Ornamental Garden Calendar for April

April is traditionally a busy month in the garden. Spring cleanup begins and there are a host of other tasks to tend to. Following is a list of tasks and things to look out for, in your ornamental gardens in April. Note: these times are approximate and can vary depending on the weather and on your specific hardiness zone. The zone I write from is a zone 6, adjust your dates accordingly.

Trees and Shrubs

  • Finish any dormant pruning that you did not get finished in March. See When To Prune Trees, Shrubs, Roses and Vines
  • Remove tree wraps from tree trunks as the weather begins to warm up this month.
  • Cleanup tree and shrub debris, especially any that could be harbouring insect pests or disease from last season.
  • Apply dormant oil spray to your trees and shrubs to kill over wintering insects and fungal spores. When applying dormant oil the temperature must be above 5 degrees C. and remain frost free for at least 24 hours. Apply the spray before before bud break. Caution: Do not spray most evergreens, maples, redbuds, hickories, beech or walnut trees as they are susceptible to dormant oil damage, as reflected by blackened branches or yellowing foliage. A few exceptions are when spraying for pine needle scale and euonymus scale, but the oil should only be applied at the lightest dilutions possible. For a more inclusive list of plants sensitive to dormant oil spray.
  • Examine shrubs for winter injury. Prune out all dead and weakened wood.
  • Shrubs and trees best planted or transplanted in spring, rather than fall, include butterfly bush, dogwood, rose of Sharon, black gum (Nyssa), vitex, red bud, magnolia, tulip poplar, birch, ginkgo, hawthorn and most oaks.
  • Prune spring flowering ornamentals after they finish blooming.
  • Begin preventative spray treatments (as buds begin to expand) for fire blight susceptible trees and shrubs including purple sand cherries, crabapples, serviceberries, flowering quinces, cotoneasters, pears, hawthorns, pyracanthas, blackberries, raspberries, mountain ash and apples. Spray options for Ontario home gardeners include streptomycin and Bordo.
  • Be on the lookout for boxwood blight, especially during the damp rainy season. If you have had it in the past spray them now with a protective fungicide like Bordo spray.
  • Examine boxwood leaves for signs of leaf miners. If quite a few leaves are affected prune them off towards the end of April to mid May (before the adults emerge from the leaves at the end of May). Remove all these clippings and burn or black bag them. Apply a systemic insecticidal soil drench, mid to late April l to target the overwintering larvae. Note only prune boxwood at this time if leaf miners are abundant other wise save boxwood pruning until mid-May or June.
  • Examine your eastern red cedar and Junipers for galls, caused by apple cedar rust, and prune them out before the orange jelly-like spore-producing structures emerge from the galls. These fungal spores spread rust diseases to apples, crabapples and hawthorns.
  • Examine the underside of viburnum branches for ovipositor sites. The viburnum leaf beetle lays it’s eggs in rows on the underside of viburnum branches. Prune out these areas before they hatch and spray the shrub with dormant oil.
  • Examine magnolia trees for magnolia scale and spray with dormant oil before bud break.
  • Scout for and remove tent caterpillar web nests.
  • Continue scouting for gypsy moth egg laying sites. Spray any masses you find on tree trunks and branches with dormant oil.
  • In late April or early May, inspect for and treat holly leaf miners as new leaves emerge.
  • Treat now to help control lace-bugs if they were a problem on azaleas or other plants last year.
  • Scout for and control common spring insect pests such as aphids, spruce mites, and pine sawflies.
  • Plant bare-root or potted trees as soon as the soil can be worked.
  • Begin fertilizing trees and shrubs once the frost is out of the ground.
  • Begin protective fungicide spray for apple, crabapple and hawthorn trees, if warranted. Begin spraying at bud break, spraying every 10-14 days (as per manufactures instructions), until pedal fall.
  • Before introducing new plants to your landscape carefully examine them for any signs of galls on roots or branches, inspect also for insects like mealy bugs and scale. Also watch for other alarm bells like weak, spindly growth, yellowing leaves, blackened roots etc.
  • Spider mites and codling moths become active on apples. 
  • As soon as young junipers begin actively growing begin protective fungicide treatments for Phomopsis blight, where warranted. Avoid pruning junipers during wet spring weather, best to wait until dry summer weather.
  • If you live in the Toronto area examine boxwoods carefully for signs of box tree moth hibernariums. If found treat with dormant oil spray.
  • Spruce spider mites begin hatching when saucer Magnolias have pink buds, begin monitoring for this pest.

