Elisabeth C. Miller Library

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Plant Answer Line Question

Keywords: House plants, Calathea

How do I care for Calathea zebrina?


The webpage of Plant of the Week have information about Calathea zebrina (zebra plant, native to Brazil):
"For the home or greenhouse. Plants reach 3 feet in containers. Leaves emerge from basal rosettes and may reach 2 feet long by 1 foot wide. Calathea zebrina need shade and temperatures above 55 degrees, but they need good light for a good, rich leaf color...use a soil mix consisting of 2 parts peat moss to 1 part loam to 2 parts sand or perlite. Good drainage is necessary or the plant will stagnate, which is a common problem. The plants should be kept moist at all times and leaves should be misted often. Fertilize every 2 weeks during the growing season and once a month during the winter months. Repot as often as necessary to avoid root bound conditions."

Date 2020-01-16
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Plant Answer Line Question

Keywords: Crassula, Insect pests--Identification, Insect pests--Control, House plants

My 100-year-old Jade plant is about 5 feet tall and recently has been producing a sap from its leaves. White and sticky. Is there anything I can do to help this? Is it normal? Or is it endangering the plant? It is in kind of a cool spot; should I move it to a warmer place? It is a succulent, right? I would also like some information about repotting if necessary.


The pests most likely to cause a white, sticky substance are aphids, whiteflies, scale or mealybugs. These are known to affect jade plant, or Crassula ovata, which is indeed a succulent. They won't destroy plants, but can weaken them and allow other problems to surface. If none of the pest descriptions below resemble what you are observing, you can take affected plant samples to a local county extension agent. Without knowing the specific pest, we can't suggest specific treatments. Northwest Center for Alternatives to Pesticides has general information on caring for houseplants. Note their description of mealybugs, which do produce a sticky substance:
"These insects look like little bits of cotton that are greasy or waxy. They are oval in shape, have a segmented body, and are about 1/4 inch long. You'll usually find them hidden between leaves and stems or under leaves. They move slowly. They make a sticky liquid called honeydew and also cause leaves to become distorted and spotted."

As for temperature and repotting, The New House Plant Expert (by D. Hessayon, 1991, p. 212), says that succulents like a difference between day and nighttime temperatures. They like to be kept cool in the winter, with 50-55 degrees F ideal, but as low as 40 is alright. Jade plants should only be repotted when essential. Repotting should occur in the spring; shallow pots rather than deep ones are preferable.

Extensive care information can be found on Succulent-plant.com. There is also excellent general information on indoor care of succulents and cacti from Brooklyn Botanic Garden.

Date 2019-05-10
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Plant Answer Line Question

Keywords: Mealybugs, Scale insects, Insecticidal soap, Crassula, Insect pests--Control, House plants

I have been nursing a Jade plant cutting that dropped off an overwatered and rotting larger plant. It has been thriving in my windowsill for 6 months or so, and has grown a lot already.

In the last week or so, I have noticed a strange white speckling on the upper surface of almost all of its leaves. Upon close inspection, it does not look like insects; it looks sort of like a detergent residue, and if I scrape my nail against the surface of the leaf, a lot of it will come off, albeit with effort.

Do you know whether this is something I need to treat?


I wouldn't assume the spots are a problem. As the following link to North Dakota State University Extension mentions, it might be salt crystals that you are seeing:
"Those dots are salt crystals and can be wiped off with a damp cloth or just ignored because they are not causing any harm to the plant. All water (except distilled) contains some salt. When fertilizer is added to the root system, the plant takes up the nutrient salts with the water. As the water moves through the leaf pores during transpiration, the salts often are left behind on the surface."

However, if you were to use a hand lens (not just the naked eye) and discover insects, there are resources with information on identifying and treating insect problems on indoor plants.

1. http://www.succulent-plant.com/pests.html

2. Washington State University's PestSense site lists several common houseplant pests, with information about treatment.

Always test any spray on one leaf before spraying the entire plant.Wait a few days after the test spray. Some plants are more sensitive to various soaps or oils.

