Soil test can help determine best fertilizer for you

Apply fertilizer to garden beds if needed. Most gardens in the Chicago area have soils with adequate levels of phosphorus, so choose a fertilizer with little or no phosphorus. The higher the percentage of nitrogen in the fertilizer you purchase means the less you need to apply to your garden, so follow instructions on the bag or ask a salesperson for advice.

Have your soil tested if you want to know for sure what the best fertilizer would be for your garden. Use soil from a few locations to mix together for a good composite sample. If your garden is large, then break up your property into sections and send in multiple samples.

If your soil is too alkaline, elemental or granulated sulfur can be added to lower the pH. Rhododendrons and azaleas are examples of plants that can benefit from sulfur applications in many gardens in the Chicago area. Apply sulfur to your blue hydrangeas that are turning pink to get the flowers back to blue. The flowers turn pink in more alkaline soils. Add 3 pounds of sulfur per 100 square feet of garden area per year. It is best to apply in the spring and fall, applying one-half of the recommended rate each time. Work the sulfur into the soil and water in. Sulfur is slow-acting. Wear protective gloves and be careful to keep the dust out of your eyes when applying.

• Be sure to keep the average last frost dates for your area in mind as you consider any early planting of cold-sensitive plants. The Garden's average last frost is May 15.

It is best not to install tropical plants and warm season annuals and vegetables early unless you can be sure the weather will not reach freezing or extended periods of time in the 40s. These are plants such as impatiens and coleus that would likely be badly damaged or killed by a frost. Warm season vegetables such as tomatoes will not do well in typical early May cold spells so do not buy them until later in the month, even though you may find them for sale.

Go ahead and plant trees, shrubs, perennials and ground covers.

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