Too Little Water
Most plants do not exhibit dramatic responses to too little water. Instead, growth slows. The foliage may wilt, and brown leaf tips may develop. Lower leaves turn yellow and fall off. The surface of the growing medium shrinks away from the growing container, leading to deterioration of the fine root system. While most plants prefer a short dry period in between watering, some require even moisture in the root zone to survive. Poinsettias, for example, need this to maintain their green leaves. Color may be lost and leaves may fall if this requirement is not met.
Too Much Water
As a rule, plants that receive too much water cease forming new leaves. The most recently formed leaves turn dark green, with the margins turning tan and cracking. The lower leaves wilt; in time, they drop or dry slowly on the plant. The stems become dark and mushy; eventually they rot. The surface of the growing medium becomes green with algae. All sorts of mosses and ferns may eventually develop on the surfaces of the growing medium and container. Watering needs to be monitored closely, as some plants need less moisture than others.
Too Little Humidity
Fortunately, most green plants thrive even in relatively low humidity, if watered properly and regularly. When grown with too little humidity, most plants cease forming leaves. The youngest leaves become yellow, smaller than normal and crinkly at the edges. The stems become wiry as the whole plant shrinks in size. The oldest leaves may dry and drop permanently.
Too Much Humidity
In general, most plants develop new leaves that are covered with yellow and tan spots. In time, the water-soaked lesions grow larger, and the centers of the spots rot. In many cases, roots begin to develop on above-ground stems. Few if any developing flower buds mature into functioning flowers. The oldest foliage on the plant may lose its color, collapse and decompose.
Temperatures Too Cool
As a rule, when plants are exposed to temperatures that are too low, the leaves curl down and around themselves. The most recently formed leaves may become colorless. Because of reduced nutrient availability to the slow-growing root system, the old leaves may turn purple. Raising the temperature, particularly at night, is the only way to correct the problem.
Temperatures Too Warm
Generally, plants grown with too much warmth do not maintain a balance between sugars gained from photosynthesis and lost to respiration. Insufficient sugar slows root development, and water and nutrient uptake are reduced. Flower production slows, followed by a rapid loss of the oldest foliage and a paling of the surviving foliage. Lower temperatures, especially at night, restore growth and, eventually, flowering.
Most plants are not sensitive to the temperatures of liquids applied to their foliage or growing media. Although the temperature of a liquid may abruptly shift that of the plant tissues, the plant temperature will rapidly return to the ambient level without any visible damage. However, some plants are sensitive. A temperature of 68 degrees is optimal for the nutrient solution .Its best to stay below 74 degrees for prevention of root diseases.
When the temperature of a liquid is 10º F. higher or lower than that of the leaf, the chlorophyll is permanently damaged. The plant cells retain their structure, but the leaves are permanently marked with colorless sections.
In general, too much light causes plant stunting. Even with no superficial symptoms, internal damage may occur that produces this. Leaves curl and may appear pale, edged with red. Plants that have been allowed to dry are susceptible to sunburn. Excessive heat and light break down chlorophyll, bleaching leaf tissues. Some plants may suffer leaf discolorations. Pruning damaged leaves, reducing light and watering properly restore good growth.
Most rapidly developing plants quickly deplete the nutrient reserves available in the growing medium. The first sign of trouble is a general yellowing of the entire plant. Do not apply only nitrogen; use a complete water-soluble fertilizer. Plants with too little fertilizer tend to turn pale green while new top leaves ascend. Few side branches develop as the oldest leaves turn yellow, dry and drop. Nutrients from older leaves are transferred by the plant to newer growth in the plants struggle to survive.
Excessive fertilizing generally boosts growth initially by favoring the development of large, deep green foliage. But it produces problems later. Leaf tips on newer growth begin to brown or burn, after which older foliage begins to turn yellow, burn and fall prematurely. Salt damage to the root system renders water unavailable to the developing roots. These dangers are especially applicable to peat-based potting mixes and rockwool. Plants may rot and die rapidly. Reducing fertilizer applications, removing the upper crust of the growing medium and leaching excess salts corrects the condition.
The optimum pH for most interior plants is 6.0 to 6.5. In this slightly acid range, most nutrients are available for uptake by the roots. Excess fertilizer will cause a dramatic drop in pH, locking up nutrients before they can be used by the plant. Raise pH in soil mixes by watering with a suspension of hydrated lime at 2 teaspoons per gallon of water. Hydroponic solutions can be adjusted with dilute solutions of phosphoric acid to raise pH and potassium hydroxide to lower pH. Do not use fish chemicals to adjust pH. Toxic buildups can occur.