Getting Ahead of Moisture-Related Issues in the Garden in Edmontons Zone 3/4
Joe Gadbois - Published July 8, 2026
Most plants love when they get consistent moisture. As it turns out though, you can have too much of a good thing. With rainfall well above average so far in Alberta this year, I thought I would compile some issues you are likely to encounter in the garden this season as a result of excessive moisture.
Slugs
Slugs tend to appear in the summer, once soil temperatures have warmed up, but they love moisture and are far more prevalent in wet years vs. dry ones. Because they don’t like hot weather, they normally come out and feed at night, but you will see them active during the day when it’s rainy. Any soft or fleshy foliage near the ground is susceptible to slug damage, and hostas are a particular favourite.
What to look for: Young slugs chew and rasp the foliage, and the damage is typically all over the leaf as opposed to starting on the edges. Rasping is the stripping of surface tissue on the leaf so that translucent “windows” are visible, so when young slugs feed, you will see a combination of these “windows” and small holes on the leaves. Adult slugs will chew holes all over the leaves. Rare severe infestations of slugs can defoliate plants; usually, the damage is cosmetic and plants will recover.
How to control it: The most effective control for slugs in-season is slug bait. Greenland carries two different products: Meta (active ingredient metaldehyde) is the most effective, but it is toxic to pets and should not be used if you have pets or children digging around the garden. Safer’s Slug Bait (active ingredient iron) is not toxic to pets and will more readily break down. It’s still an effective product that I recommend. Both baits are simply sprinkled around susceptible plants and will bait and poison the slugs. You will need to reapply every 2 weeks or as the bait is eaten or disintegrates.
To help reduce populations in subsequent years, turn your soil over in the fall to expose eggs. The eggs resemble tiny pearls; if you expose them to the elements, they may be eaten by wildlife or desiccate. Also keep in mind that spaces underneath containers, statuary, etc. placed on soil are favourite places for slugs to hide and lay their eggs.
Fungal Diseases
All fungal diseases are more prevalent in moist conditions because they need persistent moisture to reproduce. The key to controlling all fungal diseases is prevention. Plant disease resistant varieties whenever possible. Ensure plants are spaced properly to maximize air circulation. Prune trees & shrubs properly to maximize airflow through the canopy. When watering, avoid watering overhead or in the evening; this is to allow foliage to dry off faster and prevent possible infection. Tools used to remove infected plant parts are best disinfected with bleach or Lysol afterward. Finally, any plant material affected by disease should be cleaned up in the fall (do not compost), otherwise the disease can overwinter and re-infect the following year.
Here are some common fungal diseases we experience in the Edmonton area:
Powdery Mildew:
Affects many types of annuals, vegetables, perennials, shrubs, and trees. Some of the most common hosts are beebalm, columbine, cosmos, cucumber, dahlia, delphinium, ninebark, peony, rose, speedwell, spirea, squash (any type), summer phlox, zinnia, and sometimes even on lawns.
What to look for: As the name suggests, a white, powdery coating will develop on leaves and stems of affected plants. It starts as faint blotching and progresses to gradually cover the entire leaf or stem surface. Eventually, affected growth will likely die off if not removed.
How to control it: Sulphur-based fungicide (such as garden sulphur or Safer’s Defender) can be sprayed on the foliage every 7-10 days, starting in mid-June, as a preventative. Affected parts of the plant can also be removed.
Rust:
A more specific disease that bounces between two or more different hosts. Some of the more commonly affected plants include coralbells, daylily, hawthorn, hollyhock, juniper, mallow, mountain ash, rose, saskatoon, speedwell, and willow.
What to look for: Will often appear different depending on the host. For example, juniper-hawthorn rust appears relatively early in the season (June) on junipers as a gall that oozes gelatinous orange “tentacles”; later in summer (July-Aug) on hawthorns, it manifests as orange spots on leaves that have tiny pale “tentacles” underneath. On most plants, rust appears as orange spots or blisters on the leaves or stems (hence the name).
How to control it: Copper-based fungicide (such as Doktor Doom Fungicide Concentrate) can be sprayed as a preventative on susceptible plants every 7-10 days starting in mid-June. Affected parts of the plant can also be removed; on junipers, be sure to prune out galls.
Botrytis:
This disease affects only certain plants but can be very damaging. The main hosts are dahlia, lily (especially martagons), peony, and strawberry.
What to look for: On most plants (especially peonies), the disease commonly shows up as dark blackish blotches on leaves, blasted flower buds, and as a stem rot. On peonies, entire stems often wilt from being girdled at the base by the rot. In some cases, rotten areas may be covered in a fuzzy gray mold. Strawberry fruits affected by botrytis will be covered in this gray mold.
