Some Like It Sweet

By Mary Anne Michael, KC Master Gardener 1994

When I took my Master Gardener class, it seemed that every lesson started with the mantra “First, take a soil sample.”  If nothing else, we learned that successful gardening starts with a proper pH. The broad pH picture was easy: grass liked alkaline soil, perennials liked their feet in slightly acidic earth, and the major shrubs and groundcovers for our area needed definite acidic conditions.  So over the years, I routinely added lime to the lawn and acidic fertilizer to azaleas, rhododendrons, pachysandra and ivy.  The perennial bed, for the most part, did quite well with its natural, slightly acidic pH level.

The one constant problem was my boxwoods. They never had a healthy, dark green color or vigorous growth, and I assumed they weren’t able to absorb nutrients because the soil wasn’t acidic enough. I      always planned to add something to lower the pH, but never got around to it. Then about seven years ago, I bought a handbook published by the American Boxwood Society and was shocked to read that boxwoods need a pH of 6.5 to 7.2.  (That’s sweeter than grass, which likes a pH of 6.2.) Instead of iron sulphate, the struggling boxwoods needed lime.  That spring, I started a yearly regimen of sprinkling lime (powdered   because I wanted quick results) around each shrub, and after two years there was a definite improvement. I started wondering about other plants that weren’t thriving but had the right growing conditions--such as drainage and light--and the ability to handle our hot and humid summers. Maybe they also needed a pH adjustment.

I started researching individual plants and found that a few plants need sweet soil, but there are several that do better with a neutral or slightly alkaline pH. I now have an early spring ritual of lightly topdressing specific plants with lime, and it’s worth the effort.

Below are two lists:  one for plants needing alkalinity, and one for those preferring neutral or slightly sweet soil. The lists aren’t complete by any means, so if you have a plant that’s underperforming and it’s not listed below, you might want to explore further. The information isn’t easy to find; most gardening books rely on the conventional wisdom that a slightly acidic soil is optimum.  Besides reputable gardening books, sources of information include plant societies and The American Horticultural Society, which has a free online Q & A for members.

You also might want to experiment a bit, since two plants in my garden, lavender and catmint, were not mentioned anywhere as liking an alkaline pH, but they definitely have done better the last few years with a bit of lime. Let me know the results!

*Remember, take a soil test first! Call your local extension office to obtain a kit.

Definite  Alkaline Preference
Perennials    

Arabis caucasica

            Artemisia absinthium 

            Centranthus ruber

            Gypsophila

            Iris (tall bearded hybrids only)

Lilium candidum (Madonna lily)

Scabiosa caucasica

Shrubs & Vines

Buxus

            Syringa

            Clematis

 Neutral or Slightly Alkaline Preference
Perennials
Anemone pulsatilla (Pulsatilla vulgaris)
            Aurinia saxatile (Alyssum saxatile)
            Bergenia cordifolia
            Centaurea
            Dianthus