plants and spontaneous combustion

My “gas plant,” Dictamnus albus, is finally flowering for the first summer ever, and I am starting to worry: can it spontaneously combust? It’s planted close to the house. I remember stories from a couple of years ago about houses in Seattle catching fire because of sun or extreme heat igniting compost or soil in planters. Are there plants besides Dictamnus that are especially flammable?

Dictamnus is not called the gas plant or the burning bush for nothing. It is in the same family as citrus plants, and contains extremely volatile oils that can indeed reach a high enough temperature to ignite. In Defense of Plants blog describes this aspect of the plant, and asks why a plant might have this capability (to burn out competing vegetation, or merely an unintended consequence of oil production). Excerpt:
“If air temperatures get high enough or if someone takes a match to this plant on a hot day, the oils covering its tissues will ignite in a flash. The oils burn off so quickly that it is of no consequence to the plant. It goes on growing like nothing ever happened.”
Some gardeners amuse themselves and amaze their friends by demonstrating this flare of flame, but I highly recommend you not try it if your plant is up against your house!

You can read more about the flammable properties (and garden merit) of Dictamnus in the June 1995 issue of American Horticulturist. See the article “Ignite the Night” by Robert L. Geneve.

There are other flammable plants. Areas that are accustomed to preparing for summer fire season (such as Grants Pass, Oregon and Ashland, Oregon) have information about which plants are most (and also which are least) likely to ignite. The flammable list includes ornamental juniper, Leyland cypress, Italian cypress, rosemary, arborvitae, eucalyptus, and some ornamental grasses.

If you are concerned about the proximity of this plant to your house, you might consider transplanting it elsewhere in fall, though be aware that Dictamnus has a taproot and is not fond of being moved.

on growing heirloom fruit varieties

I am interested in planting fruit trees on our treeless property. Can you recommend any sources of bare-root heirloom fruit varieties grafted onto modern rootstocks? Or do people who grow heirloom fruits usually use the old rootstocks, too?

I am also interested in finding a descriptive list of how different heirlooms taste, how difficult their pests are to control, and how they do in our region (Puget Sound).

Below are some suggestions:

1. WSU’s fruit research station in Mt. Vernon is the best place to learn about history, grafting rootstocks, varieties, etc. Here is an article from the spring 2013 issue of WSU’s Washington State Magazine on heirloom apples.

2. There is an event in early October at Cloud Mountain Farm in Everson, Washington. They have a fruit festival where you can taste the fruits and talk with experts.

3. An outstanding book you will probably want to buy (or come to the library to review it first) is Fruit, Berry and Nut Inventory, 4th edition, An Inventory of Nursery Catalogs Listing All Fruit, Berry and Nut Varieties Available by Mail Order in the United States. Edited by Ken Whealy, 2009.

We also have many other excellent reference sources about growing tree fruit.

4. You might consider joining the Western Cascade Fruit Society or the Seattle Tree Fruit Society. They offer courses and events, and are very knowledgeable.

5. The staff at Raintree Nursery near Morton, WA offer a wealth of information about what grows well in the Pacific Northwest, best rootstocks, etc.

leafminers affecting vegetables

Something is damaging the leaves of my chard plants. There are blotches that appear as if the green surface has been scraped or eaten away, and there are sort of squiggly tunnel-like markings. Is this caused by an insect? Should I remove the plants and not plant chard in the same spot next time?

What you are describing sounds like leaf miner damage. Compare what you are observing to the particularly clear photos from this University of Connecticut extension fact sheet. See also this Washington State University Extension page about leaf miners on spinach and beets. Here is an excerpt:
“Control weeds in and around the garden. Rotate crops. Do not replant where crops were infested the previous year. Pinch leaves to kill larvae inside. Pick out infested leaves when noticed. Discard leaves in garbage. Screen plants with a floating row cover prior to emergence of flies in spring (April-May).”

You don’t need to remove the plants entirely–just cut off the damaged leaves–but you should plant your chard (and spinach and beets) in a different part of the vegetable garden next time.

planting over septic drain fields

I am looking for plants suitable for a septic drain field site. I have a very large north facing slope in open sun with a drain field running along the top half. I would like to plant low to no maintenance ground covers and low growing shrubs to cover this area. This is a focal point when driving up to my house so I want it to be eye catching and interesting year round.

