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Dormancy – What Happens to Your Bonsai in Winter?

We know that the deciduous trees in our yards and yes our bonsai that are deciduous, drop their leaves after sufficient shortening of the days and cooler temperatures arrive.  Even my semi-hardy trees may drop their leaves before I finally relegate them to artificial lighting in my basement.  In preparation, trees store sugars  produced through photosynthesis, so that when spring arrives they have the strength to open their buds to grow new leaves to begin photosynthesis again.  Dormancy is a self -preservation defense mechanism that protects them from the savages of winter weather. 


Conifers appear to defy logic by retaining most of their foliage, but under severe enough conditions their photosynthesis stops too.  To minimize dehydration, old needles or foliage browns and drops off.  They also store sugars for the spring push.  Unlike deciduous trees though, they carry on a low level of photosynthesis when the winter temperature rise (above 38) with the needles that they retain.  This is also why they can use poo balls to feed them at a low level over the winter months.


Tropicals seem to continue to grow all year round.  But even they need a cooling/rest period.  Most growth pauses by December and then gains speed about February after a couple of months of rest.  For all three types of trees this period of rest is almost as important as the period of growth.  I can recall when I began bonsai and didn’t understand that without this rest a tree will eventually exhaust itself and become weaker and weaker perhaps even die if they do not go through this dormancy period. 


A former club member said she’d kept her juniper continuously in the house for six years before it weakened and died.  The big exception is Jack Wikle, who has kept his mame’ collection under lights continuously for over 20 years.  His technique is described in the Brooklyn Botanical Garden’s Indoor Bonsai book in the chapter called, One Grower’s Tips for Success with Indoor Bonsai.  Three of Jack’s secrets are; 1) put the lights as close as possible (no more than 6” above them) and put them on a timer for 16 hours a day, 2) use a light fertilizer solution every time you water (he mentions Peter’s 20-20-20, at 1 tsp./5 gal of water.) and 3) use a sand bed or folded towel under the pots as a pot should never stand in water.  His soil mix is builder’s sand, sphagnum peat and sandy loam garden soil in a 1:1:1 blend.  If you haven’t seen Jack Wikle’s amazing mame’s they are all less than 6” tall.   Jack only treats his mame’ bonsai in this manner, he has or had hardy larger bonsai outdoors.  (Jack recently sold many of his larger trees.)


Interestingly some species of tropicals seem to respond to shorter days and cooling weather by blooming.  So don’t be surprised if your Beauganvilla, Rosemary or Serrisa bloom in late fall or early winter.  Also if you’re doing it right some may bloom in February when they think spring has arrived.  I had an Azalea that was bought for Valentine’s Day that bloomed every year in February.  This clued me that the tropical period of dormancy was over.  (I over-watered it to death)  So be careful with your indoor bonsai, their water use during the winter month may be much less than during the summer. 


 By Ken Schultz