Tansy in Zone 4B — Mountain West

Tansy is a worthwhile addition to any Mountain West garden. Its fern-like foliage and bright yellow flowers add visual interest, and it's incredibly low-maintenance. Plus, its insect-repelling properties can benefit your other plants.

Our short growing season and alkaline soil can be a challenge, but Tansy is well-suited to these conditions. With a 128-day growing season, you have ample time to enjoy its beauty and benefits. Proper timing is key, and this guide will help you make the most of it.

Starting Tansy indoors isn't always necessary, given its hardy nature, but it can give you a head start. If you're eager to see blooms as early as possible, start seeds indoors mid-March through early April, about six weeks before transplanting.

Use seed trays with a good seed-starting mix. Keep them warm and provide plenty of light, either from a sunny window or grow lights. Bottom watering will help prevent damping-off, which can be a problem with seedlings. Remember that our Mountain springs can be unpredictable, so starting indoors protects against late frosts.

Transplant your Tansy seedlings outdoors early to late May, once the danger of hard frost has passed. Before planting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense sun and cooler nights we experience here.

Space your plants 12-18 inches apart. Even in late May, keep an eye on the forecast for potential cold snaps. A row cover can provide extra protection if needed, especially at our high altitude.

Direct sowing Tansy is a straightforward option, especially if you prefer a low-fuss approach. Sow seeds early May through late June, once the soil has warmed up a bit. Tansy isn't too picky, but loose, well-draining soil is ideal.

Prepare the soil by loosening it and removing any rocks or debris. Sow seeds thinly and cover lightly with soil. Space them about 12-18 inches apart or thin later. Direct sowing can be a bit slower than starting indoors, but it eliminates the need for transplanting and can result in sturdier plants.

Tansy is exceptionally drought-tolerant, which is a huge plus in our dry Mountain West climate. In fact, it thrives on neglect, so avoid overwatering. Too much water can actually make it more aggressive and less healthy.

During the active growing season (roughly June through August), water only when the top two inches of soil feel dry to the touch – use the "finger test." Typically, this means watering about once a week, providing about ½ inch of water each time. Water at the base of the plant to avoid wetting the foliage, which helps prevent any potential fungal issues in our low-humidity environment.

Signs of underwatering include wilting or drooping leaves, while overwatering can lead to yellowing leaves and potential root rot. A light layer of mulch around the base of the plant can help retain moisture and suppress weeds, but avoid piling it up against the stem.

Regular deadheading is key to keeping Tansy in check and preventing it from becoming overly aggressive. Snip off spent flower heads as soon as they fade to encourage more blooms and prevent self-seeding.

Throughout the growing season, focus on removing only the dead or dying foliage. As the first frost approaches around mid- September, cut the plant back to just a few inches above the ground. This helps it conserve energy over the winter and encourages new growth in the spring.

You can expect your first Tansy harvest mid July through late September, about 75 days after planting. The best indicator is when the button-like yellow flowers are in full bloom.

Harvest by cutting the stems with clean shears, taking what you need for your intended use. Remember, Tansy is primarily used as an insect repellent and should not be consumed in large quantities due to potential toxicity. Regular harvesting encourages continued production throughout the season.

As the first frost approaches in mid- September, harvest any remaining foliage you want to use. You can dry the leaves for later use as an insect repellent.

Here are two common problems you might encounter with Tansy in the Mountain West.

Invasive Tendencies

• What it looks like: Tansy spreads rapidly via rhizomes (underground stems) and seeds, quickly taking over garden beds and even escaping into surrounding areas.
• What causes it: Tansy is a vigorous grower, and its ability to spread both vegetatively and by seed makes it a potentially invasive plant, especially if left unchecked.
• How to fix/prevent it: The best defense is a good offense. Plant Tansy in a contained area, such as a raised bed or surrounded by a physical barrier that extends at least 12 inches into the ground. Regularly deadhead the flowers to prevent seed production. If it starts to spread beyond its intended area, dig up and remove the unwanted plants, being sure to get as much of the root system as possible.

Aphids

• What it looks like: Clusters of tiny soft-bodied insects (green, black, or white) on stems and leaf undersides. Sticky honeydew residue. Curled or distorted new growth.
• What causes it: Rapid-reproducing sap-sucking insects. Populations explode in warm weather. Ant colonies may farm them for honeydew.
• How to fix/prevent it: A strong spray of water can knock aphids off the plants. Encourage natural predators like ladybugs and lacewings. For heavy infestations, use insecticidal soap or neem oil. Avoid over-fertilizing with nitrogen, as this attracts aphids.

Mountain West Specific Challenges: Our moderate-at-altitude heat and low humidity generally favor Tansy, reducing the risk of fungal diseases. However, the intense sun can stress the plants if they are not adequately watered.

Tansy can be a useful companion plant due to its insect-repelling properties. It's known to deter certain pests from other plants nearby.

Good companions include potatoes, squash, and cucumbers, as Tansy can help repel potato beetles, squash bugs, and cucumber beetles. It's also said to benefit raspberries by deterring raspberry beetles. Avoid planting Tansy near brassicas (cabbage, broccoli, cauliflower) as it may attract cabbage moths.