Ask the Master Gardener: Keeping slugs from riddling your plants

Slugs thrive in rainy weather, well-watered gardens and moist, shady areas.

Holes caused by slugs are visible on this Pulmonaria plant. Photo by Jennifer Knutson

Dear Master Gardener: The pulmonaria and hostas in my shady perennial beds have holes in them. What is causing it?

Answer: It sounds like slugs are damaging your plants. Slugs are slimy, soft-bodied creatures that look like shell-less snails. They thrive in rainy weather, well-watered and well-mulched gardens. You often find them in moist, shady areas. Slugs feed on the leaves of many plants at night and hide during the day. If you have mulch in your garden, try to keep it less than 3 inches thick and avoid large wood chips. Keep mulch at least 3 inches away from the crown of the plants. Water your garden in the morning and only when it is necessary.

Here are the recommended methods of ridding your gardens of slugs.

  • Set out flat boards, shingles, or damp newspapers in the evening, then check under them the next morning and kill any slugs that are hiding. Repeat this nightly until the slugs are no longer present. You can kill slugs by dropping them into a bowl of soapy water.

  • Sink shallow containers of beer in the ground with the rim even with the ground. The slugs are attracted to the beer, fall in, and drown.

  • Diatomaceous earth (tiny fossilized skeletons of ancient aquatic diatoms) sprinkled around plants will scratch slugs and cause them to dry up and die. It is most effective in dry conditions and needs to be repeated whenever it gets wet.

  • There are commercial products such as Sluggo (iron phosphate bait granules) that are applied to the soil, and metaldehyde baits, such as Deadline or Defender. Iron phosphate is the less toxic choice and safe to use around children and pets. Always follow the directions to the pesticide you are using โ€“ the label is the law.

Dear Master Gardener: The foliage at the top of the tree canopy of one part of my white birch clump has started to die back and is working its way down the tree. What is causing it and how can I save the birch clump?

Answer: It is probably the bronze birch borer, a beetle that feeds on birch. However, there can be other problems that cause canopy dieback too. Stressed trees are more susceptible to attacks by these beetles. Trees become stressed from sustained drought, prolonged defoliation, or physical damage to roots and trunks from lawn mower injury or construction damage. Birch typically do not do well grown in open locations where roots are exposed to heat and drying. You can minimize stress by adding organic mulch to keep soil temperatures cooler and slow the rate of moisture evaporation, as birch trees have a shallow root system. Itโ€™s important to keep trees well-watered and avoid fertilizing stressed trees.


Adult beetles that feed on the leaves of trees do not affect tree health, but the larvae create destructive galleries under the bark that disrupt the transport of water and nutrients. In addition to dieback starting at the top of the tree, look for wilting or yellowing foliage, and sawdust packed S-shaped galleries, which create raised ridges in the thin bark. You may also see 1/8-inch D-shaped exit holes on trunks and branches, but they can be hard to detect.

Treating trees with pesticides to kill borers is only effective if the tree is in the initial stages of decline and dieback. There are two systemic pesticides for treating bronze birch borer: imidacloprid is applied as a liquid drench to the soil around the trunk of the tree and dinotefuron is applied as granules to the soil directly around the tree. You may want to hire a certified arborist to accurately diagnose the problem. Professional applicators can apply imidacloprid as a soil or trunk injection or dinotefuron as a bark spray, soil drench or soil injection. Be aware that plants located near soil treated with a systemic pesticide will also uptake the chemical, endangering bees and other pollinators.

Dear Master Gardener: I was at the garden center and impulsively bought three cheerful looking Iceland poppy plants. Are they perennials or annuals? How long do they bloom?

Iceland Poppy.JPG
An Iceland poppy. Photo by Jennifer Knutson

Answer: Iceland poppies make an easy-care, colorful addition to the garden. Their tissue paper-like blooms are exquisite and they make gorgeous cut flowers. When cutting them for a vase, pick them just as the buds begin to open. They are technically perennials and hardy to zone 3, but can be short-lived. Luckily, they self-seed and typically come back year after year, so allow the plants to set seed toward the end of the bloom season. Plant them in a sunny area and keep them well-watered until they become established. Once established they are quite drought tolerant. Removing faded flowers will encourage more buds to form.

End of June Gardening Tips

  • Protect your apples by the end of June from apple maggots. One option is to enclose each marble-sized apple in a plastic sandwich bag with a zipper closure. Snip the bottom corners off each bag with a scissors so condensation can drip out. Leave the bag on until harvest. Another option is to put up apple maggot traps, which are red spheres coated with Tanglefoot (a super-sticky substance). You will need one or two spheres for a small tree and five or more for larger trees.

  • Early vegetables such as English peas, radishes, spinach, and leaf lettuce can be replaced with broccoli, cabbage, and cauliflower to ripen in the fall. Be sure to add fertilizer when you replant.

  • As temperatures rise, allow your lawn to grow taller before mowing it. Taller grass blades help shelter the crowns from heat and wind, protecting them from excessive drying. Also, there is evidence that roots grow deeper when grass is taller. Aim for a lawn that is 2 ยฝ to 3 ยฝ inches tall. Allow clippings to fall back into the lawn where they will break down rapidly and recycle nutrients.

  • If you grow raspberries, strawberries, and/or blueberries, hang traps to monitor for the Spotted Wing Drosophila (SWD). Adult SWD flies can be trapped by taking a 32-ounce plastic cup and poking several 3/16-inch holes near the top of the cup. Leave a three to four-inch section without holes to hold the bait. Pour one inch of apple cider vinegar into the trap as bait. Place a small yellow sticky card inside (found in garden supply stores). If you canโ€™t find sticky cards, add a drop of unscented dish soap to the vinegar. Hang traps from branches in the shade near your fruit and begin monitoring before the fruit begins to ripen. Replace the sticky card and vinegar bait at least once a week and dispose of the old vinegar away from the trap location. The most effective organic pesticide option for SWD at this time is Spinosad.

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at and I will answer you in the column if space allows.
University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.

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