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Houseplants Proper Care and Management of Pest Problems (PP744 (revised))

This publication summarizes how to properly grow houseplants and control the insect and mite pests and diseases that infest houseplants. Pest identification, damage symptoms and pest management strategies are described for insect and mite pests and diseases.

Janet Knodel, Extension Entomologist

Esther McGinnis, Extension Horticulturist; Kasia Kinzer,former NDSU plant diagnostician; Ron Smith, retired NDSU Extension horticulturist


Houseplants

Houseplant problems often arise when a new plant is brought into a home that previously was free of problems.

Upon acquiring any new plants, whether it’s one you purchased or a gift, isolate that plant from others in the house for at least 30 days. During that time, inspect the plant carefully for insect or disease problems.

If problems are minor, take corrective action that we suggest in this publication and be sure the problem is under control or eliminated before placing the plant anywhere near your healthy plant collection. If the problem is too far along, you really are better off discarding the plant.

Growing houseplants is a challenging and rewarding hobby that everyone can enjoy and need not be difficult. Give plants what they need and they’ll do well for you. Deny too many of their needs and plants fail.

Take time to learn the cultural needs of a particular plant when you purchase it and keep a watchful eye out for possible disease and insect problems. If a plant has poor color or distorted leaves or flowers, or if the plant tends to droop, something is wrong. These distress signals tell you the plant is having problems and you need to take corrective steps.

The leading causes of houseplant failure are overwatering and/or too little light for the species. Check these conditions first before suspecting anything else.

Cultural Considerations

Soil

Plant roots must have air, food and water. Potting soil must be porous enough to allow drainage of excess water and admit the oxygen (soil aeration) the roots need.

Garden soil may appear ideal for potting indoor plants, but it actually causes problems. This soil may be wonderful for outdoor gardening under natural conditions, but after a few months, garden soil becomes hard and almost rocklike in a plant pot.

Plants in garden soil grow satisfactorily for a month or two, but soon the lower leaves turn yellow and the plants become weak. This problem is the result of poor drainage and the lack of soil aeration due to improper soil structure. In addition, garden soil may harbor plant diseases and insect pests.

Therefore, a commercial “soilless” potting soil is recommended. Potting soil usually contains a mix of peat moss and perlite. Vermiculite or shredded bark sometimes may be components.

Potting soil is preferred over garden soil because potting soil is well-aerated and drains well. In addition, new potting soil does not contain disease organisms, insect pests or weed seeds.

Potting

Always use thoroughly cleaned containers with proper drainage holes. If the container lacks drainage holes, carefully use an electric drill to make them.

Plastic containers are easier to drill than ceramic containers. A masonry drill bit works well on terra cotta, but a more specialized drill bit such as a spear-headed glass and tile drill bit may be required to drill glazed ceramic. Be sure to wear safety goggles.

To minimize chips, cover the drilling area with tape. Score the area first and then drill slowly.

Before reusing any pots, they should be scrubbed clean and disinfected in a solution of one part household bleach to nine parts water to kill disease organisms. Be sure to rinse the pot with water after disinfecting.

Next, add enough potting soil to bring the soil level of the finished planting to about ½ inch below the rim of the pot. This top space serves as a reservoir for watering.

Repotting

Symptoms of plants needing repotting include wilting within a day or two after watering, very little or no new growth and a general lack of vigor, foliar discoloration and water flushing out of the bottom of the pot as it is being watered.

To confirm whether the plant needs repotting, hold it and the soil ball with one hand, invert it and lightly tap the edge of the pot on a hard surface such as a table or countertop. If the soil is completely surrounded by a mass of roots, the plant needs to be repotted.

To begin the process of repotting, be sure the plant is watered thoroughly a day or two ahead of time and water again just before removing it from the pot. If the plant can be picked up, knock the pot against a table edge or other hard surface.

