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Yellowing, wilting, or drooping leaves can be a direct result of either under-watering or over-watering the plant. The best way to water the plant is to thoroughly soak the soil and then do not water it again until the soil's surface is dry. The pot must never sit in the excess water that runs out of the containers drainage holes. The soil or medium must be one that is gritty and drains freely to prevent possible over-watering of the plant. In winter, if the room is quite cool, the plant must be watered even more sparingly as Yucca plants are very susceptible to stem and root rot diseases under cold, wet conditions.
If the soil is allowed to over-dry, the plant will not be able to absorb enough water, so the plants cells will loose turgor (the normal size or swelling of a cell caused by internal pressure of water taken in by the cell) due to transpiration, and thus, the leaves will wilt or droop. With prolonged dryness of the soil, some leaves (usually the lower older leaves, but not always) will quit functioning, turn yellow and die. This is natural. The older leaves are aborting in order that the younger leaves will recieve the water. The solution is to water the plant more frequently, so as not to allow the soil to over-dry.
If the plant is being over-watered, the same effect is taking place, but in a somewhat different manner. Firstly, a plants roots must have oxygen for both the roots and the plant to be healthy and survive. A good potting media should be porous (have little spaces between the soil particles), and ideally, half of the pores should be filled with water and the remaining half filled with oxygen (air). If a soil has poor structural content (has few pore spaces) and is kept constantly wet, the plants roots are harmed due to a build up of poisonous gases (carbon dioxide, etc) in the soil given off by the plants roots as they transpire. The damaged roots (many of which begin to decay) can no longer absorb enough water to meet the plants demands, and the same effect occurs as with an over- dry soil, the leaves turn yellow and droop. The difference here is that a plant that wilts from drought will bounch back if it is watered soon enough, whereas, an over-watered plant that suffers from root decay will only get worse and eventually collaspe (die) if it continues to be over-watered. The solution is to reduce watering! It may be necessary to check the plants root ball for decaying (mushy, brown, sometimes foul smelling) roots. If present, they should be cut back to healthy, white colored roots and the plant should be repotted in fresh, well-drained soil (media). Do not water the plant for at least two days so as to allow the root ends to callus. It is also important not to over-water the plant during the recovery period - it takes a while for the new roots to grow.
There could be other factors causing the problem such as the plant being potbound. In this case, the plant can not be supplied enough water to meet its demands due to the large amount of roots in the pot, and it will wilt between waterings. Repot the plant in a one size larger container.
An excessive build up fertilizer (salts) in the soil can cause the leaves to yellow and droop. Water molecules cling to the salt which prevents the roots from absorbing that water. Hard white, crusty deposits located near the pots drainage holes or a white crust on the soil's surface indicates a high sodium build up. The solution is to repot the plant in fresh, well-drained media, or leach out the excess salt by watering the plant thoroughly to dissolve the salts, and then water the plant again in half and hour to wash out the dissolved salts. Scrape off any salt deposits found near the drainage holes or on the pots sides. On the other hand, a plants leaves will turn yellow or pale green if it is lacking nutrients, especially nitrogen. Feed the plant a well-balanced plant food such as 20-20- 20 two or three times a year during the growing season.
Other possible causes are a sudden change in light, temperature or relative humidity. Remedies are to move the plant to a brighter or dimmer location, increase or decrease the temperature and maintain an even temperature range, and increase humidity levels in the room if they are very low. Often, over a period of time, the soil will become compacted and the roots may decay, again from a lack of oxygen - repot the plant in fresh soil.
Finally, a heavy insect infestation such as red spider mites can quickly turn a plants leaves yellow and cause them to droop. Examine the leaves and stem with a hand held magnifying glass and administer proper eradication procedures if insects are present.
