Adding sulfur to your soil will help flowers thrive

  • Azaleas are examples of plants that can benefit from sulfur applications in the Chicago area.

    Azaleas are examples of plants that can benefit from sulfur applications in the Chicago area. Courtesy of Chicago Botanic Garden

By Tim Johnson
Chicago Botanic Garden
Posted5/13/2018 6:00 AM

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.

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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 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.

• It's a beneficial garden practice to deadhead (manually pinch off spent flowers) of rhododendrons and azaleas after they finish flowering. This is also the correct time to prune their branches to reduce the size of the plant as needed.

It is best to be conservative with pruning them back. You can increase the flower count for the following year by very carefully pinching off one-half of the sticky new green growth emerging from the spot where the flowers once were.


• 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 40-degree range. 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 groundcovers.

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

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