Leaf Degradation

• Detergent will strip away at the waxy coating on your plants, the same waxy coating that helps them retain water. Over time, the plant will slowly begin to dehydrate. This dehydration is evident by a gradual degradation of the leaves, which will begin to turn shades of brown or yellow, curl up and become crunchy before eventually falling off completely.
  
  

Water Blocking

• When detergent-laced water is put into the plant's soil, it will block out pores and crevices that the plant would otherwise use to channel water into its roots. The more detergent in the soil, the faster your plant will dehydrate until it withers into death.
  
  

Soil Poisoning

• Most laundry detergents contain large amounts of sodium, which when added to the soil will increase the saline content. Houseplants cannot survive in saline conditions and will slowly die. The detergent also washes away nutrients and other beneficial particles that the plant would otherwise use, creating a poisonous soil that will eventually choke the plant to death.
  
  

Insect Repellent

• In very small doses, detergent can be beneficial to plants. If sprayed on in a very small amount, usually diluted with water, it can kill bugs and other pests that invade the plant. With the biting pests gone, the plant is then free to grow larger and faster. It is nearly impossible to determine "how much is too much" when it comes to detergent concentration, however, and even a minuscule amount may do more harm than good.