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Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

Reviving Philadelphus

I just bought a house with a garden which has good bones, but has been untended for a long time. I believe the garden has 4 Philadelphus X virginalis ‘Minnesota Snowflake’ plants. They are deciduous in the winter, and they have greened up nicely in the summer. They are about 6 feet tall. On the 4 plants, this first summer, I’ve only seen 2 flowers. Can these shrubs be salvaged by using a blooming (high in phosphorus) fertilizer? Or do they need something else?

The three things I would ask about Philadelphus with few flowers:

  • Are they in full sun? (Sun is needed for best flowering results.)
  • Have they been pruned and, if so, when? (Pruning is best done in late summer, after flowering.)
  • Are they growing near a lawn or other area which receives fertilizer that is higher in Nitrogen (N) than Phosphorus (P) or Potassium (K)?

I would recommend that you test the soil before embarking on a plan of fertilization, unless you are adding a mulch such as compost, which releases its nutrients slowly. Philadelphus is usually considered a light feeder (i.e., it doesn’t require a lot of supplemental fertilizer).

As far as a future pruning regime for the shrubs, Jacqueline Heriteau’s Complete Trees, Shrubs & Hedges: Secrets for Selection and Care (2005) says that Philadelphus “blooms on the previous year’s growth. A light annual pruning of older branches right after flowering keeps mock orange shapely and productive. Branches more than five years old should be removed in winter or early spring.”

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Disposing of seeds in compost

When is a seed a seed? My wife and I are in agreement on not putting weeds with set seeds in the compost (and the “Easy Compost” book says just that). However, we are less sure about weed flowers (probably OK), and what about seed cases that haven’t formed seeds yet? I’m thinking in particular of foxgloves right now, as the flowers are coming to an end and leaving behind the undeveloped seed cases. I’m unsure whether to compost them or not. Just an aside: our compost pile doesn’t get superheated.

That is a very good question. I found an article in Fine Gardening magazine which discusses harvesting wildflower seeds. It is relevant because it suggests that some unripe seeds may continue to ripen even after being harvested from the plant before maturity. Whether unripe seed will eventually germinate may have to do with the permeability of the seed coat: the more permeable, the more likely the chance it will germinate.

The book Seeds by Peter Loewer (Macmillan, 1995) says that plants with tough seed coats (like legumes and morning glories) “are virtually impermeable to water and must be nicked by the gardener or soaked in warm water for twenty-four hours before they germinate. If these jackets are not broken, scratched, or eroded, water never enters and germination never begins.”

I have found several references to the immature seeds of invasive plants (Ailanthus, teasel, yellow flag iris, to name a few) being capable of germination. The Complete Compost Gardening Guide by Barbara Pleasant and Deborah Martin (Storey, 2008) “weeds that show up in your garden are fair game for compost, even if they are holding seeds. […] Weeds that have not yet begun to bloom and lack viable root buds that help them grow into new weeds can be added to any compost project, but it is important to keep weed seeds to a minimum every chance you get. […] In every climate there are plant criminals known as noxious weeds […] Unless you are confident and committed to processing the compost made from noxious weeds with a high-heat procedure, collect them in a black plastic garbage bag and subject them to various forms of torture before dumping them in an inhospitable place. Cook bags of them in the sun, add water and let them soak into slime, and keep track of what works and what doesn’t. If your superweeds survive your torture methods and you don’t have a spot in your landscape suited to use as a little landfill, discard them as garbage.”

If you want to be on the safe side, avoid putting anything seedy (even green and immature) in the compost, especially if the pile is not going to get especially hot and speed the decomposition process.

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Trees and turf roots

I have two mature cherry trees on my parking strip that are about 20 feet tall. Their roots protrude above the sidewalk level and are quite prominent. There used to be grass planted above them but it wasn’t well maintained and I took it out several years ago. After trying different plantings, I’d like to put sod back on the strip and wondered if it was best to cover the roots fully with soil and lay the sod on top of that, or if it would be okay to let the roots be partly exposed and lay the sod around them.

