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It's alarming to see a plant suddenly losing leaves or wilting. In many cases this can be a sign ill health, but it might also just mean the plant is starting to go dormant. Knowing your plant's specific seasonal needs can help you determine if your plant is entering dormancy.
WHY HAS MY PLANT LOST ITS LEAVES?
Many plants have a period of dormancy at some point during the year. For temperate species this will be during the winter months, as a consequence of the temperatures dropping and the days turning shorter. As the season changes, your plant will enter a dormant period to preserve energy. Deciduous trees and bushes lose their foliage during the autumn, leaving only bare branches. Deciduous perennials die down to the soil surface and spend the winter resting as thick roots, rhizomes, bulbs or corms.
Some tropical and subtropical plants also have dormant period, but not due to the cold or shorter days. Because they're adapted to warmer climates, they may go dormant to survive periods of drought. Almost all tropical plants that produce tubers, rhizomes, bulbs or similar growth under the soil line are adapted to this. Some can still be grown all year around without issue, while others go dormant on their own.
SIGNS TO LOOK OUT FOR
Signs of dormancy vary from plant to plant so it's helpful to understand your plant's specific needs. Signs can include:
Growth slows or stops
Leaves turn yellow before dropping or drying on the plant
After the leaves drop, branches don't shrivel or feel hollow
WHAT TO DO NOW
For temperate plants that overwinter outdoors:
Your plant and its root system is much more exposed to the weather and temperature changes when grown in a pot versus in the ground. It means that plants that are supposed to be hardy in your zone might still get damage from winter conditions and hard freezes. To minimise the risk of this you can do the following:
Bury the pot so that it's level with the ground through the winter. This will provide a stable temperature for the roots. Add extra protection by mulching the surface.
Reduce watering when temperatures begin to cool and days shorten
Bury the pot in a leaf pile for the winter
Insulate the pot with burlap and place it against a wall or some other sheltered spot
For non-hardy plants that need a cooler winter dormancy:
There are many plants that need a cooler period during the winter but can't handle frost or low outdoor temperatures. Some might need a bright, cool spot while other can be placed in a dark garage or basement that's frost free. You find this information in the info tab for your plant. In general, evergreen plants will need lots of light, even in the dormant period, while decidious plants can overwinter in darker areas. The watering must be reduced to prevent root rot.
For tropical plants that require dormancy:
Many of the tropical houseplants that have a dormancy period due to drought don't have to be forced to go dormant. The plant will determine whether it's needed. For plants that don't go dormant by themselves, winter is a good period to let them do it by slowly decreasing watering. That's because the light is weaker and the days are shorter anyway.
Place the caudex, tuber, rhizomes or corms in an area of normal room temperature and stop watering almost completely. Many caudexes are then woken up in spring by carefully watering them - Stephania for example also needs higher humidity to break dormancy.
Plants that die down to the soil line can be kept in dark areas while caudexes and tubers planted above the soil line should be kept in a bright spot.
Before restarting them in spring, plants that produce a number of new corms or rhizomes each season should be separated to avoid crowding.
Species with smaller rhizomes and tubers can be harvested in autumn and placed in zip-lock bags with vermiculite. Be careful to check on them now and then so that there's no moisture trapped in the bag as it might lead to rot.