Keep the frost date (May 15) in mind when planting

Even though this spring has been usually mild, with many plants starting growth and flowering much earlier than in other years, you should keep the average last frost dates for your area in mind as you consider any early planting of cold-sensitive plants.

The average last frost date at the Chicago Botanic Garden in Glencoe is May 15; sites farther inland have a greater risk of late frosts.

It is best not to install tropical plants and warm-season annuals and vegetables early unless you can be sure there will not be extended periods of time in the 40s or a frost. Plants such as impatiens and coleus that will not grow in cold weather and would likely be badly damaged or killed by any touch of frost.

Warm-season vegetables such as tomatoes also would not do well in typical early May cold spells, so do not buy them until late in the month, even though you may find them for sale. But it is fine to go ahead and plant trees, shrubs, perennials and ground covers.

General garden care

Gradually move houseplants outside to protected areas when nighttime temperatures are consistently above 40 degrees. Leave them outside for a few hours more each day. Gradually acclimating these tropical plants to the sun and outside growing conditions will help prevent any damage to the leaves. Large houseplants in plastic pots should be slipped into larger, heavier pots to prevent them from falling over in the wind.

Many garden beds have irregular edges. Beds will look better and it will be easier to mow alongside them if they have smooth, flowing curves. Use a pliable garden hose or rope to lay out your new bed line. Remove all clumps of grass from inside the new line. Break up and spread out any clods of soil.

Mulch can help trees by keep lawn equipment such as mowers and string trimmers away from their vulnerable bark. But avoid the “volcano effect” of piling mulch or soil against the trunk; it can do just as much damage, because it traps moisture that can cause rot and allows rodents, insects and diseases to reach the bark. Spread mulch in a wide, even doughnut-shaped layer 3 to 4 inches deep, keeping it 5 to 6 inches clear of the tree trunk.

It’s a beneficial garden practice to deadhead — pinch off spent blooms — from rhododendrons and azaleas after they finish flowering. You can increase the flower count for next year by very carefully pinching off one-half of the sticky new green growth that is emerging from the spot where the flowers once were. This is also the correct time to prune their branches to reduce their size if needed. It is best to be conservative when pruning these plants.

If your soil is too alkaline for some plants that prefer more acid soil, elemental or granulated sulfur can be added to lower the pH. Rhododendrons and azaleas are examples of plants can benefit from sulfur applications in many gardens in the Chicago area. If you prefer blue flowers on your bigleaf hydrangea (Hydrangea macrophylla) but they have been turning pink, the cause is alkaline soil. Adding sulfur will encourage blue flowers. The recommended rate is three pounds of sulfur per 100 square feet of garden area per year. Based on that rate, calculate the correct amount to apply to the area where you want to amend the soil. It is best to apply one-half of the recommended yearly amount in the spring and the other half in the fall. Wear plastic gloves and be careful to keep the dust out of your eyes when applying.

Work the sulfur into the soil and water it in. Sulfur is slow-acting, so be patient when waiting to see its effects rather than applying more than recommended.

Planting

If you have a lot of digging to do in your garden, it is important to have underground utility lines located several days before starting the work. It can be extremely dangerous to encounter a gas line or electric line while digging.

Having the lines located and marked is a free service. In the suburbs, call JULIE at (800) 892-0123 or see illinois1call.com. In Chicago, call DIGGER at (312) 744-7000. It generally takes two working days for these services to locate your utility lines, so allow plenty of time before your project. These locating services will not mark any lines you have installed yourself, such as a gas line to a grill or wires for landscape lighting.

If you must dig within 18 inches of either side of any marked underground utility line, dig carefully by hand and pause frequently to check for obstructions. Also, take care when digging in gardens with underground sprinkler systems, whose lines can easily be punctured.

Rotate the crops in your vegetable garden to control pest problems. This is the easiest way to practice organic gardening or integrated pest management, which calls for reducing the causes of disease and vulnerability to pests rather than spraying.

Many insects and diseases attack all vegetables that are genetically related within the same plant family. By planting vegetables from a different family in a problem area, you deprive the pest of a host plant and minimize the chances for a repeat infection.

The basic groupings are the cabbage family (broccoli, cabbage, cauliflower, Brussels sprouts, radishes and turnips); the cucumber family (gourds, melons, squashes and cucumbers); the nightshade family (eggplant, potatoes, tomatoes and peppers); the goosefoot family (spinach and beets); the onion family (leeks, garlic and onions); the legume family (all peas and beans); and a group that includes carrots, parsnips and celery.

Nip back any unshapely growth on your shrubs to improve their appearance and encourage the development of a fuller plant.

Many trees are planted too deeply. To determine the proper planting depth for your new tree, locate the trunk flare — the place where the trunk widens out into the roots. When you place the tree in its hole, the flare should sit at ground level or higher. Generally, in heavy clay soil it is best for the flare to sit 2 to 3 inches higher than the surrounding ground level.

If the trunk flare is not obvious on a balled-and-burlapped tree, open the burlap to find it so that you can accurately measure the root ball and plan your hole. If the flare is obscured by soil, you may have to carefully remove excess soil to expose it. A hole for a tree should be wide and saucer-shaped and no deeper than necessary. It is best not to amend the soil; simply backfill the hole around the root ball with the soil you dug out.

Water plants grown in containers so their soil is moist when you transplant them to your garden. When you remove the plant and its root ball from the container, gently spread out or cut any roots that have been circling inside the pot. They could eventually girdle or choke the plant unless they are redirected to grow out into the soil of the garden. This will help the new plant get established in the ground more quickly.

Monitor the watering needs of recent transplants carefully. Often, plants are grown in a lightweight container soil that will dry out faster than the surrounding garden soil, stressing the plants. Generally, plants transplanted from containers need frequent, light watering until they are well-established, with roots growing out into the garden soil.

Any containers you plan to use for seasonal plantings need at least one bottom hole for drainage. Fill the container with a lightweight, fast-draining soilless potting mix, not with heavy garden soil.

Leave enough space between the top of the growing medium and the rim of the pot for water to briefly pool and soak in when you water. A layer of gravel in the bottom of the container is not necessary for drainage, but very large containers can be partially filled with wood chips or empty plastic pots to conserve potting mix.

Separate the wood chips from the growing mix with landscape fabric. Most plants’ roots need no more than 10 inches of soil depth in a container.

Lawns

Use a mulching mower that leaves the grass clippings on the lawn. This means less work for you and recycles the nutrients in those clippings back into your lawn.

Regardless of which mower you choose, you need to mow more frequently during spring due to rapid grass growth. Mulching mowers, in particular, do not work well in tall, wet grass. Rake out any wet clumps of grass that remain on the lawn.

Keep the blades on your mower sharpened for a better cut. A good general rule is to sharpen them after every eight hours of use.

Most lawns are cut too short. Mow grass at 2½ to 3 inches in height, and frequently enough so that not more than one-third of the total height is cut each time. Maintaining grass at a taller height promotes a strong root system and helps choke out weeds. In hot weather, short lawns can turn brown more quickly and weeds are more likely to move in.

Ÿ Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.