I am not sure that it would be wise to reinstall sod over your trees’ roots. You may want to mulch them lightly (no more than 2-3 inches, and keep away from the trunk), as described by a document formerly available from North Carolina State University Extension:
Excerpt:
“Exposed surface roots can become unsightly or in the way. Roots do not suddenly grow on the soil surface. Roots increase in diameter over a period of years. Soil erosion can speed their exposure. Exposed roots need protection from pedestrian and vehicle traffic including lawn mowers. Mulching exposed roots physically protects them as well as conserves soil moisture and prevents direct sunlight from heating the roots. Cutting off or covering roots with top soil are temporary solutions that can cause long term damage to tree roots.”

There is excellent, clear information in this link to the International Society of Arboriculture’s page on trees and turf. Ultimately, the best thing for your trees and your grass is to create a grass-free zone under the trees’ dripline, mulch that area, and restrict lawn grass to the area beyond the dripline. Here is an excerpt from the link:

“Trees, shrubs, ground covers, and lawn grasses all require sunlight, water, and rooting space for growth. Each plant in the landscape competes with neighboring plants regardless of type or species. Some even produce chemicals that are exuded from roots to restrict growth of nearby plants. For each plant to do well, it must have adequate space. Because perennial woody plants increase in size each year, they require additional space over time. The landscape design should provide adequate space for these plants to mature.

“While shade is the biggest, most obvious problem trees create for turf growth, a tree’s roots also contribute to poor turf performance. Contrary to general thinking, most tree roots are in the top 2 feet of soil. More important, the majority of fine, water absorbing roots are in the top 6 inches of soil. Grass roots ordinarily occupy a much greater percentage of the soil volume than tree roots and outcompete them for water and nutrients, especially around young trees. However, grass root density is often much lower in areas where trees were established first. In these situations, tree roots compete much better for water and nutrients and prevent or reduce the success of establishing new turf.

“Competition is especially important when transplanting, seeding, or sodding. The newest plant in the area must be given special treatment and must receive adequate water, nutrients, and sunlight, which frequently means that competing sod should be removed from around transplanted trees and shrubs or that some of the lower branches should be removed from existing trees above a newly sodded lawn. In any case, do not do any tilling around trees.

“Mulching is an alternative to turf around trees, and its use eliminates potential competition. A 2- to 4-inch layer of wood chips, bark, or other organic material over the soil under the drip line is recommended because it

  • helps retain soil moisture
  • helps reduce weeds and controls grass
  • increases soil fertility when mulch decomposes
  • improves appearance
  • protects the trunk from injuries caused by mowing equipment and trimmers that often result in serious tree damage or death
  • improves soil structure (better aeration, temperature, and moisture conditions)

 

“Maintenance practices for trees and turf are different. Because tree and grass roots exist together in the upper 6 to 8 inches of the topsoil, treatment of one may damage the other. Fertilizer applied to one plant will also be absorbed by the roots of a nearby plant. Normally that is good, but excessive fertilization of either trees or turf can result in tree crown or grass blade growth greater than desired.”

Identifying seeds at the cotyledon stage

I have some seedlings coming up in my compost. They smell a bit like basil, but they could just as easily be weeds. I’d like to know if there are resources for identifying plants at this early stage of growth.

You could wait a week or two and allow the seedlings to develop, which might make identification a little easier. However, there are various resources online for identifying plants, especially weeds, at the cotyledon (seed leaf) stage.

