So you soaked your corms, you planted them and now the winter is coming!
Ranunculus are cool-loving flowers and if you are in zone 7-9 (or have covered space) you probably planted them in the fall. Overwintering ranunculus takes a little bit of work to ensure their survival. But don’t fret, it will be worth the flowers in the Spring.
I have made some big mistakes with ranunculus last winter. My plants were yellowing, their heads were breaking off, and the flowers were small. I was convinced they had some mysterious disease. And then it hit me, my plants were stressed! Stressed because I didn’t take good care of them.
Here are my biggest lessons. I hope they are useful to you.
PROTECT FROM ACCESS RAIN / DRAINAGE
It tends to rain a lot more during the winter. Ranunculus corms are very susceptible to root rot if they are not protected from the access water. You have to prepare a very well draining spot in a raised bed. Sometimes planters do not drain as well as you think, so check your planters often and avoid soggy soil!
(A poorly draining planter with a dead peony)
If you are planting in a greenhouse, avoid overhead irrigation and water manually so you can check (by sticking your finger into the soil) to confirm that water is actually needed. Overhead irrigation will leave water on the foliage and if that doesn’t dry out it can lead to rot and disease.
PROTECTION FROM FROST
Ranunculus plants and even flower buds can endure freezing temperatures, probably as low as 23F (-5C). The flower buds might get damaged after the freeze but the plant should bounce back. However, prolonged freezing temperatures are not recommended.
The easiest way to protect your plants from frost is with a floating row cover. It is a white, lightweight piece of fabric that can be purchased relatively inexpensively online and re-used each year. For a small bed you can build a low tunnel using garden hoops and secure frost cover to the hoops with clips or sandbags. You could also use a tomato cage or a wire cloche as a support for your frost cover. Basically, use any support mechanism to avoid the frost cover sitting directly on your plants. It’s not just the frost cover but the trapped air underneath that will help insulate the plants. If the frost cloth is directly on the foliage, there will be little insulation.
The same support system should work if you want to use greenhouse film.
(Frost cover over thin metal hoops)
(Our daughter checking on plants under a frost cover.)
If your winters are extra cold or extra wet, you need to consider using greenhouse film to build a little cover for the ranunculus plants. You can use the same hoops to secure the greenhouse film. I would recommend using sandbags or rocks to hold it down as the plastic makes a great sail when it catches some wind. You can place the frost cover under the greenhouse plastic - directly on the plants for some extra warmth if the temperatures dip into the 20s at night.
(Snow on top of greenhouse fiIm. The thin metal hoops were used to hold the frost cloth under the plastic. Since we already had flowers I didn't want the frost cloth sitting on top of them.)
You can mulch around the plants using leaves, compost or straw. The mulch will provide insulation for the corms in the ground. Corms are actually more susceptible to frost than the foliage and will turn to mush once thawed. Don’t put the mulch too close to the base of the plant to avoid rot and the layer of mulch shouldn’t be deeper than 2”. Avoid leaf mulch if it came from an unknown source, leaves that lay on sprayed lawns absorb the chemicals and can damage your plants. I prefer to stick to just compost!
If you are in the rainy PNW like me (or somewhere else very wet) and it’s going to be rainy for days on end, move the container somewhere under cover so the plants aren’t going to be doused non stop. And if you can’t move it, consider putting a piece of greenhouse film over them to keep them dry.
OK, this is underlined because I don’t think it’s emphasized enough in any literature I’ve read and this was my biggest mistake last year. Ranunculus gotta be babied - you don’t get those beautiful flowers by planting and forgetting about them. After you’ve covered up the plants with frost cover and or greenhouse film you’ve built a little microenvironment. That microenvironment will be essentially closed off from the outside and the trapped air will become stagnant. Imagine if an airplane didn’t have proper ventilation! Stagnant air and lack of ventilation can cause many mold and rot diseases. Some of those are Crown rot (Bortrytis), Southern blight (Sclerotium), and various molds. And if the air becomes too warm the plants will go dormant early! I had all of those problems last year and it was very stressful trying to identify and resolve. To avoid these diseases, I recommend that you vent daily and anytime the sun is out or the temperatures are above freezing. Uncover the tunnel or open up one side to let fresh air in during the day. Then don’t forget to cover back up for the night.
(Our ranunculus tunnel in January - half open during the day for ventilation)
(Last December during a snow storm the soil temperature under the plastic stayed at 40F, which is perfect!)
You won’t need to worry much about watering during the winter, but you can’t let the plants dry out completely either. Check your soil by sticking a finger about 2” inches into the soil and feel if it’s damp. It’s a bit tricky to feel as the cold soil in the winter because it already feels kind of wet although it is dry. Aim the water at the base of the plants and the soil around them and avoid watering the foliage. Ventilate after watering to allow any water on top of the plants to dry out.
Don’t water in the evening or while it’s freezing outside, the water will freeze and with it it will freeze your plants and corms.
Don’t give up, experiment! Isn’t that part of the joy of gardening - to come up with new ways to take care of your plants and then woohoo see the flowers?! Or perhaps it didn’t work out that one time, but guess what we learn and adjust and try it again. Ranunculus are absolutely worth it!
I will be interplanting my ranunculus with allium this year. Alliums are part of the onion family and will help detract aphids from ranunculus! I did not have aphid issues with ranunculus last year but I will be planting them in a high tunnel where I did have aphids this summer so I am hoping that this will help prevent it.
These are things I found on the web for small gardens, if you want a recommendation for what professionals use, please feel free to reach out.
- Garden DIY Hoops
- Frost Cover
- Greenhouse film
- Frost Cover Cloche - perfect for containers and planters!