If the plant is too large to pick up, a couple of tactics have proven successful. Tip the pot and knock the high side with a rubber mallet or the heel of your hand. Roll the pot and repeat this process at least three more times in a different area of the pot. This should help loosen and break the adhesive character of the side of the pot. Then carefully grab the base of the plant and pull it loose.

Another tactic, if this doesn’t seem to work (never damage the plant in trying to extract it from the container), is to strike or break the bottom of the container and push the plant out from the bottom. As you may have guessed, this could be a two-person job.

Next, shave off 1-inch-thick slices from the sides of the root mass with a sharp knife. The result is a cube that easily can slip back into the same pot or, if desired, a slightly larger one.

Using new potting soil, cover the bottom of the pot with about 1 inch of the soil. Adjust the thickness so that the plant will be at the same depth it grew to in its previous container. Fill in the space between the edge of the roots and the sides of the container with potting soil, using fingers, a slender trowel or even a knife to get the soil in place.

Watering

Close observation and good judgment are essential for proper houseplant watering. Growing conditions vary from home to home and room to room due to variations in light, temperature (day and night) and humidity. Plants with roots in shallow containers may need daily watering, while plants in large tubs may go several weeks between watering. Succulents, such as cacti, and other dry soil plants require less watering than moist soil plants such as African violets and ferns.

Proper watering keeps the soil moist enough to supply the plants’ needs without drowning the roots. Saturated soil drives out air, and roots can die from lack of oxygen.

Proper pot drainage is critical. All the soil in a pot should be wetted thoroughly each time the plant is watered. Always empty the drainage water from the catch basin beneath your plant container after each watering. This will reduce the possibility of water logged soil and prevent the dissolved salts in the water from being drawn back into the soil. Soil should dry to the point that the plant approaches moisture stress between watering intervals.

Do not use water that is unusually high in salts or has been run through a water softener to water plants. Rain or melted snow are good alternate sources of water for houseplants. Some plants are sensitive to the chlorine in city water systems. Letting a container of tap water sit overnight before use will allow most of the gas to escape.

Fertilizing

Many brands of fertilizers are designed for houseplants. Follow the manufacturer’s directions and do not assume that twice the recommended amount is better than the recommended amount; overfeeding may damage your plants.

Plant injury can be reduced by leaching or rinsing out part of the dissolved fertilizer with clear water if overfertilization occurs. Use a container with holes in the bottom to allow thorough drainage. Place the pot in a sink and water liberally three to four times at half hour intervals, allowing the water to flush out the dissolved fertilizer and other accumulated salts.

A white, flaky material on the soil surface of potted plants often is observed one to several months after potting. These are mineral salts that accumulate in the soil. Well water in North Dakota contains varying amounts of dissolved salts, as does the fertilizer you apply. With continuous watering, these dissolved salts accumulate in the soil and appear on the soil surface.

The salts can be flushed out of the soil from time to time to prevent salt injury to your plants. Leaching (rinsing) the soil of most houseplants every three to six months is a good cultural practice and will reduce the accumulation of salts in the soil. Empty clay pots that have accumulated salts should be soaked in hot water for 24 hours before reuse.

Fertilizer will not cure all ills. It will not help a plant that is suffering from poor drainage, insect infestation, disease or overwatering. Fertilize only when new growth is evident. Eliminate or reduce the frequency of fertilizer applications during winter months.

Interstate Transportation of Plants

If plants are to be transported across state lines, determine the regulations of the states en route to avoid possible transportation of harmful plant pests to areas not previously infested. For more information on interstate transportation of plants, contact the North Dakota Department of Agriculture, 600 E. Boulevard Ave., Dept. 602, Bismarck, ND 58505-0020; toll-free at (800) 242-7535; by email at ndda@nd.gov.

Houseplant Trouble Signs

Wilting or partial wilting often will be the result of improper watering in the plant. If sudden wilting is diagnosed, check the roots, pot or soil for the trouble. Some common causes are a lack of water, excess water, root rot, too much fertilizer and/or a salt buildup.