There are also print resources which illustrate plants at the seedling stage:

  • Seeds of Woody Plants in North America by James A. and Cheryl G. Young (Dioscorides Press, 1992)
  • Woody Plant Seed Manual prepared by the Forest Service, USDA (1948), also available online
  • Seeds: The Ultimate Guide to Growing Vegetables, Herbs & Flowers by Sam Bittman (Bantam Books, 1989)
  • Park’s Success with Seeds by Ann Reilly (Geo. W. Park Seed Co., 1978)
  • Park’s Success with Herbs by Gertrude Foster and Rosemary Louden (Geo. W. Park Seed Co., 1980)
  • Weeds of the West edited by Tom Whitson (Western Society of Weed Science, 2000)
  • Weeds of California and Other Western States by Joseph DiTomaso and Evelyn Healy (University of California, 2007)

Propagating Euphorbia trigona from cuttings

I have a Euphorbia trigona houseplant which is about 6 feet tall and quite thin. It has gotten weak and is bending. It looks like it is going to break. I have read it can be propagated by stem cuttings. I don’t know if this means cut the end off OR cut several sections of the stem. If I just cut off the end will the original stem survive?

According to Indoor Gardening by Kate Jerome (Pantheon Books, 1995), Euphorbia trigona often becomes top-heavy, which sounds like what is happening to yours. They do tolerate pruning well and may branch out from the place where you prune. Do be very cautious in handling this plant, and be sure to wear gloves: it has a toxic sap that can cause skin irritation. Generally when cuttings are taken, they are from the tips, where the new growth is occuring, but you can also try several cut pieces of stem. Ordinarily not every cutting is able to form roots, but according to The Complete Houseplant Book by Peter McHoy (Smithmark, 1995), you can increase your chances of success by letting the cut ends of the stem dry out in the air before putting them into potting soil.

Deadheading rhododendrons

Should you remove old blooms from rhododendrons and, if you should, which is best: to prune or snap them off?

Here is what the American Rhododendron Society says on the subject:

“It is desirable, with the large flowered rhododendrons, to remove the withered flower clusters after the blooming season. This is fairly easily done as the central axis of the cluster, usually called a truss, will break free from the plant with a quick snap of the thumb pushing on the side, or can be cut off with a hand pruner. With the smaller flowered rhododendrons and azaleas, dead-heading is labor intensive and and generally is not required.

Dead-heading is usually done to make the bush look more attractive, to reduce the prevalence of fungus and to prevent a heavy set of seed. If it is not possible to remove the old flowers, it is usually not too detrimental, but flowering the next year may be reduced.”

I have several mature rhododendrons in my own garden, and I deadhead the parts of the shrubs which are easily reachable, leaving the other areas to their own devices. For me, it’s an aesthetic choice, and I would probably do them all if I could reach and if I didn’t get very tired of the task. (It’s hard to do well with gloves since you can’t easily feel the right place to snap off the flower head, but it’s sticky work without the gloves.) I’ve never tried pruning them off, because it seems less precise (leaves a bit of a stub), but if rhododendron experts approve (as indicated above), I may just try it this year.

Growing rhubarb with other edible plants

I just moved into a house with a beautiful vegetable garden with lettuce, kale, arugula and rhubarb (planted next to the lettuce). We have a 7-month old who will likely be all over the back yard in the next few months, and we have a few concerns about the rhubarb leaves, as we’ve heard they are poisonous.

Could rainwater roll off the rhubarb leaves and contaminate the lettuce? Could leaves left on the ground contaminate the soil? And if we touch the leaves, will the rhubarb’s poisonous properties contaminate our hands?

We aren’t huge rhubarb fans, so we will likely be taking them out at the end of this summer. Any tips on removing them to make sure they would not grow back?

Lastly, unrelated to rhubarb, we have some arugula that’s bolted (flowered). Is there anything we can do, like cutting it back, so it won’t be bitter and we can eat it? I assume we have to just replant it.

The toxic parts of rhubarb (Rheum x hybridum) are the leaves and the roots, as described by University of Illinois Extension:

“One characteristic consistent with all rhubarb is the toxicity of the leaves and roots. The rhubarb leaves contain high amounts of oxalic acid, a toxic and potentially deadly poison.”