How to control it: It’s best to remove affected parts of the plant as soon as the disease is noticed. Copper-based fungicide can also be applied preventatively every 7-10 days starting in early June.
Black Knot:
A common disease on mayday and chokecherry that appears every year but will spread even more in wet years.
What to look for: Branches develop a canker that is often described as “poop on a stick”.
How to control it: Prune out affected branches as soon as cankers are noticed. The cut should be at least 12” down from the canker, and pruners should be disinfected with bleach between cuts. Burn, bury, or dispose of the material in the garbage; do not leave it laying around the yard. Once it develops on the trunk, it’s best to cut the tree down.
Root & Crown Rots:
Can affect essentially any type of plant because of soil being too wet for too long.
What to look for: Plants will commonly turn a sickly yellowish colour and/or wilt; often, it looks like the plant is too dry but in fact is not getting moisture because the roots or crown are rotting.
How to control it: Once the disease settles in, it often can’t be treated. When symptoms are first noticed or for plants that don’t like wet feet, pull back mulch and especially landscape fabric to allow the soil to breathe and dry out faster. Landscape fabric should always be kept outside the dripline of any plants in your garden if it is used at all. If only part of the plant has rotted, it may be possible to dig the plant out, remove the rotten part, and re-plant. Consider moving the plant to a different location with better drainage if all else fails. Replace heavy clay soil with lighter soil and amend with organic matter (such as compost or Sea Soil) to improve soil structure and help prevent issues with rot during wet weather; in low-lying areas, raise beds up above the surrounding grade to allow them to drain better.
Fire Blight
This bacterial disease affects mainly edible and ornamental fruit trees/shrubs of the rose family (see below) and is most prevalent in warm, humid weather. In these conditions, it is best to avoid pruning susceptible trees as that increases the risk of exposure.
Susceptible plants include apples & crabapples (including flowering crabapple), chokecherry, hawthorn, mountain ash, pear, and saskatoon.
What to look for: Young branch tips wilt and develop a “scorched” appearance. Later, cankers form that exude an amber coloured liquid.
How to control it: Prune affected branches out at least 12” below the affected area as soon as noticed; disinfect pruners with bleach between cuts. Bury, burn, or dispose of the material in the garbage; do not leave it laying around the yard.
Mushrooms
Mushrooms are the sexual reproductive structures (termed “fruiting bodies”) of some types of fungi. Because fungi generally love wet weather, mushrooms are more likely to be seen in these conditions. They can occur anywhere moisture is abundant and where there is decaying organic matter for the fungi to feed on. Mushrooms are usually harmless to plants, but they can be an eyesore, especially when they appear in lawns. Many mushrooms are toxic if ingested, so never eat mushrooms you find without first having them identified by a mycologist or mushroom expert that you trust. The Alberta Mycological Society is a great resource.
How to control it: Remove mushrooms as soon as you see them to prevent spores from spreading. Turning over mulch or soil where mushrooms are found to allow the area to dry out will help to kill the fungus. For fairy rings, which are distinctive rings of mushrooms that appear in lawns (along with discolouration of the lawn inside the ring), aerate the area with an aerator or pitchfork. This helps dry out the mycelia (fungal “roots”) growing in and under the sod. Often, when the weather dries the fairy ring will disappear on its own. If the fairy ring persists in dry weather, soapy water can be soaked into the area to help allow water and fertilizer to penetrate the lawn and improve its overall health. In severe cases, it may be necessary to dig out the affected turf and soil underneath to a depth of 10 in. (25cm) and replace with fresh soil and new sod or seed. It’s best to disinfect any aerators, shovels, and other tools after treating fairy ring. Keeping your lawn healthy with aeration once or twice per year and spring, summer, and fall fertilizer applications is the best way to prevent fairy ring.
Mosquitoes
It’s well-known that mosquitoes love wet weather, as they breed in still water and they love to hide in grass and plants where there is moisture and protection from the elements.
How to control them: Start by reducing or eliminating still water so they don’t have places to breed in your yard. Install screens on rainbarrels and agitation for ponds and water features.
Mosquito Dunks are a biological control (not a chemical) that can be added to still water which will kill mosquito larvae but are harmless to wildlife and the environment.
Garlic repellant sprays (such as the one by Doktor Doom) are a natural way to repel mosquitoes by spraying on lawns, plants, and outdoor structures. It will leave a temporary garlic odour that dissipates after a few hours. If you are looking for mosquito repellants to apply on skin, Greenland carries Jungle Juice by Doktor Doom which is deet-free but still very effective.