Has this ever happened to you? You dig out a rose in your garden to make way for a gorgeous variety you saw on a garden tour. You plant, water and tend to the new rose with loving care. And then — big disappointment — the rose turns out to be a stunted dud and a major under-performer.
You give it a few years hoping it will get over its growing pains. It doesn’t and, your patience exhausted, you yank it out. You carefully select and plant another rose. Oh, no! The same situation replays.
Are you doing something wrong or did you just get unlucky with two “lemons”?
Myth or fact?
Rose replant disease has been a controversial subject. Lucky rosarians who have not experienced it often maintain that it is a myth. They attribute the bad outcome to several possible causes, including soil compaction, microorganic and mineral deficiencies or destruction of beneficial microbial life brought about by excessive use of fertilizers.
But when none of these issues apply, what could be going on?
The ‘poison hole’
When new roses are planted where old roses used to be, they often struggle to establish and may not grow as well as they would if they were grown in soil never planted with roses. This scenario occurs even when the soil in the hole is amended. When the rose is removed, another species of plant can grow well in the planting hole.
A recent article written by Dr. David Zlesak discusses advances in replant disease and ongoing research in Europe. The Kordes rose company in has been proactively addressing this problem by renting neighboring agricultural land and practicing crop rotation, with multiple years between rose crops on any given piece of land.
Not just a rose problem
The phenomenon is not isolated to roses. Many plants — including tomatoes, apples, pears, plums and cherries planted in soil previously occupied by of a related species — suffer decline and sometimes even die. This problem is called replant disease or soil sickness. Two microorganisms, nematodes and Actinomycetes are being investigated as possible causes for soil sickness in the European control studies.
Pat Nolan, plant pathologist for the county’s Department of Agriculture, Weights and Measures, dissuades growers from putting a plant in a hole where the same type of plant has recently died. She believes nematodes can be a common cause of plant failure, and especially fruit tree failure, in these situations.
Soil-dwelling nematodes are microscopic, worm-like creatures. Most are harmless, but some are serious pests that feed on and stunt root growth and can grow to large populations in the soil.
Why some gardens and not others?
Nematodes thrive in sandy soil and don’t do well in clay soils as they breathe through their skin and clay inhibits the air exchange.
What should you do if your plant is affected by replant disease?
Dig the rose out, shake and wash off the existing soil and either replant the rose in another part of the garden where you have not grown roses before, or pot it in a container using a good potting mix. I have personally noticed that both these courses of action result in a tremendous improvement in the vigor and health of the removed and replanted rose.
How to avoid replant disease
In smaller gardens, we have limited space to grow roses so soil sickness can pose a real dilemma. Here are some tactics you can try:
1. Alternative rootstocks: Some plant varieties and rootstocks have better resistance to nematodes. Florida has a big nematode problem, so rosarians in that state plant roses grafted on nematode-resistant Fortuniana rootstock. You can buy roses grafted on Fortuniana rootstock online at K & M Nursery.
2. Meticulously remove all the previous rose’s roots and replace the old soil in the hole with fresh soil from another part of the garden where roses have not been grown. In order to increase aeration, improve root growth and provide competition to the pathogens in the soil, add organic matter with its many beneficial microorganisms.
3. Adding worm castings and mycorrhizae fungi to the soil may make you feel you are doing something beneficial, but to date there appears to be no real evidence they can help to counteract replant problems. Mycorrhizal fungi are useful to plants but are of much less use when a plant is well watered and fertilized as it typically is in a rose garden. In addition, with the wide variation in mycorrhizal fungi, it is hard to predict their viability and usefulness in different soils and environments.
4. Plant your rose in a container. Only transplant it into the “sanitized” hole when it has developed a solid root ball in hopes that a healthy, well-established plant might be better able to withstand damage. Alternatively, plant it in a biodegradable container with extra drainage holes and hope that the root ball will be developed before the container decomposes or the pathogens can get to it. Evaluating whether the rose is a keeper before you put it in the ground will minimize the number of times you have to deal with replant issues.
5. Nematode-suppressive plants: Field experiments to control nematodes are being done in Europe using tall-growing marigolds,Tagetes erecta, commonly called Mexican or African marigolds. University of California’s Integrated Pest Management Pestnotes bit.ly/2N0uksT include the varieties of French marigolds named Nemagold, Petite Blanc, Queen Sophia and Tangerine as nematode suppressing. However, Pat Nolan warns that nematodes can be very partial to marigolds, so look specifically for only these varieties.
Rose replant disease is challenging, but keep experimenting to find out what works best in your garden. So far, I have discovered Rhapsody in Blue, growing on its own roots, and Lady of the Dawn grafted on Fortuniana rootstock are both winners. These roses laugh in the face of rose replant disease and show off gorgeous blooms with great spirit and vigor.
Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension.