3. The Northwest Center for Alternatives to Pesticides also has a guide to Growing Houseplants Without Using Pesticides.

Date 2019-05-10
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Plant Answer Line Question

Keywords: Hoya bella, Tropical plants, House plants, Container gardening

My Hoya bella was recently moved outside. It flowered nicely, but now the leaves are a light yellow/green and the soil surface in the pot is covered with moss. What is wrong and what can I do?


Here is some information I found in the book, Subtropical plants: a practical gardening guide (by Jacqueline Sparrow and Gil Hanly, 2002, p. 107), quoted below:

Hoyas do very well in pots. They need bright light, but not sun...Hoyas strike fairly easily from cuttings, taken at the warmest time of the year.

About the yellowing of the leaves...I am pretty confident that this is due to the plant getting too much water (rain, whatever source, while it was outdoors) and the soil not drying out, which also explains what happened to the top of the soil--the moss or algae growth there. I would just gently scrape off the soil surface and put a thin layer of potting soil over it. If the plant starts getting what it needs again (as it did before it was put outdoors), it will hopefully return to its former healthy self.

During its growing season, Hoya bella prefers temps between 64 and 68 degrees; during its rest season, 59 degrees is the recommended minimum temperature (so here in Seattle, right next to a window may be too cold).

University of Florida provides additional information about Hoya bella.

Date 2019-09-11
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Plant Answer Line Question

Keywords: Jasminum, House plants

How can I find out about the best way to care for a jasmine plant indoors. The plant is without a species name and I know there are many types of jasmine. Mine has rather robust leaves, and is an active "entwiner". The flowers are white and about the size of a nickel.


The Houseplant Encyclopedia by Ingrid Jantra (Firefly Books, 1997) says that Jasminum likes a full sun, airy location, and should be taken outdoors in summer. During the winter it prefers temperatures of 46-50 degrees Fahrenheit. In summer, keep the root ball moist, and feed every two weeks. In winter, water just enough to keep the plant from drying out. If it is kept in too warm a spot in winter, it may be susceptible to aphids.

Here is some information from British gardener Alan Titchmarsh:

    Indoor jasmine

  • Flower time up to 6 weeks
  • Which room? east or west window, south in winter
  • Temperature max 15C (60F), min 4C (40F), humid

The house plant jasmine (Jasminum polyanthum) bears loose sprays of delightfully fragrant flowers. It is an ideal plant for a cool conservatory or porch which is kept frost free during the winter months. Otherwise, keep it on a well-lit windowsill. Jasmines like a moist atmosphere so mist the leaves regularly and stand the pot on a tray of moist gravel. They are vigorous climbers, so you will need to prune them to keep them small or provide a larger support in subsequent years.

Here is a link to some general information on caring for jasmine plants, from the Chicago Botanic Garden.

Date 2019-07-17
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Plant Answer Line Question

Keywords: House plants, Dracaena

I recently repotted my Dracaena fragrans 'Massangeana.' Some of the leaves have turned brown. Can you tell me what might be causing this?


We cannot diagnose plant problems via email. However, we may be able to give you some ideas of what might be happening. The browning leaves could be the result of too much or too little water. The soil should be kept lightly moist, but avoid overwatering. Avoid giving fluoridated water, as this link from University of Vermont suggests this plant is sensitive to fluoride. It is also sensitive to temperature changes (up or down) and should not be near heating or air-conditioning vents. Repotting may have caused some stress to the roots. According to The Complete Houseplant Survival Manual by Barbara Pleasant (Storey Publishing, 2005), brown spots on leaves can be the result of sun scorch. This plant prefers moderate to bright indirect light.

Here is a link to discussions from a gardening forum sponsored by the University of British Columbia. Here is another from the same site.

Here is a link to information about diagnosing problems with houseplants from Penn State Extension.

