Plant Information

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Smart Gardener

April

PHOTO: Seedlings in pots.We admit to a weakness for daffodils here—last time we counted, we came up with quite a grand total: more than half a million daffodils are planted at the Chicago Botanic Garden!

April Checklist

orchid NovemberPlants brought indoors this fall might exhibit temporary “transplant shock” in their new environment due to changes in light and temperature. Sun-loving houseplants might suffer during cloudy winter season. If possible, consider supplemental artificial lights. Avoid overwatering houseplants. Cut back on fertilizer in general, except for plants intended to bloom all winter, such as miniature roses or geraniums.

Most houseplants appreciate a 10- to 15-degree difference in day and night temperature. Monitor plants for early signs of problems. When indoor heat is turned on, natural humidity disappears. Try to wash plants occasionally in a warm shower. Humidifiers and pebble trays can help raise humidity.

Pot up pretreated bulbs, such as amaryllis, paperwhite narcissus, and others, for holiday blooms.

Continue to fertilize orchids with very dilute orchid fertilizer until they set flower buds. Monitor orchid foliage to be sure it doesn’t scorch from exposure to direct southern sun.

Extra hardy bulbs not planted outside this month can be potted up and forced for indoor blooming. Plant bulbs in wide, shallow pots in a soilless mix. Large bulbs are planted side by side with just their tips showing. Little bulbs are planted with ½ inch of mix covering them. Water well and place pots in a refrigerator, cold frame, garage, or shed where the temperature remains between 35 and 40 degrees Fahrenheit. If storing in a refrigerator, cover pots with plastic wrap and avoid storing ripening fruit in same area. Some fruit releases ethylene gas, which inhibits flower formation. Major bulbs require 12 to 14 weeks of cold storage; little bulbs require a few weeks less. When pale yellow sprouts begin to show, pots can be brought out of cold storage into a bright but quite cool room (55 to 65 degrees F.) for about two weeks. As flower buds begin to develop, bring pots into a warmer room but avoid direct sunlight. Water as needed.

Pruning MarchWatch weather conditions for an appropriate window of time to spray fruit trees or large deciduous trees with dormant oil. Spray if aphids, scale, or mites were a problem in the past. Temperatures must be at least 40 degrees Fahrenheit with no chance of freezing or rain within the following 24 hours. Avoid spraying on a windy day. Certain trees have a phototoxicity to dormant oil and should not be sprayed. A few common ones are arborvitae, beech, red maple, Japanese maple, sugar maple smokebush, blue spruce, blue cultivars of juniper, and yew. Call Plant Information if in doubt.

To reduce the spread of oak wilt, all oak pruning should be completed in March, or before the oaks begin active growth. Pruning should not resume until after the first frost, or around November 1.

Prune fruit trees in early March on a dry day before buds swell. As with all pruning chores, sterilize pruning tools with a 10 percent solution of bleach before each cut. Prune out sucker growth, water sprouts, and any diseased or dead branches. Remove crossing branches, rubbing branches, or those that grow toward the center or the plant rather than outward, away from the interior. Fruit trees benefit from having their canopies opened up to permit more sunlight and air into their centers.

Prune spring-flowering trees and shrubs immediately after they flower to preserve this season’s flower display.

Prune roses when the forsythia begins to bloom. When pruning roses, make a 45-degree cut above a healthy bud, angled away from the center of the plant. If not done last fall, prune hybrid tea roses and grandiflora roses back to 12 inches to reinvigorate growth. Prune out dried, darkened, and broken canes and any dead tips. Prune shrub roses to remove dead wood and very lightly to shape to size.

Fertilize woody plants four to six weeks before they begin new growth only if they have shown signs that they could use it. These would include poor leaf color, failure to completely fruit or flower, or stunted growth. Use a slow-release granular fertilizer or an organic product and water in well. Do not fertilize newly planted trees or shrubs. Wait one year before making this application.

Plant trees and shrubs before they break bud and when soil conditions permit. If spring weather is unusually wet, consider planting in the fall when the plants begin their dormancy. With all woody plants, avoid planting too deep. Research indicates that more trees suffer from being planted too deep in the hole than any other problem. Plant with one-third of the root ball above ground. Taper soil away from the trunk back to ground level. Mulch the entire root zone with several inches of shredded or chipped bark.

