On watering

Although I consider myself a gardener and as such is interested in all aspects of biology, chemistry and physics, visitors frequently ask what kind of biology I specialize in. I explain I have a PhD in plant physiology. Quite a few look surprised and politely remark they did not know plants needed psychologists.

A plant physiologist studies the function of plant cells and tissues, and may spend time in laboratories for in depth studies of the role of e.g. plant hormones, vitamins, enzymes, mineral uptake, photosynthesis, symbioses, seed germination and reactions to pests, fungi, virus and bacteria. However, I have found that the greatest need for a plant physiologist in daily life is when it comes to the subject of watering.

For instance, an Australian friend recently told me about a governmental tree planting project somewhere on our planet, where the planting commenced just before the onset of the dry season. My friend was upset and told the organizers the plants would most likely die. People have good intentions, but many treat plants like garden furniture, not like living beings.

Therefore I thought I should share some basic advice which might be used anywhere, but particularly in a monsoon climate like at Dokmai Garden. An experienced gardener may find some of the advice below to be at kindergarten level, but at Dokmai Garden I meet with workers, students and visiting plant enthusiasts who have very limited knowledge about growing plants, so after a few years here I take no knowledge for granted. You do not have to be a plant physiologist to figure out these recommendations, you simply need experience from cultivation, but a plant physiologist may explain why a certain practice is successful and another one is detrimental. The checklist below can also be useful when instructing a new worker.

1. Plants are living beings and so they need water. All life forms; bacteria, mushrooms, plants and animals, are composed of cells which contain water to enable activity of proteins, which regulate cell activity. Some life forms may survive a dry condition, but they remain inactive until they get water.

2. Different plants have different requirements. Some 300 000 vascular plant species (flowering plants, conifers, ginkgo, cycads, ferns, angel wings, whisk ferns and club mosses) are hitherto known by science. Since they grow in different environments, they have different water requirements. A default watering management suitable for all 300 000 plant species does not exist.

That a plant should be treated differently from another can be terribly difficult to understand for a person who has never left his hometown. He only knows the local plants that survive in his local climate, and he may laugh at you when you introduce an exotic plant and explain it may require special attention, especially from a genus he is used to.

A Sumatran rain forest blood banana (Musa acuminata ssp zebrina) requires much more water and no dry season in contrast to a monsoon ‘gluay nam wa’ banana (Musa acuminata x balbisiana ABB). A mountain cherry may die at the sea shore due to salty water, a lowland breadfruit may die at high elevation due to frozen water, a Mediterranean olive may drown or die from rots if watered heavily all year round, a monsoon cassia may have a weak flowering hidden by leaves if watered in the dry season, and a rain forest nutmeg will die if left unattended in the dry Chiang Mai monsoon climate. How would you know? Trial and error is a good way, but will cost time and money. If you know the scientific name of the plant, then you can find out its origin, to learn about its requirements.

3. If you plant a seedling, do so in the early rainy season or make sure you can add water during the phase of establishment. Even if you plant a native seedling during a dry season, it will have severe difficulties establishing itself without help. Natural seed germination usually occur during rains, and to make sure an erratic shower the wrong time of the year does not awaken the innocent seed prematurely, it is often equipped with germination inhibitors which can only leach out if you water generously or if it rains heavily.

4. Terrestrial plants rarely like to expose their roots. Humans can move plants if they know what they are doing, while in nature most plants stay were they germinated. Exposing the roots in the dry air even for just a few minutes may kill some plants. I have seen workers dig up a plant, throw it on the ground in the sun, take a lunch break and then carry on. This will work for cassava and frangipani cuttings, but not for tree saplings. Plant and water the sapling immediately after transferring from a nursery, or let it recover from transportation inside a new nursery.

The roots may be severely damaged when you liberate them from a pot or net prior to planting, and the remaining roots may therefore not be sufficient for supporting enough water to the leaves. You may have to cut away all leaves of the plant to reduce water losses. If moving a shrub from one place to another, you may want to consider just moving the root ball, cutting away all branches.

