses. Some may die back to the ground but will send out new growth as the weather warms. Always prune back to good, green wood with a diagonal 45-degree cut ยผ inch above an outward facing bud, which means that the new growth at that point will be away from the center of the plant. When possible, you will want to open up the plant center for good air circulation.

April Gardening Tips

  • Donโ€™t be in a hurry to uncover roses and flowering perennials at the first signs of warm weather. Mulch keeps them cool and prevents breaking dormancy too early, when cold weather could still damage them. Begin removing mulch as it thaws โ€” usually around the end of April in the Brainerd area. Keep some mulch on hand to re-cover perennials if frost threatens.
  • Walking on the lawn when itโ€™s still soft and spongy compacts the soil, making it harder for grass to grow and easier for weeds to take over, so keep off it as much as possible until the soil feels firm under foot. 
  • As soon as soils are no longer soft and muddy underfoot, sow lawn grass seed to fill in bare spots and areas where you see vole damage, or to thicken up areas that need it. Rake out dead grass first to make sure that seed comes in direct contact with soil.
  • As soon as the soil is workable, plant seeds for cool-season crops such as radishes, cabbage, Brussels sprouts, spinach, peas, kale, and leaf lettuce. Onion sets can also be planted. 
  • Flowering sweet peas can be seeded as soon as the soil is workable.
  • Plant tomato seeds indoors early to mid-month (depending on the variety). Use a sterile potting mix and keep it evenly moist. They only need 6-8 weeks indoors under lights (ideally) or in a sunny window before they are large enough to transplant into the garden. Putting them out too early when the soil is cool will just set them back and reduce their potential yield.
  • Use graph paper to lay out your edible garden. Be sure to space plants according to their mature height and width.
  • Start seeds for annual flowers such as marigold, cleome, dianthus, nicotiana, sweet alyssum, and salvia.
  • Hydrangea arborescens (Annabelle, Bounty, Endless Summer Bella Anna, Grandiflora, Incrediball) can be cut down to the ground. Ornamental grasses can also be cut down now. Make sure to do it before new growth starts.
  • Wait to fertilize your lawn until you have mowed two or three times. Grass uses fertilizer most efficiently when it is actively growing.
  • Remove pollen-bearing stamens from Easter lilies when you bring them home. The pollen stains.  
  • Ignore daffodils, tulips, and other early perennials that are bowed down by late snowfalls.  Let nature melt the snow and the plants will right themselves.
  • Clean and disinfect your garden tools and containers to reduce the chance of spreading disease.

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.

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