Rhubarb is a frequently grown vegetable garden plant, and is often grown in close quarters (in the same soil, with the same irrigation) with other edible plants. As long as you are careful when harvesting your lettuce not to get pieces of rhubarb leaf at the same time, you should be safe. Here is what Plant Alert: A Garden Guide for Parents by Catherine Collins (Master Craftsman, 2001) says: “The leaf blade contains high concentrations of oxalic acid […] The stem is safe to eat, providing the leaf is removed with at least 2 inches of stalk below.” (By the way, rhubarb stalks or stems, spinach, beet greens, and chard all contain lesser amounts of oxalic acid–that’s what gives the chalky sensation you get on your teeth sometimes when you eat them.)

You will not be affected by the plant’s leaves or roots unless you ingest them.

There aren’t any special precautions you need to take in order to dig up your rhubarb, although gloves are always a good idea–you never know if your skin may be sensitive to particular plants.

About your arugula (Eruca vesicaria ssp. sativa): I usually cut the bolting stalks back, and new (and less bitter) leaves grow lower down on the plant. You could let one or two plants flower and go to seed–then you wouldn’t need to buy more seed. If you still have extra seed, you can sow more (called “succession planting,” described in this Fine Gardening article), in addition to cutting back the leggy stalks.

Controlling milk thistle

Silybum marianum: Should I give away baby plants? I bought the original “rare” plant from a local Perennial Society (can’t remember exact name)15 years ago. 4 feet tall, and prickly. I decided to get rid of it, but baby plants still pop up every year. Variegated, dramatic, but too prickly for a small garden!

I’m glad you asked about your Silybum marianum (milk thistle) starts. This plant is considered an invasive species in many places, including Washington State. King County lists it as a Class A noxious weed, meaning that eradication of the plant is required by law. Below is information excerpted from King County’s site:

This Class A noxious weed has a very limited distribution in Washington State, and eradication is required. The largest infestations in the state are in pastures in the southeastern section of King County but infestations are occasionally found elsewhere. Early detection and rapid, effective response is of the highest priority for this noxious weed.

Although occasionally found in gardens, it is illegal to sell or buy milk thistle in Washington State and all existing plantings should be removed in order to prevent accidental spread.

Milk thistle is toxic to livestock when consumed in large quantities, and it forms dense stands in pastures and rangelands. California reports up to 4 tons per acre in heavily infested areas. The leaves are very distinctive, with white marbling on the shiny green leaves.
Control Methods:
Manual: For small sites with few plants, pull or dig up rosettes or the bolted plants before seed heads form. Use a shovel to cut the plant off about one inch below the ground so the plant will not re-sprout. Chopping the leaves from one side of a rosette can provide access to the central growing point. Wear protective clothing. To be fully effective, all mature seed heads need to be bagged and removed so no new seeds are left on the site. Immature seeds can continue to develop in cut plants, and the less stem that remains attached to the flower head, the faster the seed head will dry out.

I highly recommend you do what you can to eradicate this plant and prevent its spread. Do not put it into the compost but instead bag and dispose of it as trash.

Ornamental grasses and their risk of reseeding

I have neighbors who are asking questions about ornamental grasses and their risk of reseeding. They are specifically interested in Pennisetum ‘Hameln’ and Cortaderia ‘Richardii.’ I have known Stipa (Nassella) to reseed but I have never noticed any reseeding from either of these two groups in the Seattle area. I know they can reseed freely in other parts of the country, but I suspect the seeds rot in our winters. How can I find out?

I consulted online lists of invasive species and found a few references to Pennisetum species (but not the cultivar you mention) which are problematic in Oregon and California.

According to the University of Florida Extension, the variety Pennisetum alopecuroides ‘Hameln,’ is not known to be invasive.

Although not classified as invasive, the species P. alopecuroides is still a spreader by self-sowing. Rick Darke, author of The Encyclopedia of Grasses for Livable Landscapes (Timber Press, 2007) and a Pennsylvanian, says it “self-sows, usually at a manageable level; however, some of the fall-blooming varieties such as ‘Moudry’ and ‘National Arboretum’ are particularly fertile and can be very weedy if conditions are suitably moist. They often become established in irrigated cool-season turf.”

