Just as with humans, any factor whether external or internal that affects a plant is known as a stressor. The stressor can be beneficial or harmful or both. For example, rhinovirus causes the common cold in humans which can be harmful if a secondary infection sets in. At the same time the common cold helps to strengthen the immune system which is beneficial. The same is true in plants. However, when we talk of stress it is usually in a negative way, referring to any factor that is harmful.
How a plant responds to adverse stressors depends on the health and type of plant. The effects of plant stress range from minor to severe and may even result in the death of the plant. If corrected early enough the plant may recover fully or it may be weakened. A stressed plant is not a healthy plant making them more susceptible to disease and insect infestation. The following is a list of plant stressors with an explanation and tips to correct.
- sudden temperature change - Simply bringing a new houseplant from the store to the car then into the house on a cold winter day is enough to cause stress. Leaves may wither and die. If the plant is place in a warm, sunny location in the house then watered well new growth should appear but the old damaged growth will not recover. It should be carefully trimmed from the plant.
- extreme temperatures - Plants exposed to too hot or too cold of temperature even briefly are adversely affected and often will die. In general, keep houseplants within your normal household temperature comfort zone (68ºF to 72ºF). If frost is forecast, cover outdoor plants and bring potted plants indoors. Extreme summer heat is difficult to avoid for outdoor plants. You can use shade cloth to help cool as well as move potted plants into the shade. If there is a heat wave be sure to keep all outdoor plants well watered especially potted plants.
- incorrect watering - One of the most common mistakes home gardeners make is with respect to watering. They either over water or under water, both of which can kill the plant. Over watering can cause mold in the soil, mildew on the leaves and root rot. It makes a plant more susceptible to fugal diseases and will cause dampening off in seedlings. Over watering will cause nutrients to leach out of container planters including raised beds. Under watering will cause a plant to wither, drop leaves and kill the plant if not corrected immediately. Ideally, both indoors and outdoors keep the soil moist but not wet. When pinched the soil should just hold together. Some houseplants like African violets are susceptible to water damage on their leaves so should always be watered from the bottom. In general, it is better to water the soil not the leaves. Wet leaves spread plant diseases. Water droplets on the leaves serve as mini magnifying glasses causes burn marks on the leaves on sunny days. Don't water outdoor plants after dusk or during the heat of the day (1 PM to 4 PM). The exception to this is emergency water if a plant is obviously heat stressed. Move the plant to a shady location then water the soil well. Prevent watering problems in container plants by using self-watering pots. Use a soaker hose for garden beds outdoors. Mulch is your best friend in helping to conserve water in your garden beds while preventing them from drying out. Choose a mulch that will naturally repel problematic insects (eg. red cedar) and avoid those that will create other garden problems.
- fertilizing - Incorrect fertilizing or too much fertilizer will cause fertilizer burn. The plant may not survive. Do not fertilize container plants potted in soil containing slow release fertilizer. In general it is better to fertilize every 2 to 4 weeks if using granular fertilizer. A small amount of liquid fertilizer can be added to the reservoir of self-watering pot.
- chemical exposure - Chemical exposure can very much adversely affect plants indoors and outdoors. Exposure can be accidental or on purpose. Outdoors most chemical exposure aside of herbicides is accidental, often caused by over spray drifting from one property to another. If your neighbours are spraying for weed control it may affect your garden beds. The problem is unless you know they are going to spray, it is difficult to protect your plants. If you know, they are going to spray simply protect via sheeting, gardener's cloth or even tarps while they are spraying. Once they are finished spraying remove the protective cover. Over spray can come from salt trucks in areas where snow removal is necessary and home owners can cause salt burn by using salt to melt ice on their property. In areas where homes back onto farm land as ours does, over spray can come from the farm land when herbicides and fertilizers are applied. Lessen this exposure by privacy fencing or plant a row of cedars along that property line. Nicotine is quite toxic to plants as well. I lost a rhubarb plant when a house guest emptied the ashtray beside it. Rhubarb is almost a weed that when established is quite hardy yet the nicotine killed it and nothing would grow in that spot either. Houseplants are exposed to chemicals via household cleaners. Years ago, I removed the wax from the kitchen floor using household ammonia. Not only did I damage my lungs, the green in most of my houseplants turned turquoise and most did not survive. I lost most of my window garden plants and several floor plants. That served as a very valuable lesson as to how dangerous household cleaners even those used for natural cleaning can be. Avoid all harsh household cleaners. Use baking soda, soap, vinegar, household ammonia (sparingly and with window open) and rubbing alcohol for household cleaning.