How Trees Grow - a talk by Andy Hardman on 11th June, 2018.
Tap roots are needed in times of drought and for support. Usually we wire bonsai into pots, so do not need tap roots. One root feeds one branch, or part of one branch. Roots store nutrients and water. In arid soil there are fewer, thicker roots. In shallow, moist soil there are more roots which fork more finely. Visible roots transport and store sugars, salts and hormones. The lower roots grow all around the trunk, with branches that are microscopic. There are millions of root hairs which take up water and nutrients. Andy showed a diagram of the cambium layer showing the phloem and xylem layers, which take up water and reserves of energy from photosynthesis.
It is important to start repotting as late as possible, waiting until buds have swollen due to the increase of daylight and warmth. He prefers to repot trident maples in September. They can be defoliated if kept under cover. Trees can die over winter if they weren't fed enough. In August and September trees are growing buds for next spring. Give feed containing all 3 main elements - NPK - not tomato feed, which does not contain nitrogen. Low nitrogen feed is much better. In spring leaf pruning will produce several lots of buds.
Root hairs take up water. Each cell will pass water on to the next cell. This process is initiated by the leaves. In hot weather leaves wilt like a string of sausages. A row of cells can spiral around the trunk and you can kill areas by cutting across. Millions of root hairs can be ripped off by bare-rooting a tree, leading to poor vigour. The Japanese wash off the roots when they first get the tree. They should not be jet washed. Leaving trees pot-bound limits growth and Andy repots tiny conifers on his display table every four years. Water expands roots and can crush cells in roots. When repotting, there need to be air spaces between particles and it is important to find the best size: the more surface area of a particle, the more root hairs grow. By drawing a cube and dividing it up, it is evident that smaller pieces have a larger surface area and water surface tension will bridge the gap. Free draining soil gets frozen and ice crushes roots. 2-3mm is best. He uses Akadama with organic bark humus and sieving reduces dust and large particles. Horticultural grit doesn't hold water and shouldn't be used for drainage.
Cell pressure gives rigidity. Photosynthesis occurs when the chlorophyll in leaves is exposed to sunlight, binding water and carbon dioxide to make sugars, giving plants nitrogen, phosphorus and potassium, protein and salts, which give disease control. The more leaves, the more sugars, proteins, etc. are produced. Pinch leaves out of maples which need more ramification, giving smaller leaves, and leaf prune each year. Buds are activated to give new shoots instead of leaves. Feed really well in spring then reduce the feed in late summer. Tap water may contain lime, so trees should be kept outside to receive rain.
In the tropics, there is little change in temperatures throughout the year. Our trees have periods of dormancy in cold weather. Transpiration raises salts and ph, which can cause brown edges on leaves in salt sensitive species such as maple, hornbeam, spruce, yew and hawthorn. Don't feed maples until the end of May.
WATERING AND NUTRIENTS
BRANCHES AND LEAVES.
Branches support structures. Leaves grow on branches to get sun. Prune out downward shoots as they die off in the shade. On a cross section of trunk the growth rings are visible. Xylem cells grow fast and rings are wider apart in years of good weather.
Pines and junipers are resinous. Resin waterproofs trees so branches can't rot. Cell structure on deciduous trees rots if water soaks into the wood. Don't leave jins, put hollows in. The cambium layer has latent buds. At the apex the apical buds produce auxins and latent buds stop growing. When cutting a branch out, cut a tear drop shape underneath so it repairs quickly.
Tree longevity depends on the length from the top to the roots. Bristlecone pines in the Rocky Mountains live 5000 years, growing in the debris in rocks and only reach 3 metres tall. Hawthorns used as hedges have all shoots near the ground. Bonsai in Japan live for 600 years. Xylem only lives for 18 months and phloem for 3 or 4 years. Repotted bonsai are constantly renewing. Sugars and hormones build up at the site of an air layer and roots form there. Trees left in the shade for a couple of weeks will look greener for a display.