If you see yellow foliage on your gardenia bush and, after trying everything to ameliorate the situation, still see yellow and, even worse, fear that your plant may soon be ready for the compost heap, you can take heart from these words from Shari Armstrong (gardeningknowhow.com): “A gardenia bush with yellow leaves is a common problem and can be very difficult to ultimately fix. If, after your best efforts, your gardenia still does not survive, don’t be too hard on yourself. Even master gardeners with years of experience can lose gardenia bushes despite their best efforts. Gardenias are a beautiful but fragile plant.”
Let’s stop here for just one moment. Many gardeners who have gone through the stages of gorgeous gardenia selection at the nursery through painstaking planting where soil was meticulously prepared to precisely match gardenia soil needs, only to helplessly watch as those efforts went for naught, may well bristle at the suggestion that they should consider planting gardenias again.
Gardenias are somewhat like orchids, however, in the sense that you may see any number of them die before catching onto what their proper care involves: part art, part science, part experience, part love.
Let’s say you planted your gardenia in a recommended soil mix. But the soil seems to be constantly wet, which could be due to fine-textured soil mix ingredients, especially peat moss. Gardenias like a somewhat moist soil but abhor wet soil since good drainage is crucial to them. In such a case surround your plant with a 3-inch layer of finished compost. This will help to open up the soil below.
I was prompted to write about gardenias after receiving emails from correspondents in Florida and in San Diego, much more favorable locales for growing gardenias than Los Angeles. Yet both expressed enormous frustration. Based on the dialogue conducted and photos received from each, it became apparent that both had erred in exposing their plants to insufficient light.
Gardenias should receive half a day of sun and a bit more. If you can surround them with plants of similar stature, it will help in increasing ambient humidity. Native to sub-tropical China, the dry air of Southern California makes gardenias feel woefully homesick.
Yet, when it comes to gardenias, it may be just as much a matter of nature (genetics) as it is of nurture (plant care). When you propagate gardenias from cuttings, the plants that develop will tend to show the classic mineral deficiencies for which gardenias are known, lack of magnesium and lack of iron. With these two mineral deficiencies, you see chlorosis, identified by yellow spaces between green veins.
Chlorosis comes from the word chlorophyll, the molecule that makes leaves green. At the center of this molecule is an atom of magnesium, surrounded by four atoms of nitrogen. (Incidentally, hemoglobin, the molecule that makes blood red is identical in its fundamental structure to chlorophyll, except that an atom of iron is surrounded by the four nitrogens). Magnesium deficiency is expressed in older leaves at branch bases even while terminal leaves remain green. The reason for this is that magnesium is mobile and will seek out the newest, which is always the most important, growth when its supply is limited. Add magnesium with Epsom salts (2 tablespoons in a gallon of water once a month) and add iron with powdered iron chelate in the spring.
Regarding nitrogen deficiency, it is the easiest of all deficiencies to diagnose. Like magnesium, nitrogen is highly mobile and when you see a branch with leaves that are pale green to yellow excepting green terminal leaves, you known there is a nitrogen shortage. Unlike magnesium or iron deficiency, however, the color of the entire leaf is a pale green to yellow color and leaf veins are not greener than the surrounding tissue. Meanwhile, since iron does not move well in the plant, it’s deficiency is first noticed on new growth.
Getting back to genetics, keep in mind that popular gardenia varieties have been selected for flower quantity or fragrance or both. However, nursery growers have learned that when these varieties are clonally propagated, the root systems of the new plants struggle to provide robust growth once plants are shifted from nursery containers to harsher garden conditions. The solution to this problem has been provided by grafting onto a rootstock species that keeps foliage green despite difficult soil conditions and that is highly efficient at extracting magnesium and iron from the soil.
With plants in general, pale green to yellow foliage may not indicate nitrogen deficiency but rather nitrogen imbalance that may be seasonal and self correcting. Two plants — common hibiscus (Hibiscus rosa-sinensis) and blue potato bush (Lycianthes rantoneii) — are famous for displaying sickly foliage in late winter only to have it change to a lush green, sans fertilizer, as weather warms.
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You can special order grafted gardenias through Monrovia Nursery, the mega plant grower. Check to see which retail nurseries in your area buy from them at Monrovia.com. The rootstock species that is grafted upon is starry gardenia (Gardenia thunbergia).
Around 20 years ago, before the Sherman Oaks Library was demolished and rebuilt, there was a stunning hedge, around 8 feet tall, of this species. Flowers are highly fragrant trumpets that end in swirling stars. In its native habitat of South Africa, starry gardenia, due to its prickliness, is used as a living security fence. Leaves of starry magnolia are always deep green and fertilization is not needed.
No wonder this dry climate gardenia succeeds as a rootstock for the moisture craving garden varieties that are grafted upon it.