Colorado Gardening Calendar for December 2023

By Molly Gaines, CSU Extension-Denver Master Gardener since 2019

Grasses left uncut provide great visual appeal throughout the winter months. Photo: Molly Gaines

A few snowfalls have already blanketed Denver, making old man winter feel official. This, and the fact that the holiday season is upon us, probably means garden to-dos are not top-of-mind for you. Yet, there are still some important tasks to consider, along with a handful of indoor activities, as we head into these darker days and colder months.

December’s Checklist 

Trees, Shrubs and Perennials 

• When there is no snow cover to provide trees, shrubs and perennials moisture, it’s imperative to their short- and long-term health to water them one to two times monthly. Here’s a great fact sheet from CSU with helpful guidelines for fall and winter watering.
• If you haven’t wrapped the tree trunks of newly planted trees, don’t forget this critical task. They need it for the first few years, until their bark develops texture. For more on this, visit this helpful article from North Dakota State University’s extension office. 
• When the snow falls heavily, shake snow from bent tree limbs and branches. This will avoid breakage. As needed, lightly prune broken limbs to avoid further damage.
• Add mulch to spots where it’s thin in your garden beds. This will help plants retain moisture and protect them from harsh winter winds and freeze/thaw cycles.

Sunflower still standing. Photo: Molly Gaines

Compost

• Yard waste, such as broken branches, and other remaining garden debris can be placed into your city compost bin. If you have a backyard compost bin or pile, continue adding brown and green materials, breaking larger debris into smaller pieces. These items will continue decomposing in the winter months, albeit at a slower pace. In Colorado, it’s best not to turn compost after December, according to this PlantTalk Colorado fact sheet on composting. Turning allows valuable heat to escape, and may stop processing in cold weather. To restart active processing, turn and mix in fresh materials with the winter compost. 

House Plants 

• Winter is the dormant season for non-blooming indoor plants. Reduce watering, stop fertilizing, and keep them away from drafts for the next few months.
• Increase humidity around your plants. Ignore misting with a spray bottle; for this to be effective, you’d need to mist for hours on end. Instead, group plants together on a pebble-lined tray and add water to just below the top of the pebbles.
• Check plants regularly for pests such as mealy bugs and spider mites. If present, treat and quarantine the infected plant. Find remedies here.  

Amaryllis adds a festive flair in December into late January. This trio should produce showy red blooms. Photo: Molly Gaines

Paperwhites take 4-6 weeks to bloom. Start them now for blooms in the new year. Photo: Molly Gaines

Speaking of house plants, ’tis the season for live Christmas trees and holiday plants. Here are some links to keep all of these at their best: Keeping the Ho Ho Ho in Holiday Plants, Tips for Caring for Your Christmas Tree, A Year in the Life of an Amaryllis, and Christmas Cactus Care. 

Holiday plants you may wish to consider for your home, or as a gift to others, include Christmas cactus, poinsettias, amaryllis or even paper whites. I’ve made it a tradition to send my mom a set of amaryllis (pictured above) each December so she can watch it grow through the holidays. Christmas cactus, which can be passed down for many years, can live to be 100 years old! They bloom in the winter and despite being cacti, need regular watering. Poinsettias add a beautiful pop of red, white or pink during the holidays and beyond. Their large size can make them a dramatic focal point of a room. 

Thank you for reading our blog. We’ll be back with one more post in December before wrapping up 2023. In the meantime, we wish you a beautiful, joyous start to your holiday season!

 As always, CSU is available in every season with gardening advice at the CSU Extension Yard and Garden website.

Outside to Inside – Hardening Off Houseplants

By Lori Williams, CSU Extension-Denver Master Gardener since 2016

Bringing houseplants in from gardens is all about timing. Reverse planning from outside to inside starts with your area’s average first frost date. Denver’s average FFD is roughly September 15^th! Colorado summers can have a long wind down or be very abrupt, either way the goal is to avoid shocking houseplants or worse, letting them suffer damage or freeze due to temps they can’t withstand. It’s best if plants have time to “harden off”, meaning they adapt from warm days to the interior temp of your home, with less cold night exposure.

A wonderful friend and esteemed gardener I know diligently lines up houseplants for thorough garden hose spray downs for 2-3 weeks, horticultural oil applied the next week, all with time allowed for sitting and drying or airing out. Then and only then are plants welcomed back inside the house. 

I respect the time management, scheduling expertise, and TLC regimen and strive for such skills and discipline! However, down to my toes I know I am amazingly less organized and reactionary in my garden.  Last night’s a great example: windows wide open I woke to grab a quilt at 3am, the fresh air was so (deliciously) chilly! This morning my first thought: get the houseplants ready to come inside before it’s too cold at night. Waa laa – the planning committee just hit town!

I use the following steps, completed in a production line, that can usually be completed in a couple of hours. This year, with 17 houseplants outside, in a range of sizes, I got them processed in about 2 hours, including time when plants were soaking or draining. Big plants can need 2 people to move, but overall, it’s a manageable job solo.

Here’s how to get your plants looking sharp and bug-free:

1. Watch weather forecasts for predicted temps and storms then pick a day when you have time and go for it.
2. Group plants in shaded warm area, shooing away Daddy Longlegs as you go. Pick a spot where you can get them saturated and they can also drain.  If they haven’t seen direct sun all summer, now is not the time to let them boldly sit in it as leaves easily scorch. 

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  Houseplants ready for their turn at soaking /debugging. Photo by Lori Williams

3. Quick-clean plants of dried leaves and debris.  With a hose spray plants and pots from every angle to knock off dirt, dust, cobwebs, debris, buggy-spidey-like things that hang on foliage. Up-spray undersides of leaves.  Delicate plants? Adjust hose pressure or use a spray bottle.
4. Run enough water in a kiddy pool, garbage can, or bathtub to submerge containers; a tarp, drop cloth and towels can also be useful.  Add a small amount of soap to water (detergents or degreasers can damage or kill sensitive plants), I like Dr Bronner’s castile soap.  You can use diluted insecticidal soap for this step, too.
   

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  Ficus tree enjoying the end of season clean up. Photo by Lori Williams

5. Fully submerge each container (for tall pots set in water, splash water in pot until water sits on top of soil). Soak 10-15 minutes until no more bubbles come up from soil.  Now the soil’s completely hydrated and in turn should drown little buggy organisms (instant in-pot composting, right?). Gently wash all non-submerged leaves, stems and branches with soapy water while the pot soaks by splashing water over the plant. It’s also an easy time to wipe down container’s sides, rim and bottom so it’s house-clean.

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  Shade cloth cones in handy if, say, midstream on soaking time, the sun comes out & the shade recesses while you’re blogging. Photo by Lori Williams

6. After thoroughly soaked, remove container and let it drain. Gently spray plant & container with hose until soap-free.  Pleas no direct sun during this step, either, as water sitting on leaves magnifies the sun’s impact. While plants are still outside, rough up top 1-2” of soil and add fresh potting soil. My favorite tool for this is a cocktail fork.  It’s tiny enough to not damage roots, sturdy enough to get the job done. Plants that are root-bound can be up-potted at this time.

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  After the last spray down these plants are almost ready to go back inside for fall & winter. Photo by Lori Williams

After completing the steps, protect your plants and effort. Mud rooms, covered porches, and garages are made for this. From there plants can be moved out during warm daytime temps and back in overnight for a couple days – truly ‘hardening’ them ‘off’.  

Additional resources:

Sunburned House Plants

Bringing House Plants Inside

Putting the Garden to Bed: End-of-Season Advice