Ask the Garden Guru

Ask the Garden Guru a question on the form (at right), and read below for the answer!



Rose Pruning

Dear Garden Guru,

I'm pruning my roses. Some are very tall. Is it true that if a branch of a rose bush has no thorns, it is a sucker? I've tried to get down to the bottom of the bush to see if the branch is coming from the base of the bush or from the ground. I can't always identify so I was wondering if the absence of thorns makes a difference. Any help would be appreciated. THANKS!!

Lorraine in Walnut Creek

Hi Lorraine,

Unfortunately, the lack of thorns does not indicate whether you have a sucker or a cane.  Many rose bushes will throw almost thornless canes occasionally that produce an umbel of flowers. They are difficult to prune in that they don’t always have a well oriented leaf bud scar to make a cut above.  You really do have to see if the origin is coming from beneath the bud union or the root zone. These thornless canes are often skinnier than there thorny sisters which makes it even harder to tell.  You won’t be hurting anything if you feel you must remove it just to be sure.

How to make Wisteria bloom

Dear Garden Guru,

I have a wisteria vine. It barely blooms and produces just leaves. I want to thin it out to bloom. Will that help?

Larry in Rio Vista

Hi Larry,

Dear Larry,

If you’re pruning your Wisteria each fall or winter, you may be pruning off the flowering wood. Wisteria only blooms on old wood. It is recommended that only the long, looping tendrils be cut back. Flower buds can be seen as largish, oyster shell or triangular shaped “scales” on the older wood behind these newer growths.

Another reason for a lack of bloom is over-feeding. Your Wisteria can get by without any feeding at all. Really! Your thought about opening the vine up with the chance of providing more sun can be done, but again, you may be removing flowering wood without knowing it, and pruning out the older wood will result in a large flush of new growth in the spring.

Meyer Lemon Tree leaves are chewed up

Dear Garden Guru,

I have a Meyer Lemon Tree that looks like something has been eating the leaves. Can you tell me what this could be and how to treat it?

Shirlee in Tiburon

Hi Shirlee,

I suspect your problem could be citrus leaf miner, especially at this time of year. The moth’s larvae mines through new leaves, causing them to distort and get ragged.  Read our handout about this pest:  Another possibility is young snails.  Applying Sluggo or Sluggo Plus around the base of the tree may help.

Gardening in sandy soil -- help!

Dear Garden Guru,

Help, I am in the outer Sunset and have very sandy soil. Can you give me recommendations on soil amendment and suggestions to plant? Thank you.

Maureen in Outer Sunset

Hi Maureen,

You have the first step right re: getting the soil amended. Indeed, there are little to no nutrients in sandy soil.

Our recommendation is to mix Sloat Planting Mix into the soil, along with E.B. Stone Sure Start when planting — this will allow plant roots to acclimate to the sandy soil, and give them the best chance to spread out and take up any nutrients in the soil.

You will need to “top dress” twice a year in February/March, and again in June/July with Sloat Forest Mulch. This is a great mix that has chicken manure and bark to help hold in moisture and keep nutrients in the soil.

Plant recommendations for your soil: lavender, sage, achillea, erigeron, Ceanothus, all cacti and succulents, escallonia, Festuca and rosemary.

Also see our Mediterranean style plant list- this should offer a good selection of options.

And a link to all our soils –

Getting rid of powdery mildew

Dear Garden Guru,

My neighbor gave me two beautiful rose trees in large pots. The stems/flowers have white powdery mildew on them. How to I get rid of it?

Nancy in Danville

Hi Nancy,

You can treat the powdery mildew with Monterey Bi-Carb. This is an old-fashioned but effective wettable powder (potassium bicarbonate) that kills the mildew on contact. It is safe for use in organic gardens. Another remedy is using Serenade spray. However, to prevent mildew in the future, you can spray roses with Neem or horticultural oil. The powdery mildew spores cannot penetrate the leaf through the oil film.

Lemons with thick rind

Dear Garden Guru,

For several years, my lemons have had a thick rind and little juice in them. I put a Meyer lemon tree in about 20ft away and it's starting to put out lemons this year. The rind again appears a bit thick. What's missing in my soil? I am using a citrus and avocado fertilizer. AG

AG in San Francisco

Hi AG,

There are two reasons that the rinds of citrus become thick and both are nutritional. The first is overfeeding with a food high in nitrogen.  It is recommended that a basic citrus fertilizer be fed in spring and again in fall along with a supplement that contains zinc such as FST.  The second reason is a deficiency in phosphorus ( the more likely cause).  To avoid providing an excess of nitrogen, you can supplement the recommended feedings with 0-10-10.

Help with dry lemons

Dear Garden Guru,

Why would lemons be dry and not juicy. It is sunlight?

Linda in San Francisco

Hi Linda,

There are a few environmental factors that will cause lemons to be dry. One is water stress. If the trees were not irrigated enough during the growing season, the tree will pull moisture out of developing fruits in order to keep the leaves healthy. Another is cold damage. When ripe or ripening lemons are exposed to freezing or frosty conditions, the moisture in the fruit dissipates. The most common reason for dry fruit is that the ripe lemons are left on the tree too long. Hope that helps!

Yellow lower leaves in houseplants

Dear Garden Guru,

About once every week the leaves at the bottom of my houseplants turn yellow. Why does that happen? I would love for it to grow and have leaves on the stalk.

Anna in Berkeley

Hi Anna,

When lower leaves turn yellow and drop off, it is usually caused by under watering (the plant is kept too dry) or lack of fertilizer. It could also be a combination of the two. Another factor, though less common, is lack of humidity. When you water your houseplant, be sure to add enough water so that all the soil is moistened and the planter drains. It may be that you have to do this in the sink or tub if you are concerned with water harming the floor or furniture. I normally would not advise feeding houseplants in the winter months but you will do no harm if you were to feed with a half strength solution of Maxsea All Purpose once a month through February. This may arrest that lower leaf fall. The lower leaves (older) turn yellow because the plant is pulling all the nutrients and energy out to disperse to the upper (younger) leaves.

Which fruit trees in the Richmond district?

Dear Garden Guru,

We are moving to the foggy Richmond District and would like to plant a fruit tree or two along the edge of our yard. What grows well in that area, and when should we plant? Thank you!

Lauren in SF

Hi Lauren,

You would do well with an Apple, such as Fuji or an Asian Pear (20th Century variety is self- fertile). You could also grow a European pear, but they do best in pairs (no pun intended). Lemons would also fare well. The best time to plant deciduous fruit trees is in January or February. You can plant citrus nearly year round.

Keeping palm trees healthy

Dear Garden Guru,

What are tips for keeping palm trees healthy indoors?

A gardener in San Francisco

Hi A gardener,

Here are some basics for growing palms indoors.

1. Palms like bright, indirect light, or morning sun a few feet away from a window. Too much sunlight can burn the foliage, while too little light can cause foliage to stretch, become stressed or encourage bugs such as mealy bug or scale. Some palms can take more or less light than others.

2. Ample humidity is essential; palms are accustomed to high humidity, so daily misting of room temperature water with a spray bottle will help. If the room is dry (from air conditioners or heaters), leaves may shrivel and turn brown at the tips. Maintain humidity by allowing the plant to sit in a saucer filled with gravel/stones and water. Also, group plants together.

3. Feel the soil about 1/2” below the soil surface. If it is dry then it’s time to water, if wet then leave alone for a few days. Never leave the plant sitting in water — it can lead to root rot.

4. Air flow is also important, without proper air circulation, your plant may be at risk for spider mites.