I thought of heaths and heathers as a possibility, but I’m not sure if the root system is shallow enough. I also would like to include native ground covers such as ferns, Gaultheria shallon and any others that you might think would work, as well as ornamental grasses and perennial flowers for interest. Can you please offer a resource for planting over drain fields or a list of plants that you think would work?

Trees or large shrubs should be kept at least 30 feet away from your drain field. If you do plan to plant trees near a drain field, consult an expert to discuss your ideas and needs. Trees and shrubs generally have extensive root systems that seek out and grow into wet areas like drain fields. Grass is the ideal cover for drain fields. Grasses can be ornamental, mowed in a traditional lawn, or left as an unmowed meadow. You can also try groundcovers and ferns.

The key to planting over the drain field is to select shallow-rooted, low-maintenance, low-water-use plants. When tank covers are buried, keep in mind that plantings over the tank–from inlet to outlet–will have to be removed every three or four years for inspection and pumping.

Planting your drain field will be much different from other experiences you may have had landscaping. First, it is unwise to work the soil, which means no rototilling. Parts of the system may be only six inches under the surface. Adding 2 to 3 inches of topsoil should be fine, but more could be a problem. Second, the plants need to be relatively low-maintenance and low-water use. You will be best off if you select plants for your drain field that, once established, will not require routine watering.

SOURCE: WSU Cooperative Extension – Clallam County

Information can be found here.

Thurston County, Washington, has some information about landscaping a drain field, including plant suggestions, here.

Additionally, the Pacific Northwest Gardener’s Book of Lists (1997, by R. & J. McNeilan) offers a number of groundcover lists for various situations, including groundcovers for dry sites, slopes, and sun and shade. The Miller Library has this book.

deterring rabbits from the garden

Wild rabbits invaded our garden area this year and ate 20 feet of bean plants and then ate all the leaves from my strawberry plants. What can I do next year to discourage the little creatures?

There is a helpful factsheet on rabbits from the Washington Department of Fish and Wildlife’s Living with Wildlife website. Some of the recommended methods of control include exclusion fencing and barriers, noise-making and scaring devices (if you have a dog, this will help!), and even planting plants that are mostly unappealing to rabbits. There is also information on live-trapping, but this is not ideal because “the animals typically become disoriented, which results in them getting hit by a car or eaten by a predator. If they remain in the new area, they may cause similar problems there, or transmit diseases to other animals in the area. If a place ‘in the wild’ is perfect for rabbits, they are probably already there. It isn’t fair to the animals already living there to release another competitor into their home range to the detriment of both of them.”

Anecdotal evidence from local gardeners indicates that some odor deterrents may be effective. The UW Farm has had trouble with rabbits eating their plants. The farmers there have used a product that is blood meal-based (PlantSkydd). It comes in dry pellets or it can be mixed with water and applied with a sprayer. It is more or less odorless to the human nose. There are repellents made with putrefied eggs but I’ve heard that these smell wretched to humans, too. Urban coyotes do control the urban rabbit population but also prey on small domesticated pets, so we should never leave our cats and dogs unattended in the garden. The City of Seattle P-Patch Program’s document, Rabbits in the Garden, has a recipe for homemade repellent, and a recommendation against capturing the rabbits for human consumption.

arrival of the brown marmorated stink bug in the PNW

I’ve been hearing lately that there’s a new invasive insect called brown marmorated stink bug.

What plants does it damage, and how can I prevent or control the damage?

Yes, the brown marmorated stink bug is a recent arrival in the Pacific Northwest. Washington State University Extension has a Pest Watch fact sheet that provides details on its life cycle, contrast with similar-looking bugs in our area, and potential damage the bug causes. “BMSB,” as it is called, is well-known on the East Coast for damaging agricultural/edible crops and ornamental plants, as well as seeking refuge inside houses during cold weather (they don’t harm people, but they do emit offensive odors).

The website “Stop Brown Marmorated Stink Bug” is the main information clearinghouse on controlling this pest.It includes a list of host plants (two of the most attractive to BMSB are invasives themselves: Ailanthus altissima or Tree of Heaven, and Paulownia tomentosa or Empress Tree). Washington State University is involved in research on natural predators and control methods for this invader (as well as another recent one, Spotted Wing Drosophila). Most chemical interventions attempted so far have had limited efficacy. Research into organic controls is ongoing.

