GARDENING WITH NATURE

Article by Sigrie Kendrick

Leave only healthy leaves

Many environmentally-conscious homeowners have embraced the practice of leaving fallen leaves in place each fall, while others still shudder at the perceived mess.

The benefits are well-documented as leaves provide valuable habitat for beneficial insects, enrich soil as they decompose, and reduce the volume of waste going to landfills.

However, there are dangers in leaving all leaves to overwinter in place. Instead, there are instances when this eco-friendly practice should be abandoned in favour of thorough leaf removal. Understanding when to rake isn’t just about aesthetics but about protecting the health of your landscape.

When Disease Changes the “Leave the Leaves” Rule

On the other hand, the most compelling reason to remove fallen leaves is the presence of foliar diseases. Many common tree and plant diseases overwinter in fallen foliage, waiting to reinfect plants when spring arrives.

Apple scab,(Venturia inaequalis), one of the most troublesome diseases affecting apple and crabapple trees, occurs in the Okanagan in years with higher than normal rainfall.

Spores, known as ascospores, survive winter in infected leaves on the ground. Come spring, these spores are released during wet weather and carried by wind to newly-emerging leaves, perpetuating the cycle.

If your apple or crabapple trees showed signs of dark, scabby lesions on leaves and fruit during the growing season, removing and destroying those fallen leaves is essential for disease management.

Black spot,(Diplocarpon rosae) the bane of rose gardeners everywhere, follows a similar pattern.

This fungal disease causes distinctive black spots surrounded by yellowing tissue on rose leaves. The pathogen overwinters in fallen leaves and infected canes, ready to splash onto new foliage with spring rains.

Gardeners who’ve battled black spot fungus should never leave infected rose leaves on the ground; instead, these leaves should be bagged and discarded.

I have a client with a large rose garden who requests that I manually remove all leaves from her rose plants in order to prevent disease.

Anthracnose diseases affect numerous trees and shrubs, including London plane trees, dogwoods, maples, and oaks

Anthracnose damage can be easily seen each year in the boulevard plantings of London plane trees around Kelowna. These fungal pathogens cause irregular dead patches on leaves and can lead to severe defoliation and twig dieback. Infected leaves and twigs harbour the fungus through winter, and leaving them in place ensures reinfection the following season.

If your trees displayed the characteristic brown, dead patches along leaf veins, removal of fallen foliage should be a priority.

Practical Reasons Not to Leave the Leaves Everywhere

Beyond disease concerns, there are practical situations when leaf removal is simply necessary.

• Leaves left on hardscape areas can present a slipping or tripping hazard, and they can leave stains on concrete.
• Thick layers of leaves left on lawns will smother grass, creating dead patches that require reseeding come spring. While a light covering can be mulched with a mower and left to decompose, heavy accumulations must be removed or relocated to garden beds where they won’t suffocate desirable plants.
• Leaves that collect in gutters, storm drains, and against foundation walls should always be removed. Clogged gutters can lead to water damage, ice dams, and costly repairs.
• Leaves packed against foundations stay wet and can contribute to moisture problems and provide hiding places for pests seeking winter shelter.

A Balanced Approach

When removal is necessary, proper disposal matters.

Infected leaves should never be composted in home systems, as typical home compost piles don’t reach temperatures high enough to kill all pathogens.

Instead, bag them for municipal collection, bury them deeply away from susceptible plants, or burn them where local regulations permit.

The key to making informed decisions about fall leaf management is observation.

Take note of any disease problems during the growing season.

Photograph symptoms if you’re unsure about identification, and consult with your local Master Gardener chapter or certified arborists when needed.

For trees and gardens that remain healthy throughout the year, leaving leaves in place offers many environmental benefits.

However, when disease is present, removal becomes an important cultural control practice.

Fall leaf management doesn’t have to be an all-or-nothing approach.

You can remove diseased leaves from problem areas while leaving healthy foliage in place elsewhere in your landscape.

This balanced approach protects plant health while still supporting beneficial wildlife and soil organisms.

By understanding when leaves should go and when they can stay, you’ll make better decisions for both your garden’s health and the environment.

The Okanagan Xeriscape Association is grateful for the ongoing financial support of the Okanagan Basin Water Board and is proud to be collaborating on the Make Water Work campaign. Check out the Make Water Work plant list at www.makewaterwork.ca

Sigrie Kendrick is a master gardener and executive-director of the non-profit Okanagan Xeriscape Association.

GARDENING WITH NATURE

Article by Sigrie Kendrick

Leave Seed Heads for a Winter Garden

In the world of gardening, we often focus on the fleeting glory of flowers with their vibrant colours and sweet fragrances that mark the peak of a plant’s ornamental appeal.

