Scotch Thistle: Spotting & Stopping this invasive in Idaho Landscapes

Have you noticed a massive, silver-green giant creeping into nearby pastures, roadsides, or even the back corners of your home garden? Scotch thistle (Onopordum acanthium) is making an aggressive push across Idaho landscapes, and this highly invasive weed requires early intervention.

Originally brought to the United States in the 1800s as a Mediterranean ornamental plant, it quickly jumped the fence. Today, it is an officially listed Class B Noxious Weed in Idaho, Oregon, and Washington. If left alone, it can easily form dense, virtually impenetrable stands that choke out native forage, block access for wildlife, and cause a major headache for homeowners.

Knowing the Lifecycle: Year One vs. Year Two

The secret to managing Scotch thistle is understanding how it grows. It operates primarily as a biennial plant, adapting its growth strategy based on the local environment:

• Year One (The Rosette): The plant spends its first year low to the ground, focusing energy on building a tough taproot. It forms a wide rosette of gray-green, velvety leaves covered in a thick mat of woolly hairs and sharp yellow spines. These rosettes can spread up to 2 feet wide.

• Year Two (The Giant): In its second year, the thistle bolts. It shoots up a heavily branched, winged stem that can tower over 8 feet tall and 6 feet wide. Globe-shaped, dark pink to lavender flowerheads emerge at the branch tips in mid-summer.

Why Is It Such a Problem?

A single, healthy Scotch thistle plant is capable of producing 8,400 to 40,000 slender, plumed seeds. These seeds are easily caught by the wind, hitch a ride on moving wildlife, or travel via contaminated soil, gardening equipment, and hay.

Once it establishes a foothold in wet meadows, dry rangelands, or disturbed garden soils, it quickly forms thick patches. Its formidable, spiny armor physically blocks animals from grazing and prevents access to waterways.

How to Control Scotch Thistle in Your Space

Tackling an infestation requires an integrated management approach combining mechanical, cultural, and targeted chemical methods.

1. Manual and Mechanical Removal

• Dig it out early: Small infestations are easiest to manage when the plants are still in the low rosette stage. Use a sharp shovel to slice the taproot completely below the soil surface.

• Gear up properly: The sharp spines are no joke! Always wear heavy-duty leather or chemical-resistant gloves and thick protective clothing to avoid painful punctures.

• Time your mowing: For larger patches, mowing can prevent seed production, but timing is everything. Mowing too early causes the plant to simply recover and flower closer to the ground. Mowing too late means cut flowerheads left on the ground can still successfully release viable seeds.

• Bag and trash: If the thistle has already started blooming, bag up and discard the cut flowerheads in the trash rather than composting them.

2. Cultural and Biological Methods

• Establish Competition: The single best long-term defense against thistle seeds is a dense, well-maintained lawn or pasture. Healthy, thick turfgrass or native groundcover will naturally outcompete invading seedlings.

• Managed Grazing: While there are currently no approved biological insect controls for Scotch thistle, goats will readily graze the prickly flowerheads, and sheep will sometimes feed on the younger rosettes.

3. Herbicide Control

When physical removal isn't practical, a targeted herbicide treatment may be necessary. Applications are generally most effective when sprayed during the active spring growth phase before the plant bolts. Remember to follow all pesticide safety protocols: wear proper protective clothing and avoid spraying during windy or excessively hot conditions to prevent chemical drift and volatility.

For specific chemical recommendations and timing tailored to your local county, consult the Pacific Northwest Weed Management Handbook or contact your local county noxious weed coordinator.

Remember Your Legal Obligations

Because Scotch thistle is on regional quarantine lists, it is illegal to buy, sell, transport, or distribute the plants, seeds, or wildflower mixes containing them. Keeping our boots, tools, and tires clean after visiting infested areas is a simple step that goes a long way toward protecting Idaho's beautiful open spaces.

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Resources & Further Reading:

• Identification and Management of Scotch Thistle — Oregon State University Extension Catalog

• Noxious Weed Search: Scotch Thistle — Washington State Noxious Weed Control Board

• University of Idaho Extension — Landscapes and Lawns

Curled Tomato Leaves: Stress or Curly Top?

