Page 178 - Tài liệu cây cảnh Bonsai4me Bonsai Basics

against though you should be able to anticipate which trees in your collection are

more likely to be attacked.

Yellowing Leaves/Dropping Leaves

There are only 3 ways that a healthy tree with healthy foliage will suddenly
lose leaves or have leaves that suddenly turn dry and crispy (over just 2 or 3
days):
Frost, a tropical and subtropical species being exposed to frost
Poison, the bonsai is exposed to a poisonous chemical either in the soil or the

air (directly onto the foliage). Though very rare, it isn't unknown for a tree to be
badly affected when accidentally exposed to drifting spray from weedkiller use.
Underwatering is by far the most common reason for the sudden drying up
and death of healthy foliage. Once there is no moisture left in the soil of the
bonsai, the leaves will die within hours. Was the soil allowed to dry out
completely? Was the soil watered thoroughly enough the last time the tree
required water? Was the soil dry but looked wet because you misted the tree and
the surface of the soil? Less severe under watering can also lead to yellowing of
the leaves; see below.
Yellowing leaves and/or dropping leaves can occur for a number of different
reasons;
Chlorosis is caused by a mineral deficiency and is due to a lack of
magnesium, manganese or iron. It usually only affects acid-loving species such
as Azaleas. Administer a liquid fertiliser that contains trace mineral elements
easily available at all garden centres. Acid-loving species such as Azaleas can
(and should) be fed Miracid, which contains easily absorbed sequestered iron on

a routine basis.
Dieback and yellowing leaves nearly always end up dying and falling off the
tree unless the cause is Chlorosis, this is likely to be dieback of the foliage.
Dieback of large areas of the tree can occur when a tree is traumatised for some
reason and the tree responds by dropping any foliage that is not required for its
survival. The cause is often due to damage to the rootsystem by rootrot through
overwatering or lack of watering which has allowed the root system to dry out.
Some species (particularly tropical indoor varieties) can also become stressed by
moving a tree to a new position, and they will loose their foliage. (See sections
on rootrot and underwatering.)
Natural wastage some trees such as Pyracantha/Firethorn and Ulmus/Elms
will develop new growth from leaf axils and will then naturally discard the now
redundant leaf. Check to see if new growth is appearing from the point of leaf