Yet there’s a quieter, more enduring beauty that emerges after the petals fall—one that extends the garden’s visual interest well beyond the traditional growing season.

Magical Seed Heads

The seed heads of xeric plants offer architectural interest, providing structure, texture, and movement that can transform a fading landscape from dormant to dynamic.

Among the most spectacular examples is Pulsatilla, commonly known as pasque flower or prairie crocus.

After its silky, bell-shaped blooms fade in early spring, this hardy perennial produces seed heads that are nothing short of magical. Each seed develops a long, feathery plume that catches light and moves with the slightest breeze, creating an ethereal display that can last for months.

These silvery, gossamer structures rise above the plant’s finely divided foliage like wisps of smoke. In late afternoon light, a patch of Pulsatilla seed heads becomes luminous, transforming an ordinary garden corner into something special.

The ornamental value of these seed heads extends beyond their immediate beauty. They provide crucial interest during the lean dormant months when many gardens lack structure.

While other perennials retreat underground or become mere stubble or sticks, Pulsatilla’s seed heads create focal points that draw the eye. They’re particularly effective when planted in drifts, where their collective movement creates a sense of flow that animates the garden even on still days.

Pulsatilla seedhead left to add interest to the garden

Seed Heads Making a Bold Statement

Achillea millefolium seedheads in a winter garden

The Drama of Allium Seed Heads

Globe allium seedheads in a fall garden

Sigrie Kendrick is a master gardener and executive-director of the non-profit Okanagan Xeriscape Association.

Our Plant Database features more than 500 plants tested and ideally suited for the unique challenges of gardening in the Okanagan’s extreme climate, iincluding the plants in this article.

GARDENING WITH NATURE

Article by Sigrie Kendrick

Think outside the box when prepping for winter

As gardeners, let’s remember to switch up the traditional garden tasks and think in a different direction: perhaps a more environmentally friendly one, that also provides us with more winter garden interest.

For instance, fall is the time of year when ornamental grasses shine. Many are in bloom and at the peak of their beauty.

What poet within us wouldn’t be charmed by the gentle way their graceful seed heads sway in the breeze and dance on the long stalks they’ve been growing all season? Grasses really add movement to your garden—unless you chop them down prematurely.

I recently had the opportunity to consider the importance ornamental grasses hold in our gardens as I cut down literally hundreds of Calamagrostis x acutiflora ‘Karl Foerster’, commonly known as Feather Reed grass, at a client’s property.

So often clients want their grasses razed to the ground before the onset of winter as they
perceive the standing grasses to be “messy” rather than graceful.

Often, land care providers such as landscapers are all too happy to oblige as that means one less task facing them in the spring.

Instead of this perverse desire to tidy in the fall why not take into consideration all the benefits of leaving ornamental grasses over the winter?

What about ecology?

From an ecological standpoint there also are many reasons to leave your ornamental grasses and your perennials standing over the winter. They provide needed habitat for birds and a myriad of other wildlife, as well as for beneficial insects to overwinter.

The seed heads of ornamental grasses and also annuals and perennials which have gone to seed, provide food for birds, who have to forage widely during the colder months, just to survive. They also provide great erosion prevention and slope stability, especially where wildfire has run through the previous season.

The time to shear your ornamental grasses is when you begin to see new growth at the base sometime in spring. Then, don’t toss out the cut grass. Instead, find a spot in your yard where it will be out of your way, but will provide valuable habitat for beneficial insects.

Perennials can also be pruned in early spring, when new growth begins to be visible, while annuals can be pulled out as soon as the ground softens in late winter or early spring.

Remember too that the fallen leaves from deciduous trees should also be left where they fall, rather than being neatly raked up and composted elsewhere in the fall. Those rotting leaves are like gold to a gardener and they provide habitat for insects and wildlife while they decompose over winter.

They also suppress weed growth and protect the roots of perennials over winter and what’s left can be gently dug into the soil come spring.

So, leave the leaves, don’t mow down the grasses and allow perennials and annuals to go to seed (unless they tend to be invasive!)

GARDENING WITH NATURE

Extreme weather taking its toll on gardens

For instance, do not prune your plants in the heat.
Periods of intense heat are stressful for your plants, and pruning, especially thinning, will only serve to
increase this stress.

Removing leaf matter increases the effects of heat on the remaining vegetation, decreasing the humidity
and therefore forcing the remaining leaves to transpire more to cool the plant.

This often has disastrous results.

Another no-no in periods of extreme heat is fertilizing your plants.

Try to make sure your garden has the nutrients necessary for plant health prior to any spike in
temperature.

Adding fertilizer is almost akin to adding salt to your soil as fertilizer essentially makes it harder for your
plants to access the water in the soil. High concentrations of nutrients actually reverse osmosis, the process by which a plant is able to absorb water from the soil. The osmotic pressure is reversed so that the pressure outside the roots becomes greater than inside, making plants unable to access moisture from the soil and they actually lose water back into the surrounding ground.