Check the cause by pressing your finger, up to the first knuckle, into the soil. If the soil is dry to this depth, the plant needs water. If the soil is wet, too much water in the root area may be the problem. The roots may be saturated or rotting and incapable of absorbing water from the soil and supplying it to the leaves.

Overwatered plants should be repotted into fresh soil. (Refer to the section on yellowing and death of all leaves and poor growth for more information on root rot.)

Sudden loss of leaves frequently is caused by a rapid temperature change. It also may be caused by factors such as prolonged hot or cold drafts, dry air, exposure to gas or furnace fumes, or by changing the plant from a sunny to a dark location. Ficus benjamina, commonly called weeping fig, frequently has sudden leaf drop when moved to a location with lower light intensity.

Yellowing or death of leaves may indicate a nutrient deficiency, usually nitrogen or iron. First apply a nitrogen fertilizer. If the foliage does not appear greener after three to four days, do not add additional nitrogen. Instead, apply a chelated iron product. Iron is essential to healthy, green leaves and may be present in the soil but in a form that the plant cannot use. Chelated iron is in a form that is readily available to the plant roots.

The yellowing and death of lower leaves may occur if plants become pot bound because of extensive root development. Rubber plant, dracaena, dieffenbachia and other woody plants are especially prone to this problem. When the lower leaves first start to yellow, apply extra nitrogen fertilizer or consider repotting. Occasional lower leaf drop may be normal. The length of time a plant will hold its leaves varies from species to species.

Yellowing of all leaves and poor growth may be due to excessive soil moisture and/or root rot. If the drainage hole in the pot or plant box is plugged or if the plants constantly stand in water, the soil will be waterlogged and lack sufficient oxygen.

If you suspect a root rot problem, remove the rootball and check the roots. Healthy roots and root tips will be white or cream colored. Rotted roots are brown black and may appear slimy. Severely rotted roots may be hollow and easily broken between the thumb and index fingers.

Destruction of the roots by soil borne fungi and nematodes may cause a yellowing of the entire plant. Severe infestations of mites, aphids and scale insects, fertilizer burn, improper light and temperature, or improper pH all may cause plant yellowing. Soil pH refers to the acidity or alkalinity of a soil measured on a scale of 1 to 14, with 7 being neutral. Anything below 7 is acidic and anything above 7 is alkaline.

Some plants such as the Norfolk Island pine and Boston fern require an acidic soil medium. To achieve this, use a potting mixture high in peat and an acidic fertilizer. Acidic fertilizers help reduce the pH of the soil. Most North Dakota well and river water is alkaline, so regular use of an acidifying fertilizer would be advantageous to plant growth.

Finely specked leaves with a faint mottled, lighter color indicate a spider mite infestation. When the mites are plentiful, the upper and sometimes the lower surfaces of the leaves may appear dusty due to their webbing.

To check plants for spider mites, shake several suspect leaves or branches over a sheet of white paper. Look closely at the specks that have fallen on the paper. If they are moving, they are spider mites. More information on mites and their control is given in the table.

Bronzed or abnormally reddened leaves indicate cold temperature damage or a deficiency of phosphorus or potassium. Check the recommended temperature range for the plant. If a nutrient deficiency is suspected, fertilize with a complete fertilizer or repot in new soil.

Unnaturally small, pale leaves and spindly plants are most generally the result of insufficient light. This is especially common during the winter or when outdoor or greenhouse-grown plants are brought into the home. Small leaves also might indicate a need for fertilizer.

Brown leaf tips and margins can be caused by exposure to hot, dry air, improper watering, insect feeding injury, salt accumulations (Figure 1) or objects rubbing against the leaves. Water that is chlorinated or contains added or natural amounts of fluoride can harm sensitive plants.

Figure 1.

Perlite (the white material in many potting mixes) and fertilizer products containing fluoride may release enough fluoride to harm sensitive plants. Spider plants, especially the variegated variety, are very sensitive to fluoride and often are seen with leaf tip burn. Occasionally flushing the soil should help reduce a fluoride salt buildup.

