Corrinne Hopkins has had a Wrightia religiosa, known in Thai as mok, in the same pot for more than five years. ''I really love it, especially when it is in full bloom, the fragrance is just very unique and perfumey,'' she wrote. ''Recently, maybe for the last two months or so, the flower buds dropped off without turning into flowers, and lately, even before they turn to buds they are dried out, almost burnt like. The leaves also have become smaller and shrivelled up on some of them. I am not sure what to do, or what the problems are. I really love this plant.''
a ‘Wrightia religiosa’ plant in peak condition.
I thought I knew the reason why Ms Hopkins' plant was acting that way, but just to make sure, I emailed her back to ask whether she has ever replenished the soil and how often she fertilises the plant. Her answer confirmed my suspicions.
The now one metre tall plant was repotted into its present container four years ago. It is therefore likely that the shrub is now pot bound. That is, the roots have filled the pot and grown into a circle next to the container wall. As the roots cannot absorb water or nutrients from the soil, the plant's growth has become stunted.
Ms Hopkins' mok is not lacking in fertiliser. In fact it is given one teaspoon of complete fertiliser once a week, and another teaspoon of slow-release fertiliser every month. Both fertilisers are inorganic and have the same properties, so applying both at the same time may be too much for the plant.
Have you ever held a fertiliser when it is wet? It is hot, so when more than the needed dosage is applied, the amount of chemicals released when water dissolves the soluble components may cause the tips of the roots to burn. Applying a teaspoon of NPK 15-15-15 or its equivalent for general growth, or NPK 12-24-12 if you want to improve flower quality, once every two months should be enough. Water the plant thoroughly after applying fertiliser.
slow-release fertiliser, applied every three or six months.
Slow-release fertiliser is so called because the nutrients are released from the plastic-coated pellets over a period of time. There are two kinds: one is timed for three months while the other is good for six. This means that you don't have to fertilise again until after three or six months. However, I noticed that the pellets deteriorate faster when exposed to full sun than when they are applied to plants in a greenhouse. It is best to dig the fertiliser into the soil around the root area, towards the edge of the pot, so that it does not deteriorate in the heat of the sun and to keep it from being flushed away when you water the plant.
When a plant wilts soon after watering and does not respond to fertilising, or when the roots are protruding from the drainage hole, it is time to repot. Ms Hopkins has trimmed her mok so the container looks like it's just right for the size of the plant. However, if she finds that the root ball leaves little room for soil when she repots, she will have to move the plant into a bigger container that allows for two or three inches of new soil around the root ball.
A day before transplanting, water the plant well. To remove the plant from its pot, tip it on its side, tap your hand or trowel on the container to loosen the soil then slip the plant out. Trim roots that have grown in a circle around the root ball as these will gradually strangle the shrub as it grows. To encourage a healthy pattern of root growth, tease some of the feeder roots loose from the surface of the root ball so that you can direct them outward when you plant.
Ordinary garden soil has a tendency to compact and constant watering will squeeze out all the air and nutrients from the soil. The ideal growing medium must hold moisture but drain easily. Ready-to-use packaged soil is available in garden supply stores, but make sure that the mixture contains an equal amount of loam and organic matter like compost. Alternatively, you can mix your own planting medium.
Cover the hole at the bottom of the pot with small pieces of broken clay pot to keep the soil in place but still allowing water to drain, set the plant in a bed of the new soil mix, then gradually add soil to within half an inch of the top. Tamp the surface gently to avoid air pockets, then water slowly to give the soil a chance to settle. Place the plant in partial shade while it is recovering and gradually put it back in full sun after a week or two.
Plants in containers need more frequent watering than those grown in the ground. The general rule is water thoroughly and not water again until the topsoil is dry to the touch. If saucers have been placed under the pots, place pebbles or clay bricks on them so that plants do not stand directly on the excess water.
Email nthongtham@gmail.com.
‘Wrightia religiosa’ planted in pots.
