Category Archives: Garden Invaders

Phil’s Garden Tips and Tricks for August

It is high summer, and the dry season is at its peak! We have only had one significant rain event since April, making ground moisture very low. Many plants are stressed if they have not had some additional water. Even the native plants do not look their best this year. The forests and meadows are tinder dry, so please be vigilant!

To do this month:

  • Weed. Weeds should be few and far between now that the hot weather is upon us and it has been a long time since it has rained. Do not let weeds go to seed in your yard!
  • Mow regularly.
  • Don’t forget to water. The best time to water is in the early morning using approximately one inch a week on the lawn. Use more if it has been hot and dry. Beds need half that much.
  • Control caterpillars on leafy vegetables, Geraniums and Petunias, as needed, with Bacillus Thuringiensis (BT). It is a natural product.
  • Use mulch to protect ornamentals and garden plants from hot weather damage. For spider mite control on susceptible ornamentals, hose off foliage once a day for three days, once a month in July, August and September. Arborvitae hedges are the most susceptible.
  • This time of the year fleas can inhabit dusty dry areas where suburban wild creatures such as Raccoons, Skunks and Opossums lurk. Fleas that were left from those animals can very easily jump onto your pets or you. Sometimes just watering these areas can drown the fleas, or, sprinkling some food grade Diatomaceous Earth can cause disruptions in their life cycle.

An Invasive Violet for Western Oregon

It is not uncommon for people to ask me about violets which seem to be taking over their yard.

Violets come up in planting beds and lawns in the early spring, with purplish-blue flowers and purple leaves in the sun but darker-green leaves in the shade. They can grow here in Western OR, even in gravely, dry soil where not much else will grow. They are not fragrant.

European Dog Violet
European Dog Violet

This is the European Dog Violet or technically, Viola riviniana ‘Purpurea Group’.  Sometimes V. riviniana plants are sold as Viola labradorica purpurea and sometimes even Viola hederacea, but I am told that those plants are not generally sold or even found growing in the NW.  Buyer beware!

V. riviniana can spread from its running fleshy roots which can plunge rather deeply, making this plant quite drought tolerant.

These plants can out compete smaller plants such as Mentha requenni  and can mar the look of a stand of slightly taller plants such as Ophiopogon p. ‘nigrescens’.  However bigger plants can comingle with no problems.  For instance, the currently invasive common exotic ivy, Hedera helix, can easily quash this violet but then who wants a groundcover of ivy?

V. riviniana can spread easily by seed as they shoot their seeds explosively, up to 8’ away when they are brown and ripe. Therefore, once established, they can become very numerous. On top of that fact, the seeds may stay viable for up to 5 years in the ground.

Some suggested methods of removal are:

  • Covering a densely growing area, where they are to be removed, with cardboard and 2” of bark mulch. Anything that dares to come up from far flung seeds, immediately remove with your handy Hori-Hori! This may take more than one year.
  • Hori Hori
    Phil’s trusty Hori-Hori

    I personally have removed all of my V. riviniana with only a hoe over the past 15 years. If I see one riding piggy back in the potting soil around a plant from a nursery, I immediately dispose of that violet in the garbage, not the compost pile! I wait to plant it in the ground, watching the potted plant and digging out any other unwanted Violets over the course of a year. I have seen several come up around a piggy-backed plant from a nursery!

  • There was a location in my garden, after the 10th year, where violets repeatedly came up from seed or old roots around the base of my largest old Azalea. Ultimately, I crawled on my hands and knees and surgically removed them with my Hori-Hori. They have not returned since.

Plants from the Genus Viola that you may want to cultivate in Western Oregon.   

viola sempervirens
Viola Sempervirens

We have some lovely native violets here in the Pacific NW, but many are robust growing as well. Try them where they will not cover smaller plants and you will be fine. You may find different species on-line. I have grown (and particularly like) a yellow one that is local and evergreen, Viola sempervirens. It is cheery in the spring. It spreads by stolons as well as seeds.

There are plenty of other lovely exotic violets that are not invasive. These Violas include the plants which we commonly call Violas and Pansies  in the nurseries. They have been bred to have large blooms and are available in many colors.

From what I have read, the most fragrant form of this plant is Viola odorata ‘Rosea’,  which is a bright pink form.

Final thoughts:

Clearing a property of Viola riviniana is not a task for the faint of heart.  It requires persistence and boldness and a ready willingness to dig in the rainy season, because that is when it is easiest to pull/dig out of the ground.  In the dry season of summer, it can be almost impossible to extract this Viola from the concrete-like dry ground. Your Hori-Hori is your best friend if you would like to rid yourself of these invasive plants.