Factors that Influence Watering
by Brent Walston
I like to check for dryness by lifting the pot. It takes some
experience to develop the 'feel' of a dry bonsai, but it works quite
well. It is amazing how much an adequate amount of water will increase
the weight of the pot. Of course this is only possible for smaller
plants. I don't go around lifting 5 gallon cans to see if they are dry,
or the monster bonsai. But since about 25% of the volume of a proper
soil mix is water at the saturation point, this is a significant
increase in weight, and one needs only to pick up a pot when wet, and
then when dry, to tell the marked difference. Dryness can also be
tested with a chopstick inserted into the bonsai pot as a sort of
dipstick. When the chopstick is pulled out and it is nearly dry, it is
time to water. This is a method advocated by Michael Persiano. A third
method, perhaps used by most people, is to dig half an inch under the
soil surface. If it is dry down to this point, it is time to water.
There are several factors that affect 'drying time' and they are all interrelated. For example, Malus and Prunus
can tolerate denser soils under optimum conditions because they grow so
quickly that the roots soon colonize it. This rapid top growth quickly
pulls the water out. I have to prune all the time of course, so when I
cut them back to form a new trunk section, they are now too wet until
the top grows back.
Here are some of the factors I have found that influence the drying time:
primary components holding water are: organic and inorganic fines (very
small particles), peat moss, vermiculite, and clay. If you increase the
amount of any of these amendments to your soil mix, you will decrease
drainage, and increase the water holding capacity of your soil, which
will increase the interval between watering.
Increasing the percentage of these elements to the point that the
soil will hold more than 25% of its volume as water is not recommended.
Beyond this point you may begin having root rot problems form decreased
aeration (poor drainage).
The components that increase drainage (aeration) and reduce
water holding capacity are: inorganic and some organic (bark) large
particles (greater than 1/8 inch). These include lava rock, coarse
sand, perlite, turface and other stable fired clay products.
Plant Size and PruningFast
growing leafy species colonize rapidly, drying out the soil. Root bound
or even properly root colonized plants will dry a pot very rapidly.
This rapid drying of the soil is very healthy for the plant, if you
manage to water often enough. Each time the plant dries out it pulls a
fresh charge of air into the root zone. Likewise, each time the plant
is watered and the excess water drains, another fresh charge of air
follows the water to through the root zone. I consider the ideal
watering interval during the growing season for outdoor plants to be
one day. This makes watering easy to remember or schedule, and will
almost assuredly prevent root rot problems. Cycles shorter than one day
inevitably lead to dry or wilted plants on occasion.
Plants that are well root colonized in the pot shorten the watering
interval as the top grows and demands more water. After a plant is top
pruned, transpiration is decreased and the watering interval is again
increased. For plants that are very susceptible to root rot, it is
important to pay close attention to this factor.
affecting how fast the plant will grow, fertilizer can accelerate the
drying time. It can also speed the decomposition of the organic
portions of the soil, causing premature soil collapse which increases
drying time and slows growth. Soil collapse due to decomposition is a
much overlooked factor in plant growth, and water intervals. It can be
avoided by using a higher percentage of stable inorganic material and
high quality organic material such as fir or pine bark. Decomposed wood
fiber products other than bark decompose very quickly and are generally
unsuitable for bonsai. The same is true of garden or other compost.
rot will decrease the ability of the plant to take up water and slow
the drying time. The symptoms of root damage can be very misleading.
There are several diseases that cause the blockage of the plant's
vascular system, preventing it from taking up water. The outward
symptom is the wilting of the leaves. The natural inclination is to
water the plant, but the problem is not a lack of water, it is the
inability of the roots to take it up. This leads to overwatering which
severely exacerbates the fungal problem. The solution is to let the
plant dry out, not to water it. Whenever a plant wilts, first make sure
that the soil is dry before watering it. If it is not dry, this can be
a symptom of a fungal infection of the roots.
will increase transpiration and decrease the watering interval. Strong
winds, under even moderate temperatures, can very quickly dry out a
plant. Some plants are much more vulnerable than others. In general you
should not place bonsai in an area that receives prevailing winds. This
can even be a problem in winter when the soil is frozen around the
plant's roots. The roots cannot absorb water under these conditions,
but the foliage and stems continue to lose water. It is very important
to protect plants from wind under these conditions, and to make sure
your plants are thoroughly watered before the onset of cold drying
Sunlight will heat the plant and the pot, increasing transpiration and evaporation, decreasing the watering interval.
Bonsai grown in the hottest and driest areas of the country need to
be located where they will get morning sun and afternoon shade. The
fastest growth will occur where there is bright light and optimum
(moderate) temperatures, so there is little growth loss for most plants
by placing them in the shade in the heat of the afternoon.
Optimum light levels will result in the fastest growth which will also decrease drying time by promoting increased foliage.
High temperature will increase transpiration even in the absence of
sunlight and decrease drying time. Temperatures above 80F, with morning
to full sun, and moderate to low humidity, will usually mean watering
every day for most established container plants (and bonsai).
humidity will decrease transpiration, and moderate high temperature,
which will increase drying time.
Increasing humidity can be a valuable part of extending the watering
cycle in hot dry climates. In our area, I can avoid watering twice a
day by giving the plants one or two short (several minute) bursts of
fine spray during the heat of the afternoon when the temperature
Pot Size An
extra volume of soil increases the reservoir of water and increases the
drying time. For very fast growing, water thirsty species such as Salix (Willow), this is a must.
Many of these factors may be manipulated to control the drying to suit
our conditions. I feel that the ideal drying time is one to two days
while the plants are actively growing.
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