04/05/2021
Root-knot Nematodes on Peonies:
I am crazy about peonies and have built a very substantial collection over the last 5 years. Every spring for the last 5 years I have spent planning expansions of my garden beds to accommodate the new peonies I plan to acquire that year. Each bed is carefully dug and amended with compost to ensure that I give my new peony roots the best start that I can. Which is why I was somewhat puzzled by the lack of blooms in quite a few of my peonies last year. Was it because we had a late cold spring 2 years in a row? Did I inadvertently plant them too deep? This puzzle kept me thinking throughout the winter and I developed a hypothesis that I was anxious to prove this spring. So, a week ago I headed out to the garden with a bucket and a shovel to dig up one of the peonies that has not bloomed for me in 2 years, The Mackinac Grand. Normally I wouldn’t dig up peonies in the spring, but this problem needed solving. As I started uncovering the roots of the peony my worst fears were realized, root-knot nematode infestation. One of the worst I’ve ever seen. How did this happen?
Four years ago, when we decided to create one long border along the southern edge of our septic leach bed we brought in a truckload of topsoil to build the border to the depth we needed. We purchased the soil from a local landscaping and garden supply company and had it delivered in October. We amended the topsoil with compost and I proceeded to plant my new peony purchases along with quite a few perennials. The first year all of my new peonies bloomed but because they were new, I fully expected to get only 1 or 2 blooms. The following year I connected this new bed to the border on the west side of the septic leach, but we used our own topsoil to help fill this in. That following spring, I noticed that one of the peonies I purchased and was really looking forward to seeing in bloom, The Mackinac Grand, failed to bloom at all. And this was repeated in 2020. Throughout this I continued to acquire new peonies and expanded this border to the east as well, but only with my own topsoil. I gave several peonies away last fall to a friend of my daughter’s and one that I dug up did show signs of root-knots. I cut them away, divided the peony and sent them off to their new home. But that incident started me thinking about whether or not I had a problem with root-knot nematodes in that border. I spent some time this winter reading about this pest which led to the trip out to the garden last week with shovel and trowel in hand.
What are root-knot nematodes? Nematodes (Meloidogyne spp.) are small slender plant-parasitic worms that can infect 2000 different crop plants world-wide.
“Nematodes are microscopic eel-like organisms that live in soil and water. Nematodes are the most abundant multicellular organisms on earth. Most soil dwelling nematodes are beneficial organisms that play a role in the break down and release of nutrients from organic matter. Some beneficial nematodes prey on other nematodes as well as soil-borne insect, fungal and bacteria pests.”
I suspect that the particular pest in my garden is the Northern root-knot nematode but I will only be able to confirm this when I send samples off to a pathology lab for testing. Root-knot nematodes are obligate and sedentary parasites that feed on the fine roots of plants. They survive winter as eggs which start to hatch when the soil temperature reaches 18°C. It is at this larval stage that the nematode will enter the plant roots and start to feed. This feeding will damage the roots and reduce the vigour of the plant. Over time as the nematode infection spreads, the plant develops an excessive branching structure of fine roots with noticeable galls or knots along them.
Photo 1: Peony roots showing both the excessive branching and the knots or galls on them
Since this pest feeds on over 500 different plant species, it is going to be difficult to control and removing the soil completely is going to be even more difficult as that border is extensively planted and I do not want to move infected soil elsewhere. Another OMAFRA publication suggests using cover crops for nematode suppression. Suggested cover crops include African marigold cultivars such as Crackerjack or Creole which are toxic to nematodes.
My plan is this:
1. Dig up, wash and clean all of the infected peonies, removing all, or as much, of the fine fibrous roots as I can, packing them in clean peat moss for a few days and them moving them to a newly created bed at a friend’s property.
2. Monitor these peonies over the next 2 seasons.
3. Send plant root and soil samples to a soil and plant pathology testing lab for confirmation.
4. Plant grasses and marigolds in the locations where all of the peonies were growing as well as in a line separating the east and west sides to prevent the infestations spreading into uninfected soil.
5. Continue to monitor the peonies on the east and west sides of that border for any signs that the nematodes have moved into those areas.
6. Prepare a new peony border elsewhere in my garden where the hopefully recovered and now uninfected peonies will be moved at the end of the 2-year quarantine.
7. Ongoing monitoring and vigilance.
Photos 2 & 3: Before and after photos of one of the peonies I dug up and removed the fine fibrous roots in preparation for replanting
As a master gardener, there are also a few lessons learned for me. I incorrectly assumed that purchased topsoil would be suitable for immediate planting. In hindsight, I should have had the soil tested and based on the results, planted nematode suppressing cover crops for the first 2 years or perhaps not planted peonies in that soil at all. I do hope that my story will help others recognize this problem pest, especially if they have planted gardens in soil brought in from a landscape supplier. Despite the setback, I am looking forward to peony bloom season this year as I do every year. I am also hoping that by adding more native grass plants and inter-planting them with Marigolds that I can suppress and over time eliminate the nematodes in that bed. Regardless, I will not be planting peonies in it again.
