Nifty, Thrifty Ways to Reuse Potting Compost

, written by Barbara Pleasant us flag

It takes a lot of potting compost to fill containers large enough to grow vegetables or robust flowers

Every year the containers on my deck multiply like rabbits. One pot of basil becomes three, a cherry tomato fills a pot so big I can barely move it, and let’s not get started on the petunias. While it’s great fun to grow plants up close, container gardening can cost a small fortune in potting compost. Why let it go to waste? With thoughtful handling, you can reuse potting compost in next year’s containers, or use it to solve other gardening problems.

The first step is to let used potting compost dry out, either in pots or dumped into a wheelbarrow or onto a tarp. I favour the dump method for two reasons. In my climate pots must be stored empty and dry to keep them from cracking in winter, and it’s easy to comb through a mountain of loose soil to remove stringy roots. Old potting compost need not be bone dry when you store it, but too much moisture can create cushy conditions for unwanted mouldy microbes. Dry soil weighs less, too.

“Dry
Allow wet potting compost to dry a bit before combing out residual roots

Storing Potting Compost

I like to store potting compost used to grow edibles separate from the stuff that supported flowers, which helps limits disease carryover from one year to the next the same way rotations work in the vegetable garden. The soil used for flowers this year is fair game for edibles next year, and vice versa. Any storage containers that will keep the soil dry will work, including bins, small garbage cans, or heavy-duty plastic bags.

Exposure to freezing temperatures is good for stored potting compost, because it will make life difficult or impossible for any insects that may be present as adults, pupae or eggs. When stored under warm conditions, used potting compost may give rise to mysterious hatches.

“Storage
Storing compost used to grow edibles and flowers separately is a simple way to break common disease cycles

Good Uses for Old Potting Compost

Much of my old potting compost is not used for potting up plants. Though it may lack nutrients, old potting compost still contains nuggets of perlite, threads of humus, and very few weed seeds. This makes it an ideal material for covering newly planted carrots, beetroot and other slow-sprouting seeds. A topdressing of moisture-holding potting compost enhances germination of the seeded crop, with fewer weeds competing for space.

Used potting compost also comes in handy when moles, dogs or other critters create holes in the lawn that need to be filled and patched. Grass seed that is covered with a thin layer of potting compost usually comes up strong, with few unwanted weeds.

You also can use old potting compost to pot up giveaway plants. Sharing divisions taken from asters, bee balm, daylilies and other vigorous perennials costs nothing when you drop them in a cracked plastic pot and snug them in with used potting compost.

“Topdressing
A topdressing of old potting compost promotes germination of carrots planted as seed tapes

Rejuvenating Used Potting compost

Many gardeners simply mix used potting compost with new material, using about half of each, with a few handfuls of organic fertiliser added to boost plant nutrition. Or, you can place the old potting compost in the bottoms of very large containers, and fill the upper parts with a fresh mix.

This simple practice works well with soil that hosted healthy plants, but my humid climate is rife with blights and mildews, so I take the extra step of heat-treating potting compost that was used to grow edibles. Only 30 minutes of exposure to temperatures above 120°F (49°C) will kill most disease pathogens, but you need not stink up your house by using your oven. Instead, put a few gallons of used potting compost in a black plastic bag, and place the bag inside a translucent storage bin set in full sun on a bright day. A parked car with the windows rolled up makes a good solarisation chamber, too. Once used potting compost has been heated and cooled, it’s ready to add to any type of new mixture you want to create.

I still use a fresh bag of seed-starting mix for starting seeds, but by recovering and storing much of the soil from outdoor containers, I make far fewer trips to the garden centre for store-bought dirt.

< All Guides

Garden Planning Apps

If you need help designing your vegetable garden, try our Vegetable Garden Planner.
Garden Planning Apps and Software

Vegetable Garden Pest Warnings

Want to Receive Alerts When Pests are Heading Your Way?

If you've seen any pests or beneficial insects in your garden in the past few days please report them to The Big Bug Hunt and help create a warning system to alert you when bugs are heading your way.

Show Comments



Comments

 
"Thank you for this article it's very helpful!"
Dave on Tuesday 15 June 2021
"Thanks for the tips. I always reuse my potting mix but I have never solarised it to kill any pathogens. I garden in sub-tropical Brisbane, Australia and the main problems I encounter are powdery mildew and fruit fly. Cabbage white butterfly is also a problem on my brassicas, for which I only have a very short growing season. On the plus side I can grow most things all year round I will now solarise my old mix before amending it."
Peter Turner on Sunday 18 July 2021
"Thank you so much for all your info I watch you on Utube most days although you have a lot more garden than I have I am a 76 year old grandmother that loves my garden and since Covid lockdown have rediscovered growing my own fruit and veg in containers I had a greenhouse bought me from my children for my birthday and it gives me great joy reusing compost is a game changer when you grow in large containers once again thank you"
Gerri on Monday 20 June 2022
"Exactly what I needed to know - thanks."
Frances on Tuesday 2 May 2023

Add a Comment

Add your own thoughts on the subject of this article:
(If you have difficulty using this form, please use our Contact Form to send us your comment, along with the title of this article.)



(We won't display this on the website or use it for marketing)



Captcha


(Please enter the code above to help prevent spam on this article)



By clicking 'Add Comment' you agree to our Terms and Conditions