Typing Kelowna into the website gives you a hardiness of Zone 7a for the most recent period of evaluation (1981-2010). For Armstrong you get a hardiness of 6b. Zone 6b is half a zone colder than 7a. Note, however, that this calculation was done for a period from more than ten years ago and climate variability has become much worse, keep reading.

If you look up a Purple Ice Plant on the OXA website, it tells you that it is hardy to Zone 6. Normally then it should survive winter in any area rated as Zone 6 or higher. Also remember that you can often increase your effective hardiness zone by covering plants with mulch or snow or planting them in an area that is sheltered from the cold like a calm area beside a house.

Complications

Global Warming provides the first complication. We have all experienced generally warmer winters in Canada over the past decades. Okanagan Lake has not frozen over since 1969, whereas it used to freeze over more commonly as in both 1950 and 1949. The Government of Canada website mentioned above compares the period of 1961-1990 to the period 1981-2010 and the zones are all higher on the website for the later date, usually half a zone to a full zone. Note that the Government of Canada is scheduled to come out with an updated plant hardiness zone map later this year.

Climate Variability adds a bigger complication. As our climate warms on average, it is also becoming more variable. This means that we are more likely now to have occasional extreme cold snaps. The winter of 2022/23 is a good example of this pattern. At the Kelowna airport we had 2 days at Zone 5 temperatures and 2 days at Zone 4! This resulted in an unusual amount of winter kill for our plants in the Okanagan. Looking at the last 12 years in Figure 2 you can see that these extreme low temperatures are becoming more common. In fact, we just had another extreme cold snap of -30°C in January 2024. It is beginning to look as though we should subtract one to two zones off the maps and lookup tables presented at the beginning of this article.

Calculation Methods provide another complication, but it is relatively minor. There are two common calculation methods for hardiness zones. One comes for the USDA and it is based simply on the lowest temperature experienced in a given area averaged over 30 years. The Government of Canada uses a more refined and complex formula which incorporates six other winter weather variables such as snow cover in addition to the lowest temperature averaged over 20 years. Historically the two methods produce similar results in the Okanagan. You can view a map calculated using the USDA method at this website: USDA Zones. This map probably uses data from the period 1978-2008 and there is a lack of topographic detail but it is quite close to the Canadian version. Any differences attributable to calculation method likely be minor compared to the variability from climate change and the time period used for averaging.

Because the landscape industry in the US is so much larger than Canada’s, you can safely assume that any hardiness zone given on a website, or a plant tag is almost certainly a USDA zone. Gardeners in the Okanagan can use the two methods interchangeably.

Microclimates provide the final complication. These are local variations in plant hardiness zones that are generally related to local variations in elevation but can also be caused by the moderating effect of a nearby large body of water. Looking at the zone map you can clearly see the effect of regional variations in topography as the Okanagan is a narrow valley surrounded by high hills and mountains. The higher elevations around our valley have the much lower zone rating of 3a to 3b. At a more local scale we can see an example of a microclimate in the Kelowna area where the airport is about a half to one zone colder than central Kelowna. This seems to be related to both the slightly higher elevation at the airport and the higher elevations that surround the airport. These elevation changes have created channels where the cold air sinks down from the higher elevations.

Summary

Plant hardiness is a relatively simple and important concept for Okanagan gardens, but with climate change it is difficult to predict. Gardeners should take this uncertainty into account in their planting plans.

If you want to try out an interesting perennial rated close to your maximum zone, then it might be worth the risk. On the other hand, if you are planting a tree or hedge that you want in place for a long time, it is better to choose a species two or three zones colder than your maximum.

To avoid disappointment give careful consideration to plant hardiness when selecting plants for your garden.

Gardening with Nature

Article by Sigrie Kendrick

Perennials are better planted in fall, not spring

We humans are certainly creatures of habit and often continue our habits without questioning why. This
includes seasonal garden tasks.

Why, for instance, have we traditionally tried with almost maniacal zeal to plant all of our trees, shrubs,
vines, perennials and annuals in the spring?

Of course, annuals must be planted in the spring as by nature they only live for one season and are not
winter-hardy, but what about all the other plants we attempt to cram into our gardens before the heat hits?

Fall is a much better time for planting perennials.

Your garden soil in autumn retains the warmth of the summer yet the ambient temperature is cooler
thereby allowing your plants to focus on establishing the good healthy root system necessary for a
thriving garden.

Fall plantings will be better able to establish a healthy root system and then slowly move into the
dormancy of winter, emerging the following spring better prepared for future ‘heat domes’.

Our spring-installed plants are rudely met with hot, dry, and increasingly windy Okanagan summers
before having the time to establish a healthy root system.

Fall planting allows you to work with nature and use the natural autumn and winter precipitation to your
advantage rather than running around with hoses and buckets trying to keep new garden additions
watered in the summer heat.

Nepeta Racemosa, Catmint

OXA’s First Fall Plant Sale

One of the challenges of fall planting originates within the horticultural community as it has traditionally
focused on spring planting and has ordered plants to meet the needs of gardeners starved of the chance
to get their fingers in the soil.

To welcome autumn planting, the Okanagan Xeriscape Association is holding its first fall plant sale this
year, and we hope to encourage others to embrace the idea of putting in perennial plants in the fall.
Many of the plants featured in our plant database will be available for purchase at the UnH2O Xeriscape Demonstration Garden, 4075 Gordon Dr., Kelowna on Saturday, October 1st from 10 am to 2 pm.

Many of the stars of the fall garden will be included in our sale, including ornamental grasses, sedums, and asters.

For instance, Schizachrium scoparium ‘Blue Heaven’ which has been chosen as plant of the year by the Perennial Plant Association. This ornamental grass features steel-blue foliage during the summer that slowly transitions to purple, red and orange shades and is extremely forgiving of poor soil and drought conditions.

