Track your bulb plantings with photographs

Take photographs of your bulb displays throughout the month and make notes to help you remember where new bulbs can be added in fall to improve the display. This will make it much easier to order and plant bulbs later this year.

Make a plan to add bulbs that bloom at different times to extend the season of color. Some bulbs will thin out over time and will need occasional replenishing for a display that is consistent year to year.

You may need to remove some scilla or other bulb plants if they have reseeded to areas where they are unwelcome.

If your soil is too alkaline for certain plants, elemental or granulated sulfur can be added to lower the pH, making the soil more acid. Rhododendrons and azaleas are examples of plants that can benefit from sulfur applications in many gardens in the Chicago area.

In alkaline soils, the flowers of bigleaf hydrangeas tend to be pink; if you prefer blue, apply sulfur to the soil over the shrub's roots. Sulfur is slow-acting so the result will not be immediate.

Add three pounds of sulfur per 100 square feet of garden area per year. It is best to split the recommended amount between a spring and a fall application. Work the sulfur into the soil and water in. Wear protective gloves and be careful to keep the dust out of your eyes when applying.

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