In the tropics you should always try to shade your recently planted treasure, also in the rainy season, because the plant is adapted to the shady conditions inside a nursery, and will have problems or may even die if suddenly exposed to stronger light and heat combined with root damage. Studies of leaves from the same plant individual during shady growth and during sunny growth reveal different amounts of leaf molecules used as sun screens (e.g. carotenoids). In the shade there is no reason for the plant to waste valuable material and energy on excess amounts of sun screens, but if suddenly transferred, the plant is not prepared. A gradual adaptation to stronger light is another method used for large scale tree plantations, i.e. leave the pot out in the sun for five minutes, next day for ten minutes, then 20 minutes and so on.

5. When you water with a hose, check the water temperature with your fingers first. A hose left in full tropical sunshine will not only eventually crack due to UV destruction of the material, but the water inside could be terribly hot which may cause cell death of shallow roots.

6. When you water with a hose, do not shoot high pressure water at the base of the stem. Sometimes the force of water is so strong the young bark is damaged, and the soil surrounding the stem base is flushed away, exposing the roots which may lead to infections and drought. Instead, water gently around the plant. A leaf litter like in a natural forest is good to reduce the force of rain and to keep the moisture, but make a ring free of litter around the stem base to avoid fungal rots.

Never water orchids with high pressure water, or fruits and flowers will be lost and a year’s work is destroyed.

7. Watering is most effective in the evening and night. During the hot day, a lot of sprinkler water evaporates too quickly. Sometimes it is recommended that epiphytic orchids should be watered in the morning so that they can dry out during the day. That applies to indoor or greenhouse conditions where excess moisture may cause rots in orchids from monsoon areas. In a monsoon climate you should observe nature, and here it often rains in the late afternoons, nights and early mornings, so you should do the same. Since the sky is the limit, the moisture level is rarely saturated.

8. A leaf is like a solar panel and should be kept clean. Some plants like citruses may develop more bacterial cankers if their leaves are watered. Although wet during the rainy season, the long droughts in the dry season seem to limit the cankers, while if watered all year round the cankers may kill the trees. There can also be risks with watering banana leaves, enabling spores of the sigatoka fungus to germinate. These are exceptions and due to introduced diseases. In a natural environment the leaves need the rain to cool down and get rid of dust which reduces light and speed of photosynthesis.

9. Some plants demand more than wet roots, they demand a moist atmosphere. That is why mangosteen is so hard to grow in Chiang Mai, which dry climate resembles Central India more than the wetter Bangkok. If you have a rain forest plant, grow it in the shade and consider either daily hand watering of the entire plant or install a misting system. Leaves not adapted to drought will die in dry climates, so you have to change the environment.

10. When potting a terrestrial plant, make sure the pot has drainage holes. It has happened numerous times that volunteers helping out at Dokmai Garden have been asked to repot some seedlings, and they have taken pots for aquatic plants without drainage holes. The roots of a land plant consume oxygen and if submerged under water for too long they may drown. A related problem is that you should not add a heavy clay to a pot, which will block the drainage and kill most plants. Use a porous compost with sand to allow drainage. Keeping the soil constantly wet may cause a rot in many plants. It is better to allow the pot to dry out and then water again.

11. When potting an aquatic plant, make sure the pot has no drainage holes. This may seem obvious but apparently it is not. When I noticed a Persicaria odorata stand shriveling up in a pot I watered it, contemplating what is going on in people’s heads when they ignore a plant obviously screaming for water. Next day the pot was dry again, so I suspected a crack and felt guilty for my thoughts of blame. I turned the pot and to my surprise there was a neat drainage hole in the bottom.

12. A mangrove may not necessarily demand salty water. It is always good to observe plants in the wild and then try to copy their environment for successful growth in your garden. Mangrove plants are commonly sold in pots (e.g. Nypa fruticans and Sonneratia spp) and many believe one should add salt to the water. I suggest you should not, because they rather endure it than demand it. In nature they are competitive at sea shores because competing plants can not survive being submerged into salty water.