For now, get to know what the bug looks like, and if you think you see one in your garden or home, report it as a citizen-scientist using the form on the website of Stinkbug-Info.org or contact your local WSU Extension office.

So far, the only officially reported sightings in Washington State have been in Clark and Skamania counties.

on ever-blooming lilacs

Is there such a thing as a repeat-blooming lilac?

There is a relatively new (2009) lilac cultivar named ‘Bloomerang’ which is a repeat bloomer, described in this amusing article in Macleans.ca by Anne Kingston, entitled “Ever-blooming lilac wars.” The article also mentions ‘Miss Kim’ and ‘Josee’ as repeat bloomers. Reblooming lilacs seem to appeal to some but to be an offense to others, who want lilac blooms to remain seasonal signifiers:
“The reason people love lilacs, of course, is because of their temporal timetable. Their first (and only) flowering is greeted with joy, tinged by the knowledge it will be fleeting, poetically so; no one has ever complained of being bored by lilacs. Tamper with that and you hit a primal nerve, as the Bloomerang clearly has.”

on pruning Japanese hornbeam trees

I have a small hornbeam tree, Carpinus japonica, roughly 6 feet tall. It has never been pruned and was just transplanted to a small shade garden, close to a path. It will need to be pruned to keep the canopy high. Should this tree be pruned lightly? Is it at a good age or time to prune and to make a single trunk tree? Would fall be the best time to prune?

According to the American Horticultural Society’s Pruning & Training (edited by Christopher Brickell, DK Publishing 2011), Carpinus species tolerate pruning well, but it is best to do it from late summer to midwinter, to avoid severe bleeding of sap. Your tree is quite small, and it should be fine to do light pruning and remove any branches that are going to interfere with the path. (Heavy pruning can result in twiggy growth.) It’s not uncommon to prune it to a central leader standard (more upright form).

on choosing among different lavender varieties

I have a fence around my front yard with a planting area of about two feet between the fence and the sidewalk. The fence is south facing and I would like to fill the narrow planting area with lavender, but am having difficulty choosing the best variety. Ideally, the plant wouldn’t spill too far into the sidewalk when in bloom. Also, I’ve also noticed some species retain some green growth during the nonblooming months, which would be nice. All recommendations and advice are welcomed.

Almost all varieties of lavender will grow in our area, but you should look for varieties that will stay close to your size limitations (so you won’t have to do hard pruning, just shearing). According to The Timber Press Guide to Gardening in the Pacific Northwest (Carol and Normal Hall, 2008), Lavandula angustifolia ‘Hidcote’ blooms over a long period of time, and doesn’t mind cooler summers that are not too dry. The ‘Munstead’ variety is dense and compact, and the most heat-tolerant of L. angustifolia cultivars. Lavandula dentata (French lavender) needs warm summers to bloom well and dislikes winter rain, so it would not be an ideal choice. Cultivated varieties of Lavandula x intermedia tend to be larger (3-4 feet high and wide). Lavandula stoechas (Spanish lavender) has sort of pineapple-shaped blooms, and flowers from May to July and sometimes again in fall. The authors mention a more upright and compact variety of L. stoechas, ‘Anouk.’

Most lavender will retain grey-green or green leaves over the winter, unless we have a serious freeze.

Lavandula

selecting Daphnes

Which Daphne from the Great Plant Picks lists would be happiest as a planting against a house foundation receiving full sun with a western exposure, and which would have the best fragrance?

Of the Daphne species and cultivars listed on Great Plant Picks the only one which might not want full sun against a foundation or heat- and light-reflective surface would be Daphne odora ‘Aureomarginata.’ The others, if planted with sufficient distance from the house to grow into their natural shape, all like full sun for maximum flowering.

Intensity of fragrance is fairly subjective, and all these Daphnes are described as having notable fragrance. This article, from the Arnold Arboretum at Harvard, has some detailed descriptions.

One thing you could do is test the smell some of these plants in other gardens. We have examples of a few of them here at the Center for Urban Horticulture.

Another consideration is the ultimate size of the Daphne you select. Daphnes aren’t too pleased with being pruned, so choosing a species that won’t exceed the space you have available would be helpful.