incorrect soil pH – gardenias prefer acidic soils. If growing in alkaline soils gardenias have trouble accessing all the nutrients they need and yellow leaves can develop. Apply sulphur to the soil to make it more acidic.
To acidify soil around the shallow roots of the plant, organic gardener Howard Garrett advises mixing one tablespoon of vinegar per gallon of water every time you irrigate the plant. While watering gardenias with a vinegar solution can lower the soil's pH level, the effects are short-lived.
Sulphur. This is the common acidifying material. Soil organisms convert sulphur into sulphuric acid, so acidifying the soil. The more finely ground the sulphur the more quickly the bacteria can convert it; sulphur dust is quicker acting than sulphur chips (and more expensive).
Scotts Miracle-Gro Miracid Plant Food
From one of the top names in gardening aids, the best overall pick for feeding gardenias is Scotts Miracle-Gro Miracid Acid-Loving Plant Food. This water-soluble fertilizer is designed to be mixed with water and poured over foliage or around the base of the plant.
Feeding and mulching Gardenias are fed during the growing season from spring to autumn with a complete fertiliser such as Seasol plus Nutrients Roses & Flowers. Water it in well after application. Plants also benefit from liquid feed such as PowerFeed PRO SERIES for Roses & Flowers during the growing season.
Gardenias use a lot of nutrients to produce so many glorious blossoms. Feed your shrubs by applying an acidic, slow-release fertilizer such as an azalea or camellia fertilizer. For the organic gardener, blood meal, fish emulsion or bone meal work well.
The thinking behind this practice is that the yellow leaves are a sign of magnesium deficiency. Adding Epsom salts, or magnesium sulfate, is supposed to replenish the missing magnesium, perking up the gardenia in the process. Some gardeners recommend a one-off dosing, others a feed at regular intervals.
Gardenias, along with roses, palms and podocarpus, need lots of magnesium. When the nutrient is in short supply, the older leaves turn yellow. Give the plant a boost (and maybe return some green to yellow leaves) by applying magnesium sulfate or Epsom salts.
It affects a range of plants including gardenias, rhododendrons, azaleas, camellias, roses and citrus. The solution is to get some Epsom salts. Take half a litre of water, and put in half a teaspoon of Epsom salts, then shake and stir. Either water it on plants or you can foliar spray it.
Though vinegar can be fatal to many common plants, others, like rhododendrons, hydrangeas and gardenias, thrive on acidity which makes a bit of vinegar the best pick-me-up. Combine one cup of plain white vinegar with a gallon of water and use the next time you water these plants to see some amazing results.
You can increase the acidity of your soil by adding things like compost and manures, leaf litter and mulch. Iron chelates work too. In extreme situations, you can use powdered sulphur - one handful per square metre, once a year. Sulphur works very slowly and you won't notice a change in your pH for about 6 months.
Mixing elemental sulfur with the soil is one effective way to reduce soil pH. Aluminum sulfate also can be used, but it reacts faster and may be harsher to plants. Elemental sulfur, because it takes longer to react, is the best option for soil acidification.
Gardenias require an organically-rich acidic (pH 5.0-6.5) soil with good drainage. Give plants plenty of room to stretch their roots – gardenias do not tolerate crowding and root competition.
Young gardenia leaves turning yellow is likely from low iron. All can be fixed by giving your gardenia a dose of an acidic nitrogen fertilizer containing micronutrients. Once your gardenia leaves are green again, use the fertilizer twice a month from early spring to late summer.
Are Coffee Grounds Good for Gardenias? Don't worry about adding more coffee grounds to your gardenia soil; these flowers absolutely love coffee. In addition to being an excellent source of nitrogen, the acid in coffee lowers the pH of the surrounding soil, stimulating the roots of acid-loving plants such as gardenias.
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.
Because gardenias need all three essential macro-nutrients (nitrogen, phosphorus, potassium) and also a good dose of micro-nutrients such as iron and manganese, you should feed them with an organic fertiliser such as Dynamic Lifter.
Low on Magnesium
If your gardenia needs magnesium, this will cause the leaves to turn yellow. You can remedy this with a fertilizer high in magnesium, or you can add some Epsom salts to your soil. Mix one teaspoon to one gallon of water and apply every two to four weeks.
How to use Epsom salts on gardenias. Use a watering can and add 1 litre of water. Stir 20 grams of Epsom salts into the water and mix well. Water this solution onto the soil around the drip line of your gardenias.
The yellowing is caused by a lack of magnesium. We suggest feeding with Tui Enrich Rose, Camellia, Azalea & Gardenia controlled release fertiliser in early spring and late summer, and applying Tui Organic Seaweed Plant Tonic regularly. Epsom salts will also help when there is a lack of magnesium.
Always maintain at least 2 inches of mulch to protect your gardenia's roots from heat or cold. Pine bark mulch or pine needles are the best mulches for gardenias.
If you want to spread mulch around your gardenias, go for a woodchip or sawdust variety. This will also add acid to your soil to help growth.
Plants like azaleas, gardenias and blueberry bushes need acidic soil to thrive, that's why ACV with 5 percent acidity works well as a fertilizer for certain flora.