on the safety of using horticultural oils on edible plants

I have some insects on my bay laurel, which we use for seasoning. Someone told me to spray it with horticultural oil. I wondered, though, if it would still be safe to use the leaves in cooking. Is horticultural oil petroleum based?

As you suspected, horticultural oil is petroleum-based. U.C.Davis provides information on Integrated Pest Management for the plant I am assuming you mean, the usual source of culinary bay leaves, sometimes called sweetbay, or Laurus nobilis. (Just to be clear, I don’t think you are referring to the plant known as California bay laurel, Umbellularia californica, which has leaves that are aromatic and reportedly edible, but not commonly used for seasoning.)

They also note that “…specially refined petroleum products, often called narrow-range, superior, or supreme oils. Some botanical (plant-derived) oils are also available.”

When referring to plant-derived oils, I believe they are referring to Neem oil, as described in this information from U.C. Davis Integrated Pest Management. Additional information from the Cornell University Resource Guide for Organic Inesct and Disease Management about Neem and human health is excerpted here:

Studies of azadirachtin mutagenicity and acute toxicity have shown that it likely does not pose a significant risk to human health. However, some people have exhibited skin and mucous membrane irritation from neem seed dust (Weinzierl and Henn 1991). Note that most studies have been done on azadirachtin, and may not show the
effects of a whole neem product. Neem is used in some commercial human hygiene products.

Another long excerpt on horticultural oil from Colorado State University Extension suggests the following:

Essentially all commercially available horticultural oils […] are refined petroleum products also known as mineral oils. Impurities in the oil that are associated with plant injury, such as aromatic compounds and compounds containing sulfur, nitrogen or oxygen, are removed. Filtration, distillation and dewaxing complete the production of the finished base oil. Final formulations of horticultural oils are normally combined with an emulsifying agent that allows the oil to mix with water. This mixture usually is used at about a 2 percent dilution.

Vegetable oils also can be used as insecticides, although the type of oil can greatly affect its activity. Cottonseed oil is generally considered the most insecticidal of the vegetable oils. Soybean oil, the most commonly available vegetable oil used in cooking, has often provided fair to good control of some insects and mites.

Extracts from seeds of the neem tree, Azadirachta indica, have recently attracted attention as a source of pest management products. Several neem-derived insecticides have been developed. A number of compounds found in neem seeds, notably azadirachtin, have proven useful as insecticides. However, the oil fraction of neem seed extracts, which is mostly free of azadirachtin and related terpenoid compounds, also has demonstrated effects as a fungicide and insecticide. At least one product currently on the market, TrilogyR, consists of a largely azadirachtin-free oil fraction of neem seed extracts. It is formulated with an emulsifier and mixed with water at a concentration similar to horticultural oils (0.5 to 2.0 percent). Many over-the-counter products sold in nurseries that mention neem contain the oils of neem seed extracts.

If you know the insect on your bay tree, Peaceful Valley Farm Supply is one commercial supplier which carries less toxic, of not organic, products to control specific pests. That does not necessarily mean they are safe enough to spray on leaves which will be harvested for cooking.

on pruning lamb’s ear and Himalayan honeysuckle

I have a question about cutting back plants. I have some non-flowering lamb’s ear that is looking quite scraggly. How far back do I cut these, and when?

Also, how far back should I cut my Himalayan honeysuckle? We planted it 2 years ago, and last summer it got 5 feet high!

Also, last year my Hebe plants did not flower. We have Hebe anomala purpurea ‘Nana’. I have recently checked the tags they came with, and it doesn’t mention that it flowers. Is this a non-flowering Hebe? Although the shrubs are lovely, I was hoping for the type that flowers. If we decide to move them, when would be the best time to transplant them?

Yes, Stachys (lamb’s ears) can look pretty ragged after winter. I’m guessing you are growing Stachys byzantina ‘Silver Carpet’ or a similar cultivar, which doesn’t flower. If you look closely, you should see signs of new growth. I would suggest cutting back all the tattered or dried leaves as far as you are able, without injuring new growth. March is a good time to divide the plant if you like. (I have shared this plant many times and moved clumps to new locations. It is quite tough, and will transplant easily.)

