Watering is usually the most easily satisfied requirement of plants, but moisture stress through over-watering or under-watering is very common. When little is known about the water needs of a plant species, some insight may be gained from the species' characteristics (i.e. leafiness, height and spread, annual, deciduous or perennial). Knowledge of the natural habitat might also provide useful information, e.g. soil type and whether the plant occurs in wet or dry areas, etc. Seasonal differences must also be considered; the water needs of a deciduous plant are less during cool and/or dry periods, when it is leafless and dormant, compared to when it is actively growing with a full canopy.

Cultured plants are often watered too much. Excessive watering causes depletion of nutrients through leaching in open, highly porous potting mixes, or waterlogging of less porous potting mixes. Over-watering can result when the watering regime is not adjusted with the changing seasons. Holding plant pots in drip trays that are constantly kept filled with water may also lead to waterlogging.

Reducing the volume of water being applied to the pot is the obvious solution to over-watering and waterlogging. Removing drip trays may overcome waterlogging problems. Re-potting the plants into a more open and better aerated potting mix should rectify waterlogging problems, but can lead to increased leaching of nutrients if too much water is applied. Leaching problems may be reduced by reducing the porosity of the mix (but balance this against the possibility of increased waterlogging), by increasing the humus or colloid content of the mix to provide better nutrient retentive properties, or by applying fertilizer at more frequent intervals.

Waterlogging causes direct physical damage to roots, and indirect effects on plants through increased susceptibility to diseases, production of organic toxins such as methane and organic acids, increased salinity and conversion of soluble nitrogen to forms not useable by plants. Toxicity of some elements, such as iron and manganese, increases greatly under very wet conditions. Through evaporation, dissolved salts from fertilizers may accumulate at the surface of waterlogged potting mix as white powder which may be injurious to the plant, possibly resulting in death.

Waterlogged plants exhibit wilting symptoms similar to plants suffering from lack of water or root rot diseases. Leaves often exude water from the tips and margins, where salts accumulate as white deposits and cause brown to black necrotic patches. A further indication of waterlogging is a pungent odour from the root ball, like that of rotting vegetation, when the pot is removed.

Healthy plant roots are normally white in colour with numerous lateral branches and a proliferation of root hairs just behind the growing tips. Under waterlogged conditions, roots are sparse, brown to black in colour, and tips, lacking root hairs, are in various stages of decomposition, or in severe cases, missing. Adventitious roots may begin to develop on the stem at or just above the surface of the mix. The effects of waterlogging manifest faster and are more severe at high temperatures, and in summer the effects may become rapidly lethal to some plant species (Handreck and Black 1984).

Too little water causes wilting, leaf abscission, flower abortion and withering or loss of fruits. If plants have not dried out completely they will regenerate if watered.

Water quality is important. Water with pH too high or too low, or with excessive levels of dissolved salts or sodium causes severe damage to plants. Water drawn from dams, creeks, and particularly bores should be tested to determine its suitability for use. For example, hard water contains calcium, magnesium, sodium bicarbonate and carbonates which can make growing media more alkaline, that is, raise the pH. Yellowing due to iron chlorosis is a common first symptom that something is probably wrong with the pH of the medium. Excessive watering with soft water, that is, water containing very little or no carbonates, calcium, etc. can, through leaching, strip elements from the potting mix causing it to become more acidic. In regions where good quality water is scarce, treated sewage effluent water may be used for plants. However, this water may also raise the pH of the potting mix.

Expensive water processing equipment can be used to remove the compounds that make water hard. However, the cheapest approach to dealing with water quality problems is to be aware that the water being used is imperfect. It is then necessary to remain vigilant for signs of plant nutritional deficiencies or toxicities that indicate problems due to changing pH.

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