Date 2019-11-14
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Plant Answer Line Question

Keywords: House plants, Carex, Blechnum spicant, Adiantum pedatum, Acorus

I've taken up planning plants for our office, and wondered if you could give advice. I'm looking for Northwest native plants that would be happy indoors, in an office environment. Available sunlight will vary by spot but is generally low (but I can probably swing some plant lights); air is standard low-humidity commercial-building air.


Most Northwest native plants I can think of are not ideal for growing indoors. However, I asked my colleague who used to garden for the Seattle Public Library, and she says that the library is growing native species of ferns indoors. She notes that they are especially prone to pests (whitefly) and diseases (scale), and must be watered every day.

Below is the list of plants being grown in the main (Central) library branch:

  • Acorus
  • Blechnum spicant
  • Adiantum pedatum
  • Carex elata 'Bowles Golden'(tall)
  • other fern (Rumohra adiantiformis?)

I hope this helps. If you wish to reconsider using natives in favor of more traditional choices for indoor plants, there are many more choices available. Below are a few links that may be use to you:

Low Light Houseplants from University of Vermont Extension

Growing Indoor Plants with Success from University of Georgia Cooperative Extension

Interior Plants: Selection and Care from University of Arizona Cooperative Extension

Date 2020-10-17
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Plant Answer Line Question

Keywords: Zamia, Poisonous plants, House plants, Cycads

I am interested in finding out if someone there can tell me the proper culture for Zamia furfuracea. I just acquired one that had been potted up as a bonsai and put on sale at a local grocery store. I think they may not have known or cared what it was. This is a plant I grew outdoors when I lived in California. I'm wondering what to do with it in Vancouver, WA. The options are greenhouse, patio pot, indoors, outdoors.


I found general cultural information from Florida State University Cooperative Extension. This is a zone 9b-11 plant, and your area is probably about zone 8, so I think you would want to grow this with some protection.

University of British Columbia Botanical Garden's discussion forum describes this as an indoor plant. This article in the journal of University of Arizona Cooperative Extension is about a similar plant, Zamioculcas zamiifolia, often confused with Zamia furfuracea.

Richard Langer's book, Grow It Indoors (Stackpole Books, 1995) says to grow this "handy table-sized cycad" in temperate partial sun with humusy soil that is kept constantly moist.

Another thing to keep in mind if you are growing this plant around pets or small children is its toxicity. The ASPCA lists Zamia species (cardboard palm) as toxic. Colorado State University Veterinary Teaching Hospital lists Zamia as toxic to dogs and people.

Date 2019-10-02
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Plant Answer Line Question

Keywords: Fungal diseases of plants, Fungi and mushrooms, House plants

I have a potted plant with a fungus growing in the soil. It is bright neon yellow and grows like a mushroom, but with no cap on top. The plant is in the basement near a window. The soil is damp and I've avoided watering for awhile to let it dry out. What do you think the growth is, how to get rid of it, and will it be harmful to my plant? I keep plucking them, but they grow back.


I have had questions about the yellow houseplant mushroom before, and I am guessing you are seeing the same thing. It is called Leucoprinus birnbaumii.

Michael Kuo's website, MushroomExpert.com has information about Leucoprinus. Excerpt:

"This little yellow mushroom and its close relatives are the subject of many frantic e-mails to MushroomExpert.Com, since it has a tendency to pop up unexpectedly in people's flower pots--even indoors! The brightness of its yellowness exhibits some rebelliousness, but it often creates a striking contrast to the green houseplants that surround it.

"Leucocoprinus birnbaumii won't hurt you, unless you eat it. It won't hurt your plant. It won't hurt your pets or your children, unless they eat it. There is no getting rid of it, short of replacing all the soil in your planter (and even then it might reappear). Since it makes such a beautiful addition to your household flora, I recommend learning to love it--and teaching your children to love it, too.

