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Garden Q&A: How do I change the color of my hydrangea, and why won’t my rosemary grow?

Q: Now that my hydrangea is blooming, I see it’s not the color I prefer. Can I change that? How?

A: I assume you have a bigleaf or mountain hydrangea (hydrangea macrophylla and serrata, respectively), as the other commonly grown species don’t change flower color in response to growing conditions. You’ll probably need to check and then change the soil pH, and I’ll explain why.

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The pigment in hydrangea blooms can bind with aluminum, and this compound is responsible for the blue color. Aluminum is often present in garden soil in sufficient quantities, so you probably don’t need to add any unless the plant is grown in a container. (Be careful if supplementing — too much aluminum absorption can stunt roots and kill a plant, though hydrangeas might have a higher tolerance than other shrubs.) Soil pH affects the availability of aluminum for uptake by plant roots.

These two factors working in concert are what turns hydrangea flowers blue or keeps them pink — the presence of aluminum and the soil acidity level. In soils with stronger acidity (about pH 5.0 to 5.5), aluminum is more available for root uptake and the plant can produce blue flowers. In weakly acidic soils (about pH 6.0 to 6.5), the aluminum remains chemically bound to soil particles, so the blooms are pink. The sweet spot between the two acidity levels can result in purple flowers, and occasionally plants will sport all three colors at once.

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A laboratory soil test will tell you what your soil pH is; home test kits might be sufficient instead, but they’re not always reliable. Changing pH involves applying either sulfur (to lower it) or lime (to raise it) and is a gradual process. Therefore, you may not see a resulting color change until the following year.

Due to genetics, a few hydrangea cultivars do not turn blue, even in the right conditions. The richness of the color (pale versus vibrant) depends on genetics and overall plant nutrition, so adjusting the pH won’t turn a pale blue hydrangea dark blue. The petals of white-flowered cultivars generally don’t respond to pH, though aging petals on any cultivar can turn greener or rosier as they fade away.

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Lacecap blooms have the added bonus of two potential color changes — the outer petals of the sterile flowers and the petal-less inner true flowers. In my experience, the inner flowers can be a darker, richer color than the petals no matter the pH, and they are sometimes bluer despite the petals being more purplish or even white. This is a neat trait that adds to the beauty of these plants.

Q: I keep failing at growing rosemary in my garden. What am I doing wrong? They’re supposed to be perennial, and I don’t want to bring it inside if I can avoid it.

A: Indeed, they are semi-shrubby perennials, but they are also sensitive to conditions that can cause dieback no matter how mild our winters are. My prime suspicion is that its soil stayed too wet at some point, which led to root death or infection and thus the loss of the entire plant.

Many herbs native to the Mediterranean region require excellent soil drainage and have limited tolerance for soggy soil or poor drainage, even if the roots are only oversaturated for a short period. If your garden has clay-based soil, don’t mix-in sand (counterintuitively, this can worsen drainage) but instead try incorporating organic matter to reduce compaction and improve drainage. Better yet, just grow herbs in a raised bed or container where you have more control over soil porosity. Container life brings its own risks when it comes to less winter insulation for the roots, though, so it’s not a blanket solution for all herbs.

Speaking of winter, not all rosemary cultivars are equally cold-hardy; some are more reliable than others, though we are still borderline too chilly here in the average winter for this particular herb in general. Look for those rated to USDA hardiness zone 6, which will give them an edge over those only hardy to zone 7 (which is most of Maryland).

Plant early enough in the season (May or June, ideally) so plants have time to establish a more extensive root system by autumn and the onset of frost. It will seem ironic given what I said about moisture above, but roots unable to keep a plant hydrated in winter because they can’t reach unfrozen water can also result in the death of all top growth or the entire plant, even if it wasn’t too cold. Winter drought is often an unrecognized killer of late-planted evergreens when soil oversaturation isn’t to blame.

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University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.


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