Q: Now that my hydrangea is blooming, I see it’s not my favorite color. Can I change this? How?
A: I assume you have a big leaf Where mountain hydrangea (hydrangea macrophylla and Serrata, respectively), as other commonly grown species do not change flower color in response to growing conditions. You will probably need to check and then change the soil pHand I’ll tell you why.
The pigment in hydrangea flowers can bind to aluminum, and this compound is responsible for the blue color. Aluminum is often present in garden soil in sufficient amounts, so you probably don’t need to add any unless the plant is being grown in a container. (Be careful if you supplement—too much aluminum uptake can stunt roots and kill a plant, though hydrangeas may have a higher tolerance than other shrubs.) Soil pH affects aluminum availability for uptake by plant roots.
These two factors working in concert are what turn hydrangea flowers blue or keep them pink – the presence of aluminum and the acidity level of the soil. In soils with higher acidity (around pH 5.0 to 5.5), aluminum is more available for root uptake and the plant may produce blue flowers. In weakly acidic soils (about pH 6.0 to 6.5), aluminum remains chemically bound to soil particles, so flowers are pink. The sweet spot between the two acidity levels can result in purple flowers, and sometimes plants will sport all three colors at once.
A lab soil test will tell you what the pH of your soil is; Home test kits may suffice instead, but they are not always reliable. Changing the pH involves the application of sulfur (to lower it) or lime (to raise it) and is a gradual process. Therefore, you may not see a color change until the following year.
Due to genetics, a few hydrangea cultivars do not turn blue even under the right conditions. The richness of color (pale or vibrant) depends on the genetics and overall nutrition of the plant, so adjusting the pH will not turn a pale blue hydrangea dark blue. Petals of cultivars with white flowers generally do not respond to pH, although aging petals of any cultivar may turn greener or pinker as they fade.
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Lacecap flowers have the added benefit of two potential color changes – the outer petals of sterile flowers and the inner true flowers without petals. In my experience, the inner flowers can be a darker, richer color than the petals, regardless of pH, and they are sometimes more blue even though the petals are more purplish or even white. It is a neat trait that adds to the beauty of these plants.
Q: I constantly fail to grow rosemary in my garden. What am I doing wrong? They are meant to be perennials, 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. My main suspicion is that his soil stayed too wet at some point, which led to the death or infection of the roots and therefore the loss of the entire plant.
Many herbs native to the Mediterranean region require excellent soil drainage and have a limited tolerance for soggy soil or poor drainage, even if the roots are only oversaturated for a short time. If your garden has clay-based soil, don’t mix in sand (unexpectedly, this can make drainage worse), but instead try incorporating organic matter to reduce compaction and improve drainage. Better yet, simply grow herbs in a raised bed or container where you have more control over soil porosity. However, container living comes with its own risks when it comes to less winter insulation for the roots, so it’s not a one-size-fits-all solution for all herbs.
Speaking of winter, not all rosemary cultivars are equally cold hardy; some are more reliable than others, although we are still too cold here in the average winter for this particular herb in general. Look for those rated in USDA hardiness zone 6, which will give them an edge over those that are only hardy in zone 7 (which is most of Maryland).
Plant early enough in the season (May or June, ideally) so the plants have time to establish a more extensive root system in the fall and the onset of frost. It will seem ironic considering what I said about humidity above, but roots unable to keep a plant hydrated in winter because they can’t reach unfrozen water can also lead to death of all top growth or entire plant, even if it was ‘not too cold. winter drought is often an unrecognized killer of late-planted conifers when soil oversaturation is not to blame.
The Home and Garden Information Center at the University of Maryland Extension offers free gardening and pest information at extension.umd.edu/hgic. Click “Request Extension” to submit questions and photos.