RanunculusCool-season annuals that can tolerate a light frost can be planted out early in the month after being hardened off. These include snapdragons, sweet peas, English daisies, pot marigolds, African daisies, lobelias, sweet alyssum, forget-me-nots, pouch flowers (Nemesia), baby-blue-eyes, larkspurs, love-in-a-mists, bush violets (Browallia), stocks, primroses, pansies, painted tongues (Salpiglossis), sweet Annie (Artemisia annua), and violets. Later in the month plant Shirley, Iceland, and California poppies, and Persian buttercups (Ranunculus).

Continue to remove all garden debris from last year. Shred and compost, leaving out diseased material. Add 2 to 4 inches of compost to garden beds if not done yet.

Plant trees and shrubs. Wait one year to fertilize new woody plantings.

Forsythia AprilPlant perennials, hardy ornamental grasses, and roses. When planting bare-root roses, soak the roots in a bucket of water for several hours before planting. Be sure to choose a full-sun site before digging the hole. Remember that as soon as tall trees leaf out, full-sun areas can become partly shaded.

Divide mature summer- and fall-blooming perennials when they are 4 to 6 inches tall. Do not divide day lilies (September), Oriental poppies (July), or iris (late July) at this time.

Fertilize spring-flowering bulbs with a granular 5-10-5 or 10-10-10 mix as new green growth emerges or when they finish flowering. Remove spent flowers but allow foliage to wither completely before removing. Divide clumps of older bulbs in need of rejuvenation. Replant in sunny spot and water in well. Bulbs prefer locations that are not heavily watered during their summer dormancy. Therefore do not overplant perennial bulbs with summer annuals.If botrytis blight or bud blast was a problem with peonies last year, spray newly emerging plants with an approved fungicide when the plants are 2 to 4 inches tall. When peonies reach 10 inches, stake or hoop them to support their blossoms. Avoid overhead watering. If fungus persists, consider relocating peonies to a more open, full-sun site. Move plants in the fall.

If weeds were a problem last year, spread a pre-emergent weed control over lawn as weather and temperature permit. Alternatively, consider hand-pulling weeds or spot-treating weeds after they emerge with a post-emergent weed control. Serious infestations often require several treatments to control the problem.

Rose Care AprilBegin uncovering roses. Remove mulch and carefully hose away mounded soil. Avoid using sharp tools around base of rosebush. Topdress soil around roses with a 6-2-0, organic, slow-release fertilizer such as Milorganite.

Young lettuce AprilPlant small transplants of asparagus, early potatoes, lettuce, radish, mustard, onions, peas, rhubarb, spinach, turnips, cauliflower, carrots, and all other cool-season crops as weather permits. Plant midseason potatoes in mid-April.

Plant strawberries and pinch off first-year flowers to develop strong root systems.

Later in the month, begin to harden off warm-season vegetable and flower transplants in a cold frame, or bring flats of small transplants outside to sunny, protected areas — but bring them back in at night.

If necessary, spray fruit trees with dormant oil for insect control. If apple scab was a problem last year with crabapple or apple trees, be ready to start spraying with an approved fungicide as soon as the leaf buds begin to swell and open. Spray routine will last until two weeks after petal drop. Wet weather will only aggravate the problem. Call Plant Information for exact timing and recommended fungicides.Fertilize fruit trees and brambles. If growing fruit trees for their fruit yields, begin a spray program for insects and disease. Call Plant Information for exact timing and approved chemicals.

Crab grass control can be spread on lawns in early to mid-April before weeds germinate. For severe problems, a second application might be necessary in early June.

Seedlings AprilIf not done in late March, there is still time to sow seeds indoors for warm-season vegetables like tomato and pepper plants and summer-blooming annuals. Most seeds take six to eight weeks from germination to outdoor planting.

Continue to bring houseplants inside at night but outside during warm days. Most houseplants can remain outside when night temperatures are consistently 40 to 50 degrees. When in doubt, wait until mid-May before placing houseplants permanently in the garden.

If orchids have finished flowering, begin to fertilize regularly with a very dilute orchid fertilizer or use fish emulsion with every watering. Monitor closely for scale attack.