Limestone plants do not necessarily crave calcium rich water, but unlike many other plants they can endure soils poor in phosphorus, a deficiency caused by tight bonding to calcium. As long as the gardener cares for the limestone plant, i.e. removes surrounding competitors, a calcium plant may very well thrive on a neutral soil.

13. Epiphytic orchids demand rain water. Rain water has an almost neutral pH around 6.8 and is poor in minerals, so it is a good precaution to analyze your orchid irrigation water regarding pH (acidity). A high pH (alkaline water) may lead to nutrient shortage in the epiphytic orchids and if too rich in salts it may lead to cell damage.

14. Consider the edge effect. Sometimes a gardener can not understand why a plant in his garden suffers tremendously, while it grows handsomely in the forest 5 km away at the same altitude, with the same solar intensity and with the same soil. The reason can be that if a moist-loving tree from inside the forest is grown in a small woodland in a rural area, the moisture levels are lower due to lack of canopy. Trees growing within a 2 km distance from a cut forest edge may suffer from drought stress, the more severe the closer to the edge.

15. Consider drainage. When you design your garden you need to avoid areas of water logging. If you plant a drought-loving plant such as pomegranate you may even want to make a little mound to avoid water logging, while a swamp plant such as pachira nut may appreciate a pit. Many native monsoon plants can stand shorter periods of water logging, but a general advice is to make a flat surface around a recently planted seedling or sapling. Excellent drainage can be obtained through sandy rather than clayey soils, slopes, deep holes filled with sand and gravel for desert plants, elevated flower beds and drainage tunnels for quick outlets. At Dokmai Garden we have even built an artificial hill for the most vulnerable plants, but also as a feature to break up the flat landscape. It is easier to add water than to take away water, and excess water may cause soil erosion, so of two evils I prefer drought to flooding.

16. Use a rain meter. Quite often a gardener will observe a rain shower with satisfaction but to his surprise the garden still looks dry the next day. A rain meter will give an objective measurement of how much water you actually got. In a monsoon climate anything below 5 mm should be considered ‘nothing’ and you need to irrigate in an area with water demanding plants.

Using the rain meter you can actually test your irrigation equipment and measure how long time it takes to get say 10 mm (which equals ten litres of water per square meter, or one regular watering can).

Use a spade to pedagogically show that in spite of a recent shower or a squirt from the hose the soil is still dusty dry just 2 cm down. Asking somebody to ‘soak it’ may not be a sufficient instruction, as that is a relative amount. Better say ‘leave the hose for ten minutes and then move it to the next plant’.

To penetrate to a reasonable depth it is better to ‘soak’ a plant at intervals (pay attention to the first signs of hanging leaves and you will soon know what a particular plant requires) rather than adding insufficient amounts (2-3 mm) every day. Such a daily watering regime only consumes water and money with little effect on the garden. A plant which sheds its leaves is usually dormant for a long time and must not be watered at all. If you do, it may wake up and perhaps die from a dry atmosphere, or start making leaves and flowers when you expected flowers only.

17. A young compost may remain dry even after a 30 mm shower, due to the thick layers of not yet decomposed leaves protecting the core. This is why you need to turn the compost to break up the leaf layers. Without water there will be no microbial degradation and no new soil. Composts need irrigation too.

18. Related questions are the installation of irrigation systems, but that field is more of physics and mechanics. I can just mention that you should not install a pump 500 m from the source of water, but as close as you can. A suction over a great distance reduces the pressure, and so does unnecessary pipeline bends. A trench for the pipeline sloping upwards will also reduce pressure, while it is better to use gravity and let the water flow towards the sprinklers. These advice may seem evident but I have seen the strangest irrigation systems so supervision is always crucial to avoid the mistakes mentioned above. Investing in metal sprinkler heads allowing spots, showers and sectors is highly recommended.

Is this a result of water deficiency? No, it is a herbicide landscape!

Text & Photo: Eric Danell