Ask the Garden Guru



Help: White flies!

Dear Garden Guru,

I have an enclosed urban (aka concrete) patio that I’ve spruced up with lots of potted plants - cherry tomatoes, herbs galore, miniature meyer lemon, potato bush, etc. The problem: My garden seems to be infested with white flies. It started on the cherry tomato, but they have spread to the basil, roses, and other plants. I have tried insecticidal soap weekly for months and seem to be losing the battle (nothing like going to get some basil and getting a cloud of white flies all over my hair and body in the process). Help! How can I save my plants from this pest?

Johanna in San Francisco

Hi Johanna,

Whitefly, as you are finding, are very difficult to get rid of.  They are immune to most sprays.  You can control the nymphs that are seen as small oval shapes on the undersides of leaves with a Neem oil spray.  The oil asphyxiates them and will do some good with repelling the adults.  You might consider putting out yellow sticky traps to reduce the populations of the adults as well as incorporate a reflective “mulch” (such as sheets of aluminum foil on the soil surface of the pots).  Your tomatoes and basil will likely stop producing by mid-November. Dispose of the plants rather than compost them.  A hand held vacuum, such as a dust buster can be used to suck the adults off the leaves when they are sluggish in the morning.

Tomato issues

Dear Garden Guru,

My tomato plants (2) were growing like mad and had many blooms. I was out of town overnight and when I returned the stems were starting to look slimy, then they quickly died, turning black. Can you tell me why?

Dolores in Pacifica

Hi Dolores,

I am sorry to hear about your tomatoes. Crop failure always hurts when it hits established presumably healthy, vigorous plants. I think that you were struck by either Late Blight of Tomato or Bacterial Blight. The sliminess suggests a Bacteria and this is a disease that can ravage a plant very rapidly. The spores are airborne or carried in the stomachs of beetles such as the Diabroitica or Cucumber Beetle (the green “ladybugs”). The disease is likely to occur if we have warm or hot days and mild nights suddenly interrupted by much cooler, damp weather.

Overhead watering will also stimulate an outbreak. I hope that you have removed the plants so that spores will not over winter in that spot. It is advisable to change the location of your tomatoes next year. To control the disease and reduce its spread, EB Stone Copper spray is recommended.

I hope this helps. Please feel free to visit any of our store locations to get more specific instructions on how to use the Copper Spray, or just get more instructions for this year.

Mealy Bugs

Dear Garden Guru,

I seem to have a mealy bug situation primarily in my succulents. I have used a fungal oil, but they seem to come back. Now what!!

Claudia in San Rafael

Hi Claudia,

It is common for succulents to get mealy bugs so you are not alone. The fungal oil did not work because mealy bugs are not affected by fungus treatments. The best way to eradicate these insects is to swab them first with rubbing alcohol which melts away the white waxy coat they wear. Follow up by spraying Bonide All Seasons Oil or Bonide Eight All Insect spray. Make sure that you have watered them in advance of any treatment.


Dear Garden Guru,

I have a new meyer lemon brought home in the past year. The leaves have recovered from initial yellowing, but are now being chewed on by something. I haven't seen the culprits, but a couple of leaves have been chewed almost in half. What would be a good approach for thwarting the unidentified culprits? (This is in the Excelsior area of San Francisco)

Laura in San Francisco

Hi Laura,

The main culprits are usually either caterpillars or slugs and snails (and sometimes earwigs). With slugs or snails you will generally see holes forming in the middle of the leaves. Plus there is usually a slimy trail left behind on the leaves too. All of our Sloat locations carry slug and snail controls that would help if this is the case.

Caterpillars will eat the leaves from the outside in. The outer edges of the leaves look like they have bite marks on them. They can certainly eat a fair amount of leaves on a citrus tree. If this is what is happening, there is really only one active ingredient that works against caterpillars, called Bacillus thuringiensis (Bt). You can also find this at any of our stores, made by Safer Brand.

Earwigs can do damage similar to that of caterpillars, but caterpillars are the more frequent attacker. If you find that the caterpillar killer is not solving your problem, then our Greenlight Slug and Snail bait also works on earwigs and could potentially handle multiple problems for you at the same time.

Additionally, spying on your tree late at night with a flashlight might help you discover what it is that is feeding of your plant.

I hope this helps. As always feel free to visit any of our garden centers for more expert advice.


Dear Garden Guru,

I have several rhododendrons in my backyard, and all are thriving except for one. All of the plants were planted about 4 years ago, and have done well. This one rhodendron, however, started to droop over the past month. It now has the buds growing on most of the stems, but all of the leaves are brown or yellow and are drooping. Is the plant dead? Or is there a chance it will come back in the spring? What is wrong with it?

Cheryl in Corte Madera

Hi Cheryl,

The symptoms you describe on your Rhododendron can be caused by a few things. Drooping leaves are an indication of root rot or crown rot. If the plant is buried too deeply or stays water saturated too long, the leaves will droop. If the temperatures drop below 35 and the soil is not wet enough, the leaves can turn brown and droop due to desiccation from cold. Some varieties of Rhododendron are more affected by cold than others. Sometimes gophers can be a problem. There is also a chance that Sudden Oak Death spores have infected the plant. I am concerned about the fact that the leaves are brown and yellow. Were they this color before the leaves began to droop? I would suggest that you aerate the soil around this plant to improve drainage and water penetration. If the soil is dryish, even after the rain, I would water. DO NOT feed the plant as this will often exacerbate a problem in an already stressed plant. You may want to dig up the plant and replant it again in a higher position. This will also give you the opportunity to examine the roots. If they are blackened, you have root rot and the plant will unlikely survive.  If the plant has been cold damaged, it will rebound again in the spring. Hope this helps.


