The Philosophy of Watering: Outdoor Plants.

Otago Witness, Issue 1875, 28 October 1887, Page 8

The Philosophy of Watering: Outdoor Plants.

Two acknowledged agencies in vigorous and healthy plant growth are heat and moisture ; plants growing out of doors must usually take the heat as they find it, and, as we cannot increase this element, our object should be not to diminish it. Moisture, on the other hand, is more under our control ; but if we exercise that control, and water our plants during dry weather in the evening, we do so at the expense of a great portion of the heat we decide to preserve. Two influences are thus brought into operation in cooling down the plants and retarding their growth, which we vainly endeavour to urge forward by moisture ; these are evaporation and radiation. Evaporation is the more rapid in proportion to the dryness of the air, and therefore it is most active when the necessity for watering is the most urgent ; but evaporation cannot take place without producing cold, and that cold is proportionate to the rapidity of the evaporating process. Chemistry points out the reason of ,this ; vapour having a greater capacity for heat than water, the heat, sensible in the water, becomes latent in its vapour, and the temperature falls— additional heat, to keep up the temperature, not being quickly enough supplied by the surrounding media. ' What, then, is the practical effect of an i evening's watering? The air is dry from the heat of tho day, and evaporation goes on briskly ; the temperature consequently falls, and the plants are chilled, as there are no sun's rays to communicate fresh warmth, The growth of plants thus treated js sometimes even more unsatisfactory than that of others growing in apparently arid bpil, which have been allowed to take their chance. The other source of diminished 'temperature we referred to was radiation. Every warm body tends continually to throw off its heat to all other bodies of lower warmth, near or remote. By adopting morning waterings theie can be no doubt but that evaporation will go on much more freely, but the atmosphere is becoming warmer instead of .colder, and the sun's rays exert their counteracting influence. 'JThe darkened surface of the soil— that very conr dition which makes the ground throw off its heat more readily during the night— causes it to imbibe the heat of the sun's rays by day with increased facility, so that we have the greatest amount of the fostering agencies of heat and moisture for successful plant growth. Artificial waterings, to be most effective, should be copious and abundant, and applied either in early morning or during a dull day. Water that has been previously exposed to the sun's rays is always preferable to that freshly obtained from a cold tank or well, owing to the chill imparted to the circulatory fluid of the plants by the application of such low-temperatured water.— Gardeners' Chronicle.

In case of almost all crops sown from seed where they are to stand, as beets, carrots, parsnips, and turnips, many more plants come up than can perfect themselves if left to struggle on alone. Thinning out should be properly attended to.