Bleached or faded spots on leaves sometimes are caused by direct sunlight burning plants that require shade or are not yet accustomed to extended periods of direct sunlight. Chemicals and plant-cleaning products also can injure leaves.

Other leaf spots, varying in description, may be caused by aerosol products, hot grease in the kitchen area, cold water (especially on African violets and gloxinias) or the sun shining on wet leaves. In rare cases, a pathogen may be involved. For more information on plant pathogens.

Plant distortion (leaf thickening, curling, leaf and flower drop) accompanied by leaf yellowing and browning may be due to gas fumes or pesticides that are toxic to the plant. Plants are very sensitive to gases and will show symptoms before the gas concentration is at a level detectable to humans. Garden soil that is contaminated with agricultural chemicals and used for potting houseplants can result in chemical injury to houseplants.

White substances on the soil surface may indicate two things. If crusty or crystalline, the substance probably is an accumulation of salts. The crusty surface layer of soil can be removed and replaced with fresh soil between repottings. Refer to the previous section on Fertilizing for more information.

A white or light yellow moldlike growth may indicate the presence of a saprophytic soil fungus, a fungus that lives on dead or decaying matter. The fungus will not harm the plant but may indicate contaminated potting medium or an overwatered plant.

Light brown corky scablike lesions, usually occurring on the underside of leaves and along stems, are the result of excess water (Figure 2). This condition is known as oedema. Careful watering and good drainage will minimize this problem.

 Figure 2.

Pest Management Strategies for Control of Insects and Arthropod Pests of Houseplants

Here are some of the most common insect and arthropod pests of houseplants, their damage/symptoms and pest management strategies. Also see Table 1 for specific insecticides available to homeowners and the pests controlled.

 Common insects and arthropod pests of houseplants

 Common insects and arthropod pests of houseplants

Common insects and arthorpod pests of houseplants continued

Early detection is important for proper pest management of insect and arthropod pest problems in houseplants. If an insect is detected before it has an opportunity to build or spread, pest management strategies generally can be applied in a timely and satisfactory manner.

Cultural Practices

Cultural practices often are effective in managing insect and arthropod pest problems in houseplants, especially when infestation levels are light or moderate. Some common cultural practices that are easy for the homeowner to use are:

ExclusionNew plants should be inspected closely for pests before purchase. Buy only pest-free plants. Any infested plant should be separated immediately from other plants. Keeping new plants isolated for three to four weeks is a good idea to prevent the spread of pests and allow time for eggs to develop if present at the time of purchase. Houseplants that are taken outside during warm weather can become infested with insects and should be examined closely before being brought back into the home in the fall. If a plant is heavily infested and damaged, simply disposing of it may be best.

SanitationUse new potting soil and always use clean pots and tools.

Avoid overwateringAllowing the soil to dry can reduce infestation by fungus gnats and springtails that require moist soil conditions to survive. Overwatering also can increase the incidence of fungal diseases that cause root rots. Proper pot drainage is important in preventing a wet soil environment.

Mechanical Control

Mechanical control is feasible when few plants have low pest infestation levels. However, mechanical methods are time-consuming for the homeowner and require a continued effort for a period of time.

Washing and sprayingSpraying plants with a forceful stream of water can be effective in removing and drowning insects such as aphids, mealybugs, crawlers (scale insects) and spider mites. Focusing the stream of water on the undersides of leaves where most insect pests are found is important. This tactic may not be 100 percent effective in killing pests and some may crawl back onto plants.

WipingWashing or cleaning foliage and stems with a moist cotton ball or soft cloth with an alcohol solution (one-half water and one-half rubbing alcohol) will remove most pests. The alcohol solution may burn sensitive plants, so try this on a small area of the plant first. This is an easy way to control a light infestation of aphids, mealybugs, crawlers (scale insects) and mites. This method should be repeated several times.

