Use sulfur to keep your blue flowers blue

Apply fertilizer to garden beds if needed, but only add those nutrients your plants need. Have your soil tested if you want to be sure of which fertilizer would be best for your garden. For smaller gardens, mix soil from a few locations together for a good composite sample for the test. If your garden is large, break up your property into sections and send in multiple samples.

Most gardens in the Chicago area have soils with adequate levels of phosphorus — the middle number in the ratio of major nutrients on a fertilizer package — so choose a fertilizer with little or no phosphorus.

The higher the percentage of nitrogen (the first number) in the fertilizer you purchase, the less of the product you will need to apply. Follow instructions on the bag or ask a salesperson for advice.

Many garden beds have irregular edges. Beds will look better and be easier to mow along if they have smooth flowing curves. To lay out a new bed edge, use a pliable garden hose or rope to establish the line and edge along it with a sharp spade. Remove all clumps of grass inside the line. Break up any clods of soil and spread the soil around.

When you spread mulch, avoid the “volcano effect” of piling soil and mulch up around the trunk. Use a flat spade to carefully remove any soil that has built up over time around the base of your trees, being careful not to damage the bark of the trunk or roots. In most situations, it is best to remove the excess soil and put it somewhere else in your garden.

If your soil is too alkaline for some plants, elemental or granulated sulfur can be added to lower the pH. Rhododendrons and azaleas are examples of plants in many local gardens that can benefit from sulfur applications. Flowers of mophead hydrangeas will be pink rather than blue in alkaline soils. Apply sulfur to get blue flowers.

In areas with acid-loving plants, add 4 pounds of sulfur per 100 square feet of area per year. It is best to apply the sulfur in the spring and fall, applying one-half of the recommended amount each time. Do not exceed the recommended rate of application; too much sulfur can burn plants. Wear protective gloves, and be careful to keep the sulfur dust out of your eyes. Work the sulfur into the soil and water in. Sulfur is slow-acting, so do not expect immediate results.

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