Himalayan honeysuckle, Leycesteria formosa, can be cut back to the ground (or within a few inches of the ground) in late winter or early spring,according to Sunset Western Garden Book. The website of Rainyside Gardeners (a Northwest site) has a useful page on Leycesteria formosa.

According to Hebes: A Guide to Species, Hybrids, and Allied Genera by Lawrie Metcalf (Timber Press, 2006), Hebe anomala ‘Purpurea’ is a synonym for Hebe odora ‘Purpurea’ which is supposed to have a lot of flowers. He doesn’t mention the dwarf variety, ‘Nana,’ but I assume it would have similar attributes. Even with the nomenclature confusion, there seems to be some consensus about the floriferous qualities of the plant: Douglas Chalk’s Hebes & Parahebes (Christopher Helm, 1988) lists Hebe ‘Anomala’ as a cultivar of Hebe odora, and he too says it has lots of flowers. Are your Hebes getting enough sun? Some Hebes will flower in partly shady sites, but the flowering will be diminished. Could they have been pruned accidentally, just before flowering? Another possibility is that the plants are not mature enough to flower. The Metcalf book mentions a few species which can take years to produce flowers. He also says that flowers are enhanced by chilling followed by warmth, over a period of about 12 weeks. The number of hours of daylight to which the plants are exposed is also a factor. As far as transplanting, doing it in March should be fine. It isn’t too hot, and we are likely to have the occasional rain,but you should still water well when you first move them.

propagating sedums and succulents

I am learning how to propagate plants for my yard. I am now into Sedums and other succulents. I am trying to learn how to propagate Echeveria x hybrida “The Rose.” This one has me totally baffled. Can you help?

First, here is some general information. The propagation method you choose for Sedum depends on the habit of the plant, according to the American Horticultural Society’s Plant Propagation (edited by Alan Toogood; DK Publishing, 1999). Most species will root easily from cuttings in 1 to 6 weeks.

Tender species can be propagated from leaf cuttings. Take leaves off a stem. Place on damp newspaper in bright shade at 61 degrees F. Roots and plantlets should form in 3 to 4 weeks. You can also use stem cuttings by taking 2 to 3 inches from the tip of a stem and allowing the cutting to callus for a day. With hardier forms of Sedum, use 3/4 to 1 1/4 inch stem cuttings.

The book Echeveria Cultivars by Lorraine Schulz and Attila Kapitany (Schulz Publishing, 2005) offers directions on propagating from offsets, cuttings, cuttings from crests, head cuttings, leaf and stalk cuttings, and seed.

proper time to prune Arbutus unedo and other plants

When is the proper time to prune Arbutus unedo? How much can be pruned at a given time? Same question for Osmanthus decorus, Viburnum odoratissimum, and Quercus reticulata.

According to The American Horticultural Society’s Pruning & Training edited by Christopher Brickell (DK Publishing, 1996), you can prune Arbutus unedo in spring, as soon as danger of frost is past (that would be early April in Seattle), but keep pruning to a minimum. Some people choose to remove lower branches to create a taller trunk on younger trees.

The book Pruning: A Practical Guide by Peter McHoy (Abbeville Press, 1993) says that Osmanthus decorus can be clipped in late summer. If you want to limit its size without clipping, prune back long shoots to points far inside the shrub in late spring or early summer, after flowering. If the plant is overgrown, you can spread this type of pruning over two or three years, but do not do it annually. I am not familiar with this species of Osmanthus, but I do know Osmanthus delavayi, and grow it as a hedge. It is sheared after it flowers, and then probably two more times
before winter. I did have to prune the top back quite hard last year, and this did not seem to cause any problems, but O. decorus may have different needs.

