Why are the leaves on my Gardenia yellow?
Gardenias are very useful plants that produce beautiful fragrance from their pure white flowers but can often look pretty ordinary when the leaves turn yellow. It’s all about Chlorosis or the lack of chlorophyll.
A plant can lack chlorophyll due to any number of causes- poor drainage, pH too low (<5.0), insufficient nutrients or combinations of these.
First check the soil moisture levels by using the best water meter in town- your finger! Soils can be OK when permanently moist provided the drainage is good.
Right, moisture levels good, so check the PH. We have covered this in a previous blog so wont expand on it….except to say that Gardenias seem happiest when the pH is between 5.0 and 6.5. If the pH drops lower than this, the availability of nutrients like iron, magnesium and zinc is diminished. Like us, plants depend on a maintenance level of minerals to adequately perform all cellular functions.
If nutrient deficiency is the issue you can with some practice identify which mineral is in short supply.
Magnesium deficiency shows as yellow leaf bases but the tips appear green. Correction can be achieved by applying Epsom salts (Magnesium sulphate). Try diluting about 20gms (one dessertspoon full) to 5 lt of water and applying this to the bed surrounding the plants once a month. If it is a deficiency, you should notice a change within two months.
Iron deficiency Tips of leaves may appear yellow but the leaf base and veins are green. You will notice iron deficiency as winter approaches and plant uptake becomes slower. Best to apply iron chelates in Spring and look for this with added sulphur.
Nitrogen deficiency shows as yellow between the veins at any time through the year. Aged fowl manure or Blood and bone meal applied at one handful per square metre and watered in well every eight weeks from Spring through Summer will address this deficiency.
Tip- the roots that absorb water and nutrients are not near the trunk but generally around the outer diameter of the foliage and beyond. This is where you should be watering and feeding! Fertilisers (salts of minerals) can burn the tissues of stems allowing the entry of pathogens with death resulting, so always distribute fertiliser away from trunks. Nitrogen can be easily vaporised into the atmosphere so always water in well once applied.