"You might also impart the idea that mushrooms are very, very cool--but shouldn't be eaten. Perhaps your child would like to become an awesome and famous mycologist some day. I would love to encourage your child's interest in mushrooms by putting his or her drawing of Leucocoprinus birnbaumii on this Web page (at least temporarily).

"Leucocoprinus birnbaumii is probably poisonous; do not eat it. Handling it, however, won't hurt you."

Date 2019-05-15
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Plant Answer Line Question

Keywords: Ficus, House plants

I just bought a Ficus lyrata that was heavily treated with pesticide. As the plant makes my eyes burn, how can I minimize the pesticide residues on the large leaves? Is the citrus-based Veggie Wash appropriate and not harmful to the plant, or is there a better solution? If changing the soil and pot is necessary, does Ficus lyrata like any particular soil?


Without knowing what pesticide(s) were used, it is difficult to say how the residues could be removed. Looking at The Complete Houseplant Survival Manual, I can see that you would do no harm to the Ficus lyrata by washing the leaves with a sponge dipped in soapy water and then rinsing them with fresh water (this is a remedy often suggested for spider mites). Ficus also tolerates insecticidal soap well, so I imagine that the citrus-based veggie wash would not harm the ficus either. Of course, if the plant is making your eyes burn, it makes sense to take precautions to protect yourself while you are working with it--wear safety glasses, gloves, and a mask, and don't continue to work with it if it still bothers you.

Ficus lyrata is not picky about soil, any good potting soil will do, and it is generally suggested that a smallish pot will help keep the plant from getting too big.

Date 2019-05-18
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Plant Answer Line Question

Keywords: Strelitzia reginae, House plants

I think that my bird of paradise plant is dying. Last year it had around 8 leaves, now it is down to 3 and they are all starting to yellow. I recently bought some fertilizer, but it hasn't seemed to help. I moved it to a window with more light, and it still seems upset. What can I do to revive it?


According to Rodale's Encyclopedia of Indoor Gardening edited by Anne Halpin (Rodale Press, 1980) and The House Plant Expert by D. G. Hessayon (Expert Books, 2001), the ideal growing conditions for Bird of Paradise (Strelitzia reginae is the most common species) are bright light (direct morning or winter sun, southern or eastern exposure), and a humusy soil mix with good drainage. Keep the plant at 55-60 degrees in the winter, and do not overwater. The rest of the year, it prefers indoor daytime temperatures of 65-70 degrees, and nighttime no lower than 50-55. Let the soil dry out between moderate waterings, and keep humidity at 30-35%. You can improve humidity by misting the leaves from time to time.The plant needs to be pot-bound (not overly roomy container) in order to flower. New plants take 4 to 6 years to flower.

As for fertilizing, you can feed every two weeks with a water soluble fertilizer, but do not fertilize in the winter.

There are several reasons it might have yellowed leaves. It could be due to unfavorable light or temperature, over- or under-watering, or the pot might not be draining well. I can't diagnose the problem via e-mail, but if you try to maintain the appropriate conditions for your plant, it may recover. You can also take pictures and bring a sample leaf or two to a Master Gardener Clinic.

Date 2019-09-19
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Plant Answer Line Question

Keywords: House plants, Coffea

I want to grow coffee plants but mine always end up spindly and slow-growing, with just a top-knot of leaves. I keep them indoors near my only natural light source, which is north-facing. The top foliage seems healthy and thick, in fact too thick for the thin stems to support unaided. I don't water until the soil is less than half 'wet.'
I've tried cutting the top off and replanting a couple inches of the root and stem, to no avail. I've been trying with multiple plants for 15 years to get this right and I just seem to have a perpetual 'black thumb.'
Should I toss my top-heavy plants?