Dear Garden Guru,

I just obtained a tall and heavily flowering Cymbidium orchid and would like to stabilize it by transplanting to a deeper and wider pot. Is this advisable, and what sort of growing medium is best?

Tom in San Francisco

Hi Tom,

Yes, those Cymbidiums certainly can be tippy. I would advise you NOT to transplant your orchid until after it finishes blooming in the spring. Wider is much better than deeper, as a deeper pot will hold too much moisture and could possibly rot your plant. For planting, you could use our fine orchid bark OR the orchid planting mix. The orchid planting mix is of course heavier and would act as a better ballast for a tall Cymbidium. We do have an orchid seminar coming up on the evening of February 6th at 5:30. It will be held at our store on Sloat Blvd. Glenn Smith, who will be giving the talk, is a wealth of knowledge on orchid repotting.

Eek, a snake! How do keep them out of my yard?

Dear Garden Guru,

I live on the west slope of Twin Peaks, adjacent to many open spaces. I am morbidly afraid of snakes and I have seen several in the area and one in my yard. I am landscaping the backyard and need to know the best way to keep snakes out of my yard.

Susan in San Francisco

Hi Susan,

I cannot help you with your phobia of snakes, but be assured, the snakes that you have seen are probably not poisonous. I suspect that you have seen garter snakes (black w/ yellow stripes or orangey green) and bull or gopher snakes (ochre yellow and brown to 6′ long). While there may be an occasional Pacific rattlesnake, they tend to live in Manzanita/oak chaparral by creeks. Gopher and bull snakes hunt frogs, lizards, rats, mice and birds.

To keep them from intruding, I would not have a backyard pond or plant material that uses a lot of water. Any area that is irrigated more than 3 times a week will entice little frogs and salamanders. Bird feeders may invite little field mice.

Overgrown shrubberies and ivy will be appealing to rats. You may want to consider putting out some of the new safe rodent baits (they only target rodents, not cats, dogs, birds, etc) to reduce your chances of having rats and mice about. There is also a snake repellant called, fittingly, Snake Away. Sloat does not carry it but it is available online. It is basically napthalate (the main ingredient in moth balls). You may want to consider sprinkling moth crystals or some such along your more “wild” borders.

Keeping plants healthy with beneficial insects

Dear Garden Guru,

I have a fairly shady back yard in the North Beach part of SF. I have put in Heuchera a couple of times. They seem to last/be healthy a year or so and then die. When I go to check the dead looking plant, it comes up in my hand without apparently any roots left. Is something eating the roots? Or possibly rotting them? The plants can look healthy and the next time I go down (a week or so) the leaves have all drooped and it is starting to dry out. Thank you.

Phyllis in San Francisco

Hi Phyllis,

Other than gophers, (you would see the evidence of mounds of earth) roots can be attacked by weevil larvae and cutworms. They are especially prevalent in shaded areas. Other plants that usually fall victim are Azaleas, Impatiens, Primrose, and Iberis. The best plan of action would be to apply beneficial nematodes to the soil. The microscopic nematodes search out, attack, and subsequently vanquish soil borne grubs and larvae. If the roots were rotting, other plants in the area would likely show symptoms of stress as well. These are available via mail order through any of our stores.

Peach borer pest

Dear Garden Guru,

I just noticed that my Donut Peach tree has some clear colored, sticky (sap type) substance on the branches ... what is this and how do I get rid of it safely?

Kathy in Orinda

Hi Kathy,

If the sap you see is on the trunk and lower branches of the tree, I suspect that you may have Peach borer damage. The peach borer looks like a metallic blue wasp and lays its eggs in the bark. The larvae burrow in causing sap (and you may see frass- a sawdust like substance) to leak from the holes.

You can kill the larvae by inserting a thin wire or unbent paperclip into the holes. You may also want to spray the bark of the tree with Bonide Borer Miner Killer (available at any of our stores). If holes are not in evidence, the tree may be showing signs of gummosis disease. This occurs when the tree has received too much water or has poor drainage. This can also occur due to winter cold damage of new growth triggered by a late summer pruning.

Make sure that you feed the tree this spring and again in mid summer to promote vigor. The Miracle Gro Fruit Tree and Citrus spikes are easy to use…or use EB Stone All Purpose Food around the drip line. Be sure to water after feeding. Do not mulch your tree so the soil holds too much water.

Dreading Aphids

Dear Garden Guru,

I have aphids on my anenomes and have treated them with insecticide soap a number of times such that the aphids seem to be gone, but then they come back within a couple of days. I have also tried dousing them with water, but to no avail. Besides controlling them on the plants outside, I would like to bring some cut flowers inside but am afraid of spreading aphids to houseplants.

Susan in Mill Valley

Hi Susan,

I am sorry to hear that you are being plagued. Rather than the insecticidal soap, you may want to consider trying Rose Defense. We have been very satisfied with the results of this product for years. Beneficial ladybugs are also back in the stores. It may be to your advantage to release a box so that they can hunt down the hidden aphids in your garden that are unseen.

As far as cutting flowers to bring inside, a thorough washing with the hose should do the trick. I have a feeling the aphids are hiding under the foliage or on unopened buds. Be sure to spray under the leaves as well.

Curious if we have your favorite plant or product in stock? Call one of our locations directly and we'll be happy to check.