Planting in the heat of summer is not ideal

Out of necessity, the landscape industry must continue to plant throughout the hottest summer weather but this is far from an ideal situation.

If at all possible delay your planting to the coolness of shorter days in late summer when the ambient air
temperature has decreased but the warm soil necessary for strong root establishment exists.

If you must plant in high heat, at least offer supplemental shade for new plantings by using shade cloth or, in a pinch, an old white sheet.

This shade is even more vital if you are planting in an area of ‘high albedo’. High albedo environments occur where there is a great deal of reflection such as found in a rock garden. This reflective sunlight will damage young plants that can not transpire enough in the high heat to cool their leaves and almost immediately begin exhibiting heat stress.

Unusual snowfall in early November 2022 landed on trees whos leaves had not had time to drop yet

At the other end of the spectrum, severe winter weather caused considerable havoc in Okanagan gardens, as well as in commercial orchards and vineyards.

The weather we have experienced over the last two years has had severe repercussions for our trees, shrubs, perennials, vines, and ornamental grasses.

In the Okanagan Xeriscape Association’s demonstration garden, we saw the complete loss of several Lavenders and Penstemon and extensive injury to several of our trees from the 2022/2023 winter. The Parrotia persica, the Robinia pseudoacacia ‘Twisty Baby’ and the Koelreuteria all have suffered winter damage, and the latter has had to be removed. It’s a heart-breaking loss.

I have heard other Master Gardeners around the valley sharing similar stories of witnessing first-hand deaths of these plants as well as Buddleia, Hybiscus syriacus and various ornamental grasses.

I know that the long warm fall followed by the extreme and abrupt cold experienced in the beginning of November was to blame for much of the damage, similar to your forgotten frozen water bottle exploding, but I wanted to know more. It was hard to ignore that many of our trees held their browned leaves through the winter, never having the chance to drop them as they normally would, with last fall’s sudden lurch from summer into winter.

I consulted with plant pathologist Robert Hogue of Pegasus Horticultural to gain a better understanding of the physiological process which had wreaked such havoc.

Robert explained that it all actually began with the Heat Dome of 2021 when plant tissue was damaged by the extreme heat, registering into the mid-40s.

Like people, plants go into a low-energy state in extreme heat. In survival mode plants do not process as much carbon dioxide, leading to less carbohydrates moving into the roots. This lack of carbon dioxide means the roots receive less sugars which compromises the ability of the roots to absorb the necessary water and nutrients from the soil.

This in turn leads to root death and with the death of large woody roots comes the death of the organism as a whole. This root death also occurred last fall with the abrupt arrival of winter when the roots were still actively growing.

As Robert explained, ‘The abrupt cessation of metabolic activity in the above-ground plant parts meant there was not enough nutrient flow to the roots to complete the suberization process’.

Suberization is the process by which the root walls harden off into corky tissue similar to a callus over a wound. These weakened feeder roots act as an entry for many root pathogens such as Fusarium, Pythium, and Verticillium.

The damage can be immediate, as seen this spring when the plant abruptly died after a seemingly normal leaf-out; but in other cases there will be a slow but inevitable death.

Robert has done extensive research on the hardiness of plant life in the valley and has come to the conclusion that we simply can not rely on traditional plant hardiness ratings in our changing climate.

Gardening with Nature

Article by Sigrie Kendrick

Leave the leaves!

Don’t stress over fall garden clean-up.  Just leave the leaves on the ground where they fall.

I have to question why, as gardeners, we are so obsessed with the big fall clean-up, including raking or blowing all the fallen leaves into piles, shoveling them into plastic bags and sending them off to the landfill—or even to be composted centrally.

It’s a tradition we really should reconsider.

In fact, this is counter-productive behaviour. Instead, we would like to educate everyone about the many benefits of leaving the leaves where they fall in the fall.

Let us mimic what occurs naturally, in contrast to our need to clean, control and manipulate our natural environment.

There’s a reason for everything in nature.

Leaves are nature’s nutrient recyclers.

For example, with the shorter days of light in the fall, the leaves of deciduous trees and shrubs fall to the ground and act as a mulch, which suppresses weeds, protects the roots of perennials from the winter cold and feeds the soil as they decay.

Their decomposition creates rich organic matter in the soil that helps your garden become lush and beautiful.

Leaving your tree, shrub, and perennial ‘mess’ in place over winter also provides a source of food and sanctuary for a whole host of small mammals, birds, and insects.

Sigrie Kendrick is a Master Gardener and Executive-Director of the Okanagan Xeriscape Association. She can be reached at 778-363-8360 or by email at exec_dir@okanaganxeriscape.org.