Hand RemovalIf the pest infestation is light, insects such as aphids, mealybugs and scale insects may be controlled by removing them with a toothpick or tweezers. Hand removal generally is not effective against very small insects and mites. This method should be repeated several times.

Plant trimmingIf the pest infestation is severe, the injured parts of the plants can be removed to permit regrowth and recovery. This method works best when followed by repeated washing or chemical control.

Plant disposalIf the plant is heavily infested with a pest, disposing of it may be the best solution.

Chemical Control

Chemical control is used as the last resort for pest management. Pest identification is important when selecting a pesticide because some products are effective in controlling only certain pests. Using the wrong chemical is ineffective and a waste of time and money.

Some pesticides are labeled for control of household insect pests, such as cockroaches and ants; however, these products may contain oils or other ingredients that can be phytotoxic (burn foliage) when applied to houseplants. Use products that are labeled for application on houseplants.

All insecticides are poisons and should be handled with care. Always read, understand and follow the manufacturer’s directions before using the product and always use the labeled rates.

Treating plants outside or away from people, pets, fish, birds and other animals always is a good idea. Wear personal protective clothing, such as rubber gloves, dust mask, long-sleeved shirt, pants and shoes, to prevent pesticide poisoning.

After spraying, thoroughly wash yourself and triple rinse all spray equipment. Keep pesticides in a separate storage area that is locked and labeled “Pesticides” to keep children and others safe from pesticides.

Insecticides are available in concentrated or ready-to-use spray formulations (aerosol, hand misters, hand atomizers). When spraying plants, covering all plant parts thoroughly is important, especially where insect pests are concentrated, such as the undersides of leaves or leaf axils.

To dip plants, make sure your container is large enough for the insecticide-water mixture and the whole plant to be submerged for a few seconds. Use a cardboard disk to fit around the plant stem to prevent the soil and plant from spilling when you turn the plant over. After dipping, hold the plant over the container to allow any excess pesticide mixture to drip off of the foliage.

Do not dispose of excess insecticide mixture in the sink or other drains that empty into the sewage system. The best way to dispose of excess mixture is on ground where runoff or other contamination is unlikely.

Dusting plants is effective in controlling common insect and arthropod pests, but dust is not used commonly on houseplants because it is messy and difficult to apply efficiently. A thin visible coating of dust is all that is necessary to kill pests. Do not apply a heavy coat of dust on the foliage. If possible, dust plants outside.

Many trade names (or brand names) of insecticides are available for the homeowner. The active ingredient that is listed on the label identifies the specific insecticide. In Table 1, some common insecticides that are available to homeowners for control of common insect and arthropod pests of houseplants are listed. Some labels require that insecticides be applied to houseplants outside.

 Some insecticides available to homeowners for control of insect and arthropod pests of houseplants

Diseases of Houseplants

Houseplants rarely suffer from infectious plant diseases because the indoor environment is typically too dry for plant diseases to develop. Cultural, environmental or insect problems are more common than disease problems in houseplants, and some symptoms that are caused by disease are similar to those caused by noninfectious disorders or insect injury.

Disease problems can be prevented or minimized by purchasing healthy, vigorous, disease-free plants; using sterile potting soil or media when repotting; using clean pots sanitized in a 10 percent bleach solution; and avoiding overwatering.

Diseases of indoor houseplants are caused by plant pathogens, such as certain microscopic living organisms (fungi, bacteria, nematodes or viruses). Plant pathogens can cause disease on indoor plants only if three conditions are met: 1) a plant pathogen is present, 2) the plant is susceptible to infection by the plant pathogen, and 3) the environment allows disease to develop.

Because indoor air environments are typically too dry, diseases on foliage of houseplants rarely develop; however, excessively moist soil environments can favor the development of root rots.