The Royal Horticultural Society plant profile for ‘Hameln’ says it is early-flowering (which might mean that seeds have time to establish themselves, though a droughty summer such as we sometimes have in the Seattle area might prevent this).

One sterile cultivar is Pennisetum advena ‘Rubrum’.

As with Pennisetum, there are species of Cortaderia (C. selloana, C. jubata) which are listed as invasive in various parts of the world. Here is an excerpt from Los Angeles & San Gabriel Watershed Invasive Plant-Monitoring and Outreach Program’s WeedWatch information on Cortaderia:

“It is unknown whether the cultivated and ‘sterile’ varieties of Cortaderia are able to cross with the wild species of Cortaderia and produce viable offspring. Until this scientific research is conducted, and considering the rampant ecological damage already caused by both C. selloana and C. jubata, it is not recommended to plant any members of the species, including cultivars, varieties and supposed ‘sterile’ varieties.”

Author Rick Darke says that all species of Cortaderia may be grown from seed (therefore, species are not sterile).

A Pacific Northwest palms and subtropical plant online discussion group, Cloud Forest, (no longer available online) includes some dialog on Cortaderia richardii:

“Another terrific grass is Cortaderia richardii, which is a NZ form of the humongous pampas. It is short (to 5 ft.) with elegant flowering pattern (circular and reaching instead of vertical). It’s evergreen and a great specimen. Needs a 6 x 6 space minimum. ”

“Cortaderias scare me a little, ever since my neighbor’s specimen seeded itself all over my poor rock garden (heck it is all over the neighborhood). Obviously sellowiana; I don’t think C. richardii is as aggressive. The other one is a devil to try to remove once it gets wedged in! The leaves cut like razor blades and the crown is extremely persistent. I actually think they look kinda grand, and if I lived on the coast I’d be willing to use them as windbreaks (which is how they got all over the place). I don’t think a hurricane would injure them.”
“Cortaderia richardii is the magnificent cousin of the monstrous pampas grass. Petite and elegant in comparison. Mine have never self-seeded. My friend’s seeded a bit. It may be hotter by his house.”

Stopping raspberries from spreading

We are expanding our vegetable garden, and I wanted to put in a row of raspberries (hardly vegetables but you get the idea). My wife is against it because raspberries spread so many underground roots and sap the energy of the rest of the soil and interfere with other plants. I’m sure she’s right. What do gardeners do to stop the spread of raspberries?

To keep the raspberries from infiltrating the vegetable bed, you could install a root barrier such as gardeners use to contain bamboo. Even more simply, you could set the planting area apart from the veggies with fairly deep edging material–such as one might use to keep grass from invading garden beds. I’m sensitive to proposing something that is a lot of work, but you could make a dedicated raised bed for raspberries along the edge of the vegetable bed. I’m a fairly lazy gardener, and I planted raspberries amongst all my other plants. When the berries run to areas where they are unwanted, I just pull them out. But you will get a better, easier to harvest crop if you provide them with their own area. This Fine Gardening article addresses the issue of running, as well as ideal planting conditions:

Excerpts:
“Raspberries are joyfully exuberant about procreating by underground runners, poking up impressive numbers of healthy new plants all around your original patch. I don’t consider this to be a problem, though, because one whack of the hoe takes care of them. You can also present them to a friend or use them to extend your patch. […] If you have rich, deep soil that drains well year-round, you can simply plant your raspberries in a permanent garden site. Not us. The Pacific Northwest gets rain all winter, and many gardeners lose raspberries to root rot because they make the mistake of planting their raspberries’ fussy little toes directly in the ground, which is often soggy clay covered with a skim of topsoil. We also experience a two-month drought most summers. Raised beds allow us to have deep soil that holds moisture evenly yet drains well.”