I could not find information about Viburnum odoratissimum specifically, but most pruning books have general guidelines for Viburnum species. Unless you do not mind losing the flowers, it is best to prune when flowering is done. If you are growing V. odoratissimum as a tree, then special pruning may be needed. George E. Brown’s The Pruning of Trees, Shrubs and Conifers (Timber Press, 2004) says V. odoratissimum is somewhat tender, and may grow best as a standing bush with the protection of a wall, using ties in places to keep it close to the wall. The only
pruning he mentions is cutting out older wood after flowering, and tying new growth back to the wall (if you are growing your plant in a site where you can do this).

According to the Peter McHoy book, oaks do not require routine pruning. Brown’s book says not to prune oaks between mid-spring and mid-summer, as a means of protecting against oak wilt and beetle infestation. If you
must prune, do it in winter.

Quercus reticulata is not a common tree, nor are the species of Viburnum and Osmanthus you are growing. Unless there are compelling reasons to prune harder, I would suggest sticking to the 3 D’s of pruning: take out only dead, diseased, and disordered branches. Another general rule of thumb is never to remove more than 1/3 of the plant at one time. You might want to consult a certified arborist as well. You can find arborists through Plant Amnesty’s referral service or the Pacific Northwest Chapter of the International Society of Arboriculture.

on transplanting root-bound Bougainvillea

I have a Bougainvillea that has been in a container for a year. I would like to move it to a sunnier location, but it has taken root to the ground. What should I do to move it? I don’t want to just pull on it for fear of tearing and harming the roots.

The best time to move or transplant your Bougainvillea is in early spring, before active growth begins.

Since your Bougainvillea is only a year old, it shouldn’t be too difficult to transplant, but you should dig carefully around the base of the pot to try to loosen the roots which have made their way into the ground. You may have to break apart the container to get the plant out. Try to get as much of the root system as you can. Bougainvillea has very fine roots. According to Sunset Western Garden Book (2007 ed.), “the roots do not knit soil together in a firm root ball, and they are highly sensitive to disturbance.” Are you repotting the plant, or planting it directly into the garden? In pots, Bougainvillea likes its roots to be somewhat crowded and potbound. The August, 2006 issue of the Master Gardener’s Newsletter from Travis County, Texas, says, “Don’t place a Bougainvillea container directly on the soil or else risk the plant rooting out the bottom and creating quite a surprise when the plant is moved.” So you are not alone in having this experience!

This document from University of Hawaii Cooperative Extension provides a lot of information about how to grow Bougainvillea.

ideal growing conditions for Bird of Paradise plant

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.

using hair as fertilizer

I have heard that hair used in a garden is good for pesticide and fertilizer. Do you have any information about this? Also does it have to be human hair or is dog hair considered to be the same?

Here is what The Rodale Book of Composting (Rodale Press, 1992) says about using hair as fertilizer. “Between 6 and 7 pounds of hair contain as much nitrogen as 100 to 200 pounds of manure. Like feathers, hair will decompose rapidly in a compost pile but only if well-moistened and thoroughly mixed with an aerating material. Hair tends to pack down and shed water, so chopping or turning the pile regularly will hasten decay.”

I would think that any hair, including dog, would respond similarly. I would not add the hair directly to any planting area without putting it through the composting process described above. Also, hair which is heavily processed with chemicals (perms, dyed hair, etc.) doesn’t seem like anything I would want to add to the garden.

I could not find anything in the literature which mentions the use of hair as a pesticide, although I have heard that hair strewn around the garden beds may discourage animals from foraging there.

root-pruning Laburnum bonsai

I have a Laburnum grown from RHS seed. I wish to bonsai the tree (it is currently about 2″ high and in the UK). I would like to know when the tree should need root pruning, branch pruning (obviously when it is much bigger). What is the best soil mixture and what should the watering regime be. I have looked in my bonsai books, but Laburnum trees are somewhat neglected. Any help would be appreciated.

I consulted the many bonsai titles in our library, and found only one which mentions Laburnum in the index, The Complete Book of Bonsai by Harry Tomlinson (Abbeville Press, 1991). The book offers no specific pruning directions for Laburnum. I will paraphrase what the author says: The bonsai specimen should receive full sun, and daily watering plus feeding every 2 weeks during the growing season. It should be repotted annually in early spring, using basic soil mix. Trim shoots to 2 or 3 buds. Appropriate bonsai styles for this plant are informal upright, slanting, semi-cascade, cascade, twin trunk, clump, and multiple trunk.