Seeing curled leaves on your tomato plants can be alarming, but curled leaves do not always mean disease. In many cases, the plant is simply reacting to stress.

Physiological leaf roll is a common response to growing conditions. It usually begins on older leaves near the bottom of the plant. The edges of the leaves cup upward and may roll inward, sometimes forming a tight, tube-like shape. Even though the leaves look unusual, the plant generally stays green, continues growing, and keeps producing fruit. Fruit quality and yield are usually not seriously affected.

Several things can trigger physiological leaf roll. Hot weather, drought, uneven watering, overly wet soil during high temperatures, heavy pruning, too much nitrogen, phosphorus deficiency, or a heavy fruit load can all contribute. Some tomato varieties are also more prone to leaf roll than others. The best response is usually simple: keep soil moisture as even as possible, avoid overfertilizing, and do not remove too much foliage at once. During extreme heat, temporary afternoon shade may also help reduce stress. There is no need to remove the plant just because the leaves are rolled.

Beet curly top disease is different. It is caused by Beet curly top virus and is spread by the beet leafhopper. Tomatoes are commonly affected, but peppers, beans, squash, table beets, Swiss chard, melons, cucumbers, and many weeds can also serve as hosts.

Curly top causes more than curled leaves. Infected plants may become yellow and stunted. Leaves can look thick, leathery, twisted, or brittle, and the veins may turn purple. New growth may be distorted, fruit production may slow or stop, and young plants can die. Unlike physiological leaf roll, plants infected with curly top do not recover.

Take a look at the whole plant before deciding what is wrong.

Is the plant still green and growing normally, with curling mainly on the older, lower leaves? Is it still setting and ripening fruit? That points more toward physiological leaf roll.

Is the plant stunted, yellowing, brittle, or declining? Are the newest leaves distorted? Do you see purple veins, leathery foliage, or very little fruit? Those symptoms are more concerning for beet curly top.

Plants suspected of having curly top should be removed because they will not improve. Managing nearby weeds may help reduce possible host plants. Young transplants can also be protected with row cover or shade cloth while they are becoming established, as long as row covers are removed when pollination is needed.

The bottom line: Leaf curl by itself does not equal curly top. Look at the plant’s color, growth, leaf texture, fruiting, and overall health before reaching a conclusion.

Interested in learning more?

• University of Idaho Extension — Physiological Leaf Roll of Tomato
• Oregon State University Extension — Physiological Leaf Roll of Tomato
• Utah State University Extension — Curly Top of Tomato

Creating a Fire-Resistant Landscape Around Your Home

The saying goes, “Where there’s smoke, there’s fire.” While that early proverb remains true, smoke is not the only danger during a wildfire. Embers—burning pieces of airborne wood or vegetation—can be carried by the wind for more than a mile. These embers may start spot fires or ignite homes, debris, fences and other objects.

Here we will cover ways homeowners can help reduce damage to homes and other structures by creating a fire-resistant landscape and choosing plants that are also suited to dry conditions.

What Are the Primary Threats to Homes During a Wildfire?

Research comparing homes that were destroyed with those that survived points to wind-blown embers and small flames as the primary ways homes ignite during wildfires.

Homeowners can prepare their property to resist embers and reduce the chance of flames reaching the house or attached structures. The condition of the home and the area immediately surrounding it can make an important difference. This space is known as the Home Ignition Zone, or HIZ.

The Home Ignition Zone is generally divided into three areas.

1. Immediate Zone: 0 to 5 Feet

This is the area closest to the house and the most vulnerable to ignition from embers.

• Remove leaves, pine needles and other debris from roofs, gutters, decks and along the foundation.
• Keep firewood, wooden planters, combustible decorations and wood mulch away from the siding.
• Cover exterior vents with corrosion-resistant metal mesh with openings no larger than 1/8 inch.
• Whenever possible, keep this zone free of vegetation and use gravel, rock, concrete or another noncombustible surface.