Root and Crown Rot Diseases

Symptoms of root rot or crown rot diseases are usually noticeable first on the foliage. Wilting of foliage, typically followed by yellowing or browning of the leaves, may indicate that a root or crown rot disease is present. These diseases are caused by various fungi, including species of Fusarium, Rhizoctonia, Phytophthora, Verticillium, Sclerotium and Pythium.

Overwatering can lead to the development of root rot diseases, which can be worsened by heavy potting soils or poorly draining pots. Because several factors not related to a pathogen can cause wilting in houseplants, examine roots by carefully removing the plant from the pot while keeping the plant, roots and soil intact as much as possible. Healthy roots appear white to cream colored and are vigorous, while roots suffering from a root rot disease are typically discolored brown to dark brown.

Many root rot diseases cause the roots or crown to become water-soaked, brown and mushy, and the outer sheath of the root may slip off the central root cortex easily in severe cases.

If the entire root system is brown and mushy, the plant cannot be saved, but propagating a new plant from cuttings may be possible for some species of houseplants. If some of the roots are still white and vigorous, the plant possibly may be saved if the brown roots are cut out with a sharp, clean knife. The plant should be repotted in a new pot with new potting soil.

Foliar Diseases

Diseases of the leaves of houseplants are rarer than root rots. However, if the foliage of an indoor plant is subjected to high humidity or frequent misting, disease problems on the foliage can occur. Symptoms of common foliar diseases can include leaf spots, lesions, blights or dieback, with or without water-soaking, and yellowing, browning or defoliation of leaves.

If detected early and diagnosed correctly, cultural practices of removing and destroying infected material often can lead to satisfactory management of the disease. Fungicides are available for some foliar diseases.

Leaf spots and lesions on indoor plants more often are caused by environmental factors, insect or arthropod injury, phytoxic aerosols or other factors. They rarely are caused by pathogens in the indoor home environment. Table 2 summarizes a few foliar diseases that may be encountered in houseplants, along with their causes, symptoms, plants affected and corresponding management strategies.

Table 2.

Table 2 continued

 

Using Fungicides to Manage Plant Diseases Caused by Fungi

Several effective fungicides are available for managing diseases on indoor plants, but they differ in how they are used, where they can be used, on what plants they can be used safely and for which diseases they are effective. Most often, the cost of purchasing a fungicide is greater than replacing the plant itself.

If choosing a fungicide, be sure it is labeled for the intended use and be sure to read, understand and follow the instructions on the label to avoid causing harm to the plant, the environment or yourself.

Compounds such as chlorothalonil (Fungonil), tebuconazole (for example, Bayer 3-in-1 Insect, Disease and Mite Control), neem oil (Triact 70), thiophanate methyl (Cleary’s 3336), potassium bicarbonate (GreenCure) and fungicidal soap are available for use as fungicides on indoor houseplants.

Always read and follow the manufacturer’s directions before using the product and always use the labeled rates. Mention of any trade names does not imply endorsement of one product over another nor discrimination against any product by North Dakota State University Extension Service or the authors.

Photo credits for Insects and Arthropod Pests

Adult fungus gnat (Johny N. Dell)

Mealybugs (U.S. National Collection of Scale Insects Photographs Archive, USDA)

Close up of longtailed mealybug (David Cappaert, Michigan State University)

Scale insects (U.S. National Collection of Scale Insects Photographs Archive, USDA ARS )

Armored scale insects (U.S. National Collection of Scale Insects Photographs Archive, USDA ARS)

Springtails (Susan Ellis)

Adult and nymph - western flower thrips (Jack T. Reed, Mississippi State University)

Close up of western flower thrips thrips (Jack T. Reed, Mississippi State University)

Whiteflies (Clemson University – USDA Cooperative Extension Slider Series)

Close up of adult greenhouse whitefly (David Cappaert, Michigan State University)

Twospotted spider mites (David Cappaert, Michigan State University)

Close up of twospotted spider mite (David Cappaert, Michigan State University)

Stippling injury to leaves caused by spider mites (Whitney Cranshaw, Colorado State University)

Revised April 2016


 

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