Note: Protect bees and other pollinating insects. Do not spray insecticides on trees and shrubs that are blooming.

Perennials and Annuals

  • Frost is still possible this month. Do not uncover plants or plant tender plants too early. 
  • Fluctuations in temperature can result in damage to flower buds of plants such as hydrangeas and magnolias.
  • Remove any garden debris to expose overwintering insects.
  • Remove mulch from roses in mid-April to limit damage from stem canker diseases and prune according to it’s type.
  • Fertilize established roses once new growth is 2 inches long and begin spraying to control black spot disease.
  • Easter lilies past blooming can be planted outdoors. Set the bulbs 2 to 3 inches deeper than they grew in the pot. Mulch well to protect from spring frosts.
  • Once the perennials get actively growing start fertilizing.
  • Once the soil dries up a bit it is a good time to do any splitting or relocating of perennials.
  • Begin planting out summer bulbs such as caladiums, gladioli and acidanthera. 
  • If you have not yet pruned your Clematis they may be pruned at this time, on a dry day, with no rain in the forecast. Type C (group 3) clematis can be prune back to a healthy pair of buds about 30cm (1′) from the ground. Type B (group 2) clematis are pruned very lightly, pruning off only broken or diseased stems.
  • As clematis begin vigorously growing check the stems regularly for reddish lesions on the stems and/or spotting on the leaves and stems, that could be an indication of clematis wilt disease.
  • Lavender plants may be cut back at this time. Do not cut back beyond the leafy area into the woody part of the plant or you may cause the plant to dieback to the ground; which will very much delay your bloom.
  • Pull the stems of any rhubarb plants damaged by frost as they may contain oxalic acid.
  • When crabapples are in bloom, hardy annuals (like snap dragons and pansies) may be transplanted outdoors.

Weeds

  • The winter annual weeds (that germinated in the fall and over wintered) begin flowering in the spring. It is important in April, May and June to keep these pulled before they set seed. Some winter annual weeds are: Hairy bittercress, yellow rocket, whitlow-grass, Henbit, purple dead nettle, common chick weed, corn speedwell, Persian speedwell, annual bluegrass.
  • A few perennial weeds also germinated in the fall such as dandelions, clover and plantain and will be in flower this month.

Soil Work

  • Annual garden beds and vegetable gardens can be dug and soil amendments aimed at improving soil structure, drainage or soil pH can be applied now. Hold off on adding compost, composted/manures or fertilizers in these annual beds until planting time.
  • Do not work wet soils.

Turf Grass

  • Rake lawns to remove dead debris from winter.
  • Apply cornmeal gluten by mid-April or by the time forsythia is blooming. Cornmeal gluten will also prevent grass seeds from germinating, wait 6 weeks before over-seeding your lawn.
  • If you fertilized in the fall hold off fertilizing until May. If you do fertilize now use a low nitrogen fertilizer. Nitrogen applied in spring encourages excess growth, which is more susceptible to disease.
  • Aerate turf if thatch is heavy or if soil is compacted.
  • Start mowing cool season grasses at recommended heights.

Miscellaneous

  • Hummingbirds return from their winter home in Central America.
  • Wasp and hornet queens begin nesting.
  • Carpenter bees may be seen buzzing around decks and other wooden structures. Although males appear aggressive when defending their territory they can not sting. Females do not defend their nests but can sting if handled.

Photo credits: all photos by the author.

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