Current wildfire guidance recommends keeping the first five feet around the home as free of
combustible materials as possible. When plants are retained in this area, they should be widely spaced, carefully maintained and surrounded by noncombustible materials.

Possible low-growing plants include:

Hardy ice plant (Delosperma species): A low-growing succulent with colorful summer flowers. It works well in rock gardens but should be kept from forming a continuous mat.

Stonecrop or sedum (Sedum species): Fleshy, water-storing plants that remain low to the ground. Many varieties are well suited to hot, dry locations.

Hens and chicks (Sempervivum species): Compact succulent rosettes that can be placed among rocks or gravel.

Turkish speedwell (Veronica liwanensis): A low, mat-forming groundcover with small blue flowers.

Even these plants should be placed cautiously. No plant is completely fireproof, and accumulated dry leaves or stems can become fuel.

2. Intermediate Zone: 5 to 30 Feet

The goal in this zone is to slow the movement of fire and prevent flames from reaching the house.

• 
  Space trees and shrubs so their branches do not touch.
• Remove lower tree limbs, generally up to 6 to 10 feet from the ground, while avoiding removal of more than one-third of the tree’s live crown.
• Keep grass mowed to no more than about 4 inches.
• Remove dead leaves, branches and flower stalks.
• Plant in small, separated groups rather than creating continuous beds of vegetation.

This zone can include low-growing perennials, lawns or lawn alternatives, and carefully selected deciduous shrubs.

Common yarrow (Achillea millefolium): A widely adapted perennial with finely divided foliage. Western native forms are available, and established plants tolerate dry conditions.

Blanket flower (Gaillardia species): A deep-rooted, drought-tolerant perennial that provides colorful flowers through much of the summer.

‘Autumn Amber’ three-leaf sumac (Rhus trilobata ‘Autumn Amber’): A low-growing selection of a western native shrub. Its spreading form can help cover soil without creating a tall wall of vegetation.

Lewis’ mock orange (Philadelphus lewisii): Idaho’s state flower, this deciduous shrub has fragrant white flowers. It should be kept pruned, separated from other shrubs and free of dead interior branches.

3. Extended Zone: 30 to 100 Feet

The extended zone is intended to interrupt the path of a wildfire and prevent flames from moving into tree crowns. On steep slopes or heavily vegetated properties, defensible-space work may need to extend beyond 100 feet.

• Thin dense stands of shrubs and trees.
• Remove ladder fuels—vegetation that allows fire to climb from grasses and shrubs into tree canopies.
• Break up large areas of brush or continuous vegetation.
• Move firewood stacks at least 30 feet from buildings.
• Remove combustible debris from around sheds, fences and other outbuildings.

Hardy native and drought-tolerant plants can be used in this zone, but they still need proper spacing and maintenance.

Arrowleaf balsamroot (Balsamorhiza sagittata): A familiar western native with a substantial taproot and good drought tolerance. Remove dry flower stalks and foliage when they begin to accumulate.

Golden currant (Ribes aureum): A drought-tolerant native shrub with yellow spring flowers and green summer foliage. It should be planted with enough space between neighboring shrubs.

Yucca (Yucca species): A low-water plant with thick leaves and a strong architectural form. Older, dry leaves should be removed because they can collect beneath the plant and become combustible.

Plant Choice Is Only Part of the Solution

Fire-resistant does not mean fireproof. Any plant can burn when it becomes dry, overgrown or filled with dead material. Placement, spacing and regular maintenance are often more important than the name of the plant.

A well-planned fire-resistant landscape should include healthy plants, open spaces, noncombustible surfaces and breaks between groups of vegetation. Choosing drought-tolerant plants can reduce water use, but those plants must still receive enough water to remain healthy and green.

By looking at the landscape as a series of zones, homeowners can take practical steps to reduce available fuel, slow the movement of fire and improve the chance that their homes will withstand wind-blown embers.

https://www.nfpa.org/education-and-research/wildfire/preparing-homes-for-wildfire

https://www.nfpa.org/news-blogs-and-articles/blogs/2025/01/06/what-is-the-wildfire-home-ignition-zone

https://objects.lib.uidaho.edu/fwres/fwres38.pdf