Are you providing as many of the optimum conditions for your Coffea (coffee plant)as possible? They may not be getting enough light. According to Rodale's Encyclopedia of Indoor Gardening (edited by Anne Halpin, 1980), coffee plants prefer bright, indirect light from an eastern or protected southern exposure. Other needs include evenly moist (but not soggy) soil, frequent misting, and monthly feeding during the growing season with mild balanced fertilizer. In the winter, reduce watering, but don't let the plant dry out. Ideal temperatures are 70 to 80 degrees in the day and 60 to 65 degrees at night. This plant dislikes being rootbound, and if it needs repotting it is best to do this in late winter with fresh potting soil. To prevent leggy, straggly growth, this book recommends pinching the stems. Here is a bit more information on this plant, from Missouri Botanical Garden.

The other issue to consider besides lack of light (which is probably the main cause of the slow and spindly growth) is the plant's age. According to Reader's Digest Success with House Plants (1979), Coffea is single-stemmed when young, and only in time becomes bushy. If yours are younger plants, they may be out of balance now but could fill out as they mature.

Date 2019-12-18
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Plant Answer Line Question

Keywords: Plant protection--Law and Regulations, House plants

I live in Seattle, but am going to Canada for an extended stay. Can I bring my houseplants across the border?


Generally, Canada allows houseplants from the mainland United States, but you may be asked to provide proof of origin at the border. The Canadian government page on guidelines for visitors and seasonal residents spells out the details. Here is an excerpt:
"Houseplants are defined as plants commonly known and recognized as such, which are grown or intended to be grown indoors. Bonsai plants are not considered to be houseplants. If you are importing houseplants from the continental United States as part of your baggage or household effects, you do not need phytosanitary certificates or import permits. For all other plants from the United States, you may require a phytosanitary certificate from the U.S. Department of Agriculture and an import permit from the CFIA."

Date 2019-09-27
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Plant Answer Line Question

Keywords: House plants, Blechnum gibbum

I recently purchased a plant which the grower referred to as Blechnum gibbum, or Dwarf tree fern. I have searched many databases including the BBC, IPNI, and fern societies. Ultimately, I would like to discern the accurate, current name for the plant and plant culture and care information for in-home.


I confirmed the name Blechnum gibbum on the Royal Horticultural Society's website, as well as in Sue Olsen's Encyclopedia of Garden Ferns (Timber Press, 2007). For now you can safely assume that Blechnum gibbum is the correct, current name for the plant commonly called dwarf tree fern. It is a member of the family Blechnaceae, native to Fiji, and commonly grown as a houseplant in North America.

According to The New Houseplant Expert by D.G. Hessayon (Sterling, 1991), tree ferns require a temperature range between 60 and 70 degrees Fahrenheit (over 75 is a problem), an east or north facing window (or some shade in a hotter exposure), moist air (misting ferns can help with this), regular watering (particularly in spring and summer), and repotting so that the crown is not buried and the pot is not overly full of roots. The House Plant Encyclopedia by Ingrid Jantra (Firefly Press, 1997) says much the same thing, adding that temperatures below 54 are also a problem and that a light feeding once a month is helpful.

Date 2019-10-31
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Plant Answer Line Question

Keywords: photoperiodism, Kalanchoe, Failure to flower, Cactus and Succulents, House plants

Why do my house plants stop flowering after I bring them home? They are by a bright, sunny window. I bought Kalanchoe in 4 colors, and none flower any more.


Can you tell me if you feed the houseplants anything? Sometimes plants (indoors or outdoors) which are given a fertilizer that is high in nitrogen will produce a lot of leafy growth at the expense of flowers.