The book Bonsai: The Art of Dwarfing Trees by Ann Kimball Pipe (Meredith Press, 1964) offers general directions on maintaining a bonsai specimen. Pruning of roots and branches is probably best done in early spring before new growth begins.

There are also online resources. The website of the American Bonsai Society, used to have a list of bonsai resources which included this list of articles on bonsai trees, written by Brent Walston. Here are links to a few of the articles:

best base soil for raised bed organic gardening

I recently moved to a new house that had what looked like a fair amount of chemicals dumped on the land around it. What do you recommend as a base soil for raised beds for organic gardening if you do not want to trust what you have?

The first thing you might consider doing is having your soil tested. There are various labs that can test for toxins as well as for soil type and nutrient deficits. Here is a link to a WSU site, which lists the labs and what they do. These labs primarily serve agriculture, so you might consider getting a soil test from a lab that specializes in home gardens, such as University of Massachusetts, Amherst. Also, if you suspect heavy metals in the soil, there is a link from Seattle-King County Public Health to labs that will test for toxins.

In her book, Backyard Bounty (New Society Publishers, 2011), British Columbia gardening expert Linda Gilkeson says that for your raised bed (or any situation where there was either no native soil, or where you are replacing the existing soil) you should “buy the best soil available, and mix in a generous amount of compost, leaf mold (well-rotted leaves) and other organic matter as you fill (1 or 2 parts compost to 9 parts soil).”

If you purchase topsoil, make sure to find out what the composition of the topsoil is. Linda Chalker-Scott’s factsheet from Washington State University Extension indicates that the ideal percentage of organic matter [abbreviated as OM] in topsoil used for gardens is 5% by weight.

Excerpt from the WSU blog discussion of this topic:
“If you’re purchasing topsoil, check out what you will be getting before it’s delivered. Ask the seller what the topsoil contains and also ask for the producer’s test data regarding pH, salt level, nutrient levels, OM content, and texture. If they don’t have that data available, you may want to consider taking a sample and have it tested yourself. Also, find out if the soil has been screened to remove rocks. Before you’re stuck with unsuitable topsoil, know exactly what you’re getting.
Garden Hint: Topsoil is usually sold by the cubic yard. One cubic yard of soil will cover about 50 square feet to a depth of four to six inches.”

pruning and training young trees

I have two 15 foot maples (2-inch diameter trunk) in our front yard. They shot up with quick growth last year, which was very impressive.

Now, with the recent rains on their thick foliage, they are sagging to an extreme degree. One tree, to the west of the driveway, has its lead trunk bending over at a 90 degree angle — it is now parallel to the ground, no longer pointing vertically.

Why can’t my trees support themselves? Where is their strength? Are they in danger of breaking? Vertical growth is the goal here, not stooping, drooping, sagging maples.

Should I prune them, or tie them up, or let them droop?

From your description it sounds like your trees could benefit from some pruning and/or staking. In general the former option is better than the latter. If you find that, after pruning, your trees still droop, you will most likely want to stake them. The goal with pruning is to slowly train the tree so that its shape fits your needs and the tree’s structure is sound. Below are some websites that will help you prune and/or stake your trees. The last two sites address in detail the issue of young trees.

The City of Seattle offers general tree care information that may be useful, as well as a page on young tree care.

The International Society of Arboriculture (ISA) has a succinct page about how to prune a young tree.

The ISA also has a section on how to plant a tree. Scroll down to find a diagram that shows how to stake a tree.

The National Arbor Day Foundation’s Nine Things to Know About Trees contains some pruning information as well.

A Grounds Maintenance Magazine column (now archived) by Michael W. Dougherty of Tree Management Co. addresses the specific needs of young trees.

Lastly, the Urban Tree Foundation’s Pruning page shows how to shape a young tree in order to strengthen its structure.