No appeal to these golden leaves
One kind of autumn leaf colour that some gardeners find worrying is the yellow leaf found on gardenias at this time of year. They immediately reach for the Epsom salts to cure what they believe is a magnesium deficiency.
If this is you, please stop and look at the whole shrub first. Are those yellow leaves the older ones, further down the stem? Are the rest of the leaves green and healthy looking? Perhaps it’s just one small shoot that’s turning yellow?
If so, this is simply the gardenia’s normal leaf turnover. Even evergreen plants have to drop old and tired leaves sometime, and in gardenias it happens when the nights get cool. Simply pick off the affected leaves if the appearance bothers you – if it doesn’t then practise Masterly Inactivity and just wait for them to drop off.
On the other hand, if all of the foliage seems a bit yellow, particularly the older leaves, and the yellowing is between the leaf veins, then the gardenia is almost certainly suffering from magnesium deficiency. Scrape back the mulch and sprinkle Epsom salts thinly over the soil surface before watering it in. To get a quicker response, mix a teaspoonful of the salts into a litre of water and spray it over the foliage.
Gardenias are not the only plants affected by magnesium deficiency. Any plant can suffer from it in the Northern Rivers, because our heavy summer rains leach this mineral out of the topsoil. Keep an eye out for the yellowing between the veins (on some plants, including grapevines, it may have a reddish tinge) and treat as necessary.
Heavy croppers like tomatoes are sometimes affected because they are being fed with a high-potassium fertilizer. If magnesium deficiency is an issue, tomatoes show weak and slow growth and crop poorly.