Make certain that you are providing ideal conditions for growing Kalanchoe. According to Barbara Pleasant's The Complete Houseplant Survival Guide (Storey, 2005), Kalanchoe grown indoors needs bright direct sunlight, and warm temperatures (70-90 degrees) from late spring to early fall. In fall and winter, it requires 50-70 degrees. It should not receive any fertilizer from late winter to early spring, and in winter, let soil dry out between light waterings. More importantly, Kalanchoe responds to changes in its exposure to light, which is referred to as photoperiodism. Pleasant says that "before a kalanchoe will make buds, it must be exposed to a series of long, sunny days followed by at least 2 weeks of short days, less than 12 hours long. This is easy enough to accomplish by placing plants outside in summer and then bringing them indoors in late fall, just before nighttime temperatures drop below about 40 degrees. After you bring the plant in, keep it in a room where no lights used at night. When brought into bloom naturally, kalanchoes flower in January and February. To speed up the schedule, cover the plants with a box for 14 hours each night for 14 consecutive days. Blooms will appear about 6 weeks later. Snip off bloom-bearing branches after the flowers fade."

Date 2019-09-11
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Plant Answer Line Question

Keywords: House plants, Air pollution

I would like to grow plants in my dorm room to improve the air quality. Which plants are most effective?


There was a NASA study on houseplants and indoor air quality in 1989. The study has to some extent been disputed and/or discredited. A January 17, 2018 article in Time magazine discusses it. Here is an excerpt:

"'There are no definitive studies to show that having indoor plants can significantly increase the air quality in the home to improve health in a measurable way,' says Luz Claudio, a professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai.

"Claudio has reviewed the research on the air-quality benefits of indoor plants. She says there's no question that plants are capable of removing volatile chemical toxins from the air 'under laboratory conditions.' But in the real world—in your home, say, or in your office space—the notion that incorporating a few plants can purify your air doesn’t have much hard science to back it up.

"Most research efforts to date—including the NASA study—placed indoor plants in small, sealed environments in order to assess how much air-scrubbing power they possessed. But those studies aren't really applicable to what happens in a house, says Stanley Kays, a professor emeritus of horticulture at the University of Georgia."

A 2019 study by Bryan Cummings and Michael Waring, published in the Journal of Exposure Science and Environmental Epidemiology concludes that "potted plants do not improve indoor air quality." The study is summarized in this article from Science Alert.

The Environmental Protection Agency has useful guides to maintaining good indoor air quality, one aimed at health care professionals, and one for the general public.
Here are excerpts from each:

"Recent reports in the media and promotions by the decorative houseplant industry characterize plants as 'nature's clean air machine', claiming that National Aeronautics and Space Administration (NASA) research shows plants remove indoor air pollutants. While it is true that plants remove carbon dioxide from the air, and the ability of plants to remove certain other pollutants from water is the basis for some pollution control methods, the ability of plants to control indoor air pollution is less well established. Most research to date used small chambers without any air exchange which makes extrapolation to real world environments extremely uncertain. The only available study of the use of plants to control indoor air pollutants in an actual building could not determine any benefit from the use of plants. As a practical means of pollution control, the plant removal mechanisms appear to be inconsequential compared to common ventilation and air exchange rates. In other words, the ability of plants to actually improve indoor air quality is limited in comparison with provision of adequate ventilation."

"Over the past few years, there has been some publicity suggesting that houseplants have been shown to reduce levels of some chemicals in laboratory experiments. There is currently no evidence, however, that a reasonable number of houseplants remove significant quantities of pollutants in homes and offices. Indoor houseplants should not be over-watered because overly damp soil may promote the growth of microorganisms which can affect allergic individuals."

All that being said, houseplants can, at the very least, provide an aesthetic improvement to a room, and as long as you are careful not to overwater, they shouldn't hurt air quality.

Date 2020-01-17
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Garden Tip

Keywords: House plants, Forcing, Bulbs, Winter gardening

If you feel emboldened after successfully growing a pot of paperwhites, why not keep the indoor flower display going through winter. For inspiration and good instructions read Forcing, etc: the indoor gardener's guide to bringing bulbs, branches and houseplants into bloom by Katherine Whiteside (Workman, 1999) Included are details on forcing bulbs and flowering shrubs, in addition to techniques for propagating house plants and tender perennials like coleus. The color photos will stimulate your enthusiasm.

Date: 2003-01-15
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