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What might cause brown tips on houseplants? Don Kinzler answers that question and more in this week's Fielding Questions

In this week's Fielding Questions, Don Kinzler answers questions about the possible causes of brown tips on houseplant leaves, if wood ash can safely be applied to gardens, and more.

Prayer plant with brown leaf tips.jpg
Gardening columnist Don Kinzler says the most common cause of brown leaf tips is buildup of salts or chemicals in the soil, left behind by the water source.
Contributed / Nathan S.
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Q: I’m trying my hand at houseplants, and so far I’ve had pretty good luck. But several different types, including the prayer plant in the photo, develop brown, crisp tips on the leaves. What am I doing wrong? – Nathan S.

A: Brown leaf tips are especially common on certain types of houseplants, such as prayer plant, peace lily, Chinese evergreen and spider plant. Many other types are less susceptible.

The most common cause is buildup of salts or chemicals in the soil, left behind by the water source. Fluoride, chlorine or water softener salts can cause leaf tip burn. Allowing tap water to sit overnight before using will release at least some chlorine gas, but not fluoride.

Repotting sensitive plants into fresh potting mix once a year will help reduce the problem. If plants aren’t ready for repotting, flush chemicals from the soil by placing the plant in the sink and watering generously until water is exiting the drain hole. Repeat immediately twice more.

Low humidity and furnace-dried air also cause leaf tip burn. Group plants for shared humidity where possible, mist occasionally, and place sensitive plants above water-filled pebble trays.

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To mitigate leaf tip burn on sensitive plants, try watering with rainwater, melted snow, or filtered water.

Q: We heat with wood, and I’m wondering if we can spread the ashes on our vegetable garden. We generate about six to eight five-gallon buckets of ashes a year. – Clarence D.

A: I first wrote about this in 1982 when woodburning stoves were highly popular. After consulting with the North Dakota State University soil specialist at the time, we developed the following recommendation: “Wood ashes do provide some soil aeration benefits in moderation, and a small amount of fertilizer. However, since wood ashes are high in lime (calcium), large quantities could be harmful to North Dakota soils already containing plentiful lime. Potential risks may outweigh the benefits. If a disposal is needed for wood ashes, one inch applied to the soil surface followed by tilling is a safe rate.”

To see if the recommendation was still accurate, I consulted a University of Saskatchewan website, and in a Dec. 2022 article they indicated the largest ingredient in wood ash is calcium carbonate (lime), with small amounts of potassium, phosphorous, iron, manganese, boron, copper and zinc, depending on the type of wood burned.

The university indicated that although it has some nutrients, wood ash has no nitrogen, so it’s not a complete fertilizer. Over-use of wood ash can be detrimental to soil bacteria, so the university advised using it sparingly.

Because of its high alkalinity, wood ash is a better fit for the acidic soils of the Southern and Southeastern states.

Q: I’m a volunteer at our high school greenhouse. We got several coleus plants last fall, and I’ve tried to start cuttings with no results. I’ve dipped them in rooting hormone, but they die. Any suggestions? – Clarence D.

A: Although many gardeners propagate coleus cuttings successfully in a glass of water, there’s a way that helps new roots transition to potting mix better, with less transplant shock than roots formed in water. I call it the ice cream bucket method of propagating cuttings.

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Fill a plastic pail half full with moistened vermiculite, after first poking six holes in the pail’s bottom for drainage. Take stem cuttings from plant tips, about four inches long, and remove leaves from the lower half. Insert the lower half of the cutting into the vermiculite, and firm well.

A pail can hold about eight cuttings, depending on size. Water thoroughly after installing the cuttings, then place the pail inside a clear plastic bag, fastening the top loosely, to convert the pail into a miniature greenhouse. Place in a window receiving indirect sunlight.

Roots should form on coleus cuttings in two to three weeks. Coleus enjoy warm temperatures, so placing the pail in a warm location will enhance rooting. Because coleus root readily, no rooting hormone is needed.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu . Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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