No. 4 Containerised trees – what time to plant
NEW ZEALAND FARM FORESTRY ASSOCIATION INFORMATION LEAFLET
Introduction
Over some 16 years we have advocated and promoted the planting of
containerised trees over late spring and summer with outstanding
success in survival and growth rates. During the summer of 1998/99 we
really put our concept to the test by planting on severely
moisture-deficient sites that had caused mortality in established trees
up to 20 years old. One planting was undertaken in temperatures that
reached 35°C in the shade! With the exception of one site that we
abandoned before completion, the results were outstanding.
In this article are some observations and techniques we feel should
help make the concept of spring and summer planting more widely adopted.
The importance of roots
Ground preparation
Effective weed control
The importance
of roots
With the trend in garden centres towards emotional sales based on
foliage and flowers, the importance of roots is often totally
overlooked. To me, the roots are the most vital part of the tree, as
initial establishment through to eventual tree stability are critical
for success.
The trees we plant – which may include containerised eucalypts,
conifers, poplars and silver birch – are quite small but have
self-contained root systems that may in mass equal the stem and leaves.
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| If carefully handled,
eucalypt (on right) will
establish very quickly
(note root length). The eucalypt (middle) and Pinus radiata
(left) are
our preferred size with greater root mass than stem/leaf mass. |
Compare this with your 2m tall specimens perched precariously in bags,
with stability reminiscent of a picnic umbrella in a gale.
If we consider the tree too tall we snap it back, often quite
ruthlessly, to balance the roots with the foliage. The greater the leaf
area, the greater the transpiration. The only way the plant can hydrate
is through the roots, so balance is critical.
All nurseries I am familiar with are located in relatively sheltered
climes with much money spent on enhancing the environment to create a
microclimate relatively free from wind and temperature extremes. I have
always found it difficult to understand why nurseries recommend winter
as the best time to remove these pampered containerised stock and plant
them in a hostile, cold environment where animals are foraging for just
the sort of tasty morsels these nutrient-rich trees provide.
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| Eucalyptus
saxatalis planted in November
from 20cm-tall nursery stock
measured 1.4m tall x 1.2m wide at six months. |
In Southland, this vulnerable period may be three or four months.
Compare how trees establish in nature. Over-winter conditioning and
subtle changes in temperature, light and moisture break the dormancy of
the previous year’s seed to enhance germination just when conditions
are ideal. This enable the seed to grow rapidly before exposure to its
first winter, and better handle the vagaries by due hardening off in
the autumn. Have you ever tried to pull these trees out
mid-season and seen their huge root system? I often get clients wanting
instant tall trees for shelter not appreciating that such trees would
each require at least a cubic metre of soil to hold the root system.
(top)
Ground preparation
To guarantee a high rate of tree survival after establishment it is
imperative that ground preparation is of a high standard.
Deep ripping with a winged ripper is a well proven method of site
enhancement, as is cultivation over the rip. There are several reasons
for this:
Temperature
When you plant a tree, exposing the bare soil creates a much freer
conduit between ground temperature and air temperature. Even in
Southland, the ground temperature rarely reaches freezing and this
higher ambient temperature goes some way towards raising the air
temperature, which can drop to -12°C at ground level during some
winters. A late frost, the greatest enemy of tree establishment, can be
completely negated by bare soil. Mulching, normally beneficial in other
ways, can cause severe losses due to frosting.
Chemical
interaction
Most tree planters regard weed control as removing or killing existing
vegetation to minimise the effect of competition for nutrients and
moisture. While obviously a factor, it is of relatively minor
importance. Of greater importance is the effect of allelopathy, the
term used to refer to certain biochemical interactions between all
types of plants, including micro-organisms.
All plants to some degree, secrete chemicals from their roots, often to
repel competing roots from other plants, be it grass or clover. These
chemicals can remain for some time after the plant has been killed. It
is important therefore, to remove the turf, dead or alive, and plant in
the loose soil beneath.
Good, deep ripping will completely shatter a wide deep profile.
Cultivation and thorough mixing of the turf profile with the soil can
also suffice. Roots also move rapidly through areas of loose soil and
will not enter areas without any oxygen such as tight clays or areas
with a high stagnant water
table. Ripping will ensure a rapid root run and efficient scavenging of
nutrient. On average sites, no amount of fertiliser will achieve better
growth than good site preparation.
Once the tree is planted with the self-contained, well soaked
root-ball, we find that root growth is more spectacular than top
growth. We have observed 1cm of root growth from all growing points
five days after planting. Weed control during this critical phase is
paramount as germinating weeds will race the tree roots for areas
containing nutrient and moisture. We have examples of eucalypts
planted during late October having a 2m diameter root mass by May where
total weed control was achieved. Interestingly, the tree was only one
metre tall but ready to achieve 2m or more growth the next season.
The
thatch effect
Assuming you choose to ignore the above recommendations and continue
the traditional spray/plant or plant/spray method, be aware that dead
turf, particularly from older pasture, is an extremely effective
thatch. Once dry, it will take rain of monsoon proportions to moisten
it. It is amazing that any trees survive. Ripping fell into disrepute
among some planters because the heave caused by ripping combined with
no turf removal or cultivation creates a profile like the roof of a
house – it is just as effective in shedding rain. Get rid of this turf.
Pamper
the tree
By December, the tree in the nursery can be putting on phenomenal
growth but so is its wild cousin. Forget about hardening off stock, but
a little help will get it through those summer gales. Make sure the
tree is planted in a depression by removing the turf and placing it on
the upwind side of the tree. The tree will adapt quickly to the site
while receiving early cost-free shelter.
(top)
Effective weed
control
By planting during spring and summer the site will have a
generous
growth of weeds. I have noticed that in Central Otago an apparent
paucity of ground cover can disguise a solid underground root system;
the poor moisture retention of the soils dictate that a substantially
greater root mass is needed to support the leaves above, and it is the
weed roots that are the real problem. Winter spraying often creates a
seed bed for masses of thistles, but if growth is vigorous, they are
easy to control with a later spray. Make your spot at least 2m across
for the reasons explained above. The smaller the spot, the less the
early growth of the tree seedling.
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| Eucalyptus
gunnii at Manapouri, 23 months
after planting. Trees were
sown in September, then planted 12 weeks later in a severely
moisture-deficient site. |
It is my experience that when you plant over summer without following
the above steps, the tree is likely to die
when it comes under extreme moisture stress. The higher rate of
transpiration of larger trees puts them under greater pressure, and
without adequate weed control the roots will quickly extend into the
pasture zone. This sharing of moisture with other often apparently
innocuous grasses will cause earlier soil moisture loss, associated
tree stress, and eventual death. By contrast, a small newly planted
tree has relatively low transpiration needs compared to its big
brother, and on a carefully prepared site it has all the goodies to
itself.
Many of our clients refused to accept the concept of planting in spring
or summer unless I offered a guarantee because they believed their
sites got too dry over summer. They are now regular clients. One
long-time tree planter of some ability in one of the toughest farm
environments in Central Otago told me he liked to water the trees in
their first year to guarantee survival but said “If I plant them in
January I only have to water them for 2-3 months. Planted in August, I
have to water for up to 7 months”.
(top)
This article by Graham
Milligan appeared in the August 2000 issue of the New Zealand Tree
Grower.
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