Also available will be Panicum virgatum ‘Blood Brothers’ which has been featured on our website as Plant of the Month. As a lover of red plants, I am absolutely enthralled with its foliage which transitions to a deep wine colour as the temperature dips.

It has been said that “nature abhors a vacuum” and if you have a sedum somewhere in your garden, you won’t have a weed in its place. We will have a variety of sedums available, both tall—such as the cultivars ‘Autumn Joy’ and ‘Matrona’—and ground covers, such as Sedum rupestre ‘Angelina’ and ‘Blue spruce,’ as well as ‘Dragon’s Blood’.

All of these sedums attract pollinators and the tall sedums are fall bloomers and good choices for extending interest in the garden—for people and insects alike.  Both tall sedums and ground cover sedums are excellent choices for sunny, dry areas of your garden and are especially well-suited for planting in rock gardens.

Asters serve as some of the brightest colour in the fall garden and none are better, in my opinion than Aster frikartii ‘Monch’. This Aster starts blooming in July and continues its delightful pale purple blossoms through to fall without the usual need for time-consuming deadheading.

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.

Article and Photos by Mark Godlewski

The following is largely put together from information provided by Jim Brockmeyer and Muriel Neale as part of their Bluestem Nursery webpage. Both have now retired, but their excellent website lives on with funding for the hosting provided by OXA.

Trimming Cool Season Grasses

Normally you want to trim your cool season grasses in early spring. During the winter, depending on the species and the weather, grasses can create a great deal of interest in your garden. However, you will want to trim all your cool season grasses down to about 3-4 inches above the crown before they start growing in the spring. If you are late and there is significant new growth your best bet is cut down only two thirds of the grass. For thin grasses you may want to use hedge trimmers. Otherwise, a coarsely-serrated knife is the best tool for this. You may be able to pick up an old bread knife from your thrift store. It is also a good idea to wear gloves and long sleeves. Some of the grasses can give you a nasty cut if they slide along your skin.

Fig. 1 – Cutting Tools

A tip regarding cutting back larger grass clumps is to tie one or two bungee cords around it before you start to cut. You want to keep all the stalks and leaves in a tight bundle. This makes it much easier to get rid of the mess later….and a mess there will certainly be if you don’t!

Trimming Warm Season Grasses

There is no great rush to prune these grasses in the spring depending on how they look in your garden. You can wait until late spring. Some warm season grasses such as Miscanthus or Saccharum have very thick stems. Here you will need pruning shears, or a pruning saw to cut them off 4-6 inches above the crown.

Fig. 2  – Warm Seson Grasses in Winter

Trimming Evergreen Grasses

Some Fescues and Blue Oat Grass Helictotrichon fall into the evergreen (everblue?) category. A few people, especially those at landscape maintenance companies, like to cut these grasses back early but that is not the best for the health and shape of the plant. Blue Oat grass responds well to being combed (with a gloved hand) in the fall and maybe again in the spring. The dead leaves and flowers will come loose relatively easily. Some of the tan-coloured leaves will remain until the next season. Some other evergreen grasses and Carex experience a fair bit of dieback in our winters. You can shear back the dead leaf ends in the spring but be careful not to cut back too much into the living part of the plant.

Fig. 3  – A nice variety of grasses in the late summer

Take care to trim your ornamental grasses appropriately to achieve the best long term results.

An excellent reference for getting the maximum performance from your plants is “The Well-Tended Perennial Garden” by Tracy DiSabato-Aust.

Varieties

Companion Plants

Great companion plants for lavender include sedums, especially the tall, late-blooming varieties such as ’Autumn Joy’ and ‘Matrona’.

Deep-yellow flowers of Achillea ‘Moonshine’ yarrow or Coreopsis ‘Golden Gain’ provide a good colour contrast.

Silvery-grey leaves of blue oat grass provide a good foliage texture contrast.

To find more plants that like the same conditions, check the OXA Plant Database. Select ‘dry’ and ‘full sun’.

To learn more about lavender, and see many varieties that thrive here, visit the Okanagan Lavender Herb Farm. They have a beautiful demonstration garden, serve tea, teach lavender crafts, and have a delightful shop.

Check okanaganlavender.com for information and special events.

Watering Ornamental Grasses

Ornamental grasses, be they plugs or field-grown clumps, need to be well watered their first year. The soil was fluffed up when you dug the hole, so it will dry out faster. The plant’s roots are not well-established and some of the roots were lost when the plant was dug up. This means that there aren’t quite as many roots to take care of the top growth as the plant had planned. Readily accessible moisture makes the job of the roots a whole lot easier.

Note: Mulch can be very useful the first year. However, do not mulch right to the crown of the plant. This can cause the crown to rot because of the constant moisture.

But the next year is a completely different story. Their root systems should be well established, meaning they have gone deep in search of water and nutrients. They can take care of themselves for fairly lengthy periods of time, especially as they get older. If you have a layer of mulch, they can fend for themselves for longer. However, excessive watering will cause them to flop. Have you ever grown Achillea (Yarrow) in rich moist soil? Same problem. These plants want to live a spartan existence.

How Deep to Plant

It is very important to plant your ornamental grasses so that the crown of the plant is level with the soil’s surface. Some grasses, in particular Helictotrichon and Pennisetum, are very sensitive to being planted too deep. The crown rots and the plant dies.

First Watering
I expect you already know, but I would be negligent if I didn’t mention that the plugs or the field clumps must be thoroughly watered after transplanting. A good soaking gets rid of any air pockets that might have formed. Those air pockets cause the roots to dry out, even though the roots are under the ground.