
How to Grow Ranunculus (in Seattle, PNW, Zone 8b/9a)
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Ranunculus are one of the queens of spring. Often with 3-4 inch blooms of ruffly petals in pinks, purples, pastels, and other bold colors, what's not to love?
And if you want to enjoy them outside of your garden, ranunculus make excellent cut flowers, boasting a vase life of 10+ days. Cut the flowers before they have fully opened to enjoy the longest vase life.
Here's my comprehensive guide on how to grow this beautiful flower.
(This is my perspective and experience as a flower farmer in the PNW, Seattle, WA, zone 8b/9a - while much of the information is applicable no matter your location, some adjustments might be needed for optimal growing in your specific climate.)
How to Grow Ranunculus from Corms
The majority of ranunculus are grown from corms, which are small, brown, octopus-shaped things. Corms are primarily bred and produced overseas (in Europe) for shipment into the United States in the fall each year. Flower farmers will often place their wholesale orders in the prior spring to secure the best colors/varieties. Everyday gardeners can place their orders in the fall/early winter.
If you are not going to plant immediately upon receiving them, store the corms in a cool, dry place prior to planting.
Where to Buy Ranunculus Corms
Not all corms at created equal. The larger the corm, the more flowers you can expect. While you can often find great deals on ranunculus corms at big box stores, they are typically smaller, generic varieties, giving you fewer blooms with only a single layer of petals. Instead, look to smaller flower farmers in your area who are selling their high-quality wholesale corms. Van Engelen/John Scheepers is another great spot for small growers/gardeners to get well-priced, quality corms in small (or large) quantities.
When to Plant Ranunculus Corms
Ranunculus corms can be planted October through early February in Seattle for blooms April-June. They say you can expect blooms in 90 days, but I've found that it depends more on when you heat up in the spring. Thus, while the October-planted corms might take 6 months to bloom, the January-planted corms may bloom in only 3-4 months.
There is an advantage to planting them earlier, however. Because ranunculus love getting established in cool weather, fall-planted corms are able to grow a more-supportive root structure, which will extend their blooming time. Fall-planted ranunculus typically bloom for 6-weeks, while the ones planted later might bloom for a month.
I like to succession-plant my ranunculus corms so I can ensure I have a great harvest all spring. I plant new corms every few weeks from October to January.
To determine the absolute latest you can plant in your area, think of when you typically start seeing temperatures consistently in the 70s or hotter, and count backwards four months. Planting then should give you several weeks of blooms. (We tend to see those temperatures in June in Seattle which is why I don't recommend planting past early February.)
Preparing to Plant
Before planting, soak the corms in room-temperature (NOT hot) water for 3-4 hours. This will plump them up and wake them from their dormant state.
Next, many people have success doing what's called "pre-sprouting" their ranunculus corms, but this is not a required step. I've personally never done it and I have great success without it so I'm happy to have eliminated the need for the extra work that goes with this step. After soaking, I plant my corms directly outside.
There are of course some benefits to pre-sprouting. It can reduce rotting in your planting space and make your space more efficient. If you have one rotting corm, there is potential that it can cause neighboring corms to rot as well. Pre-sprouting can help weed out the corms that are prone to rotting. It can also help you weed out the duds (some corms will unfortunately just never grow), so you ensure that you are maximizing your planting space. Pre-sprouting can also be useful in a colder climate as it allows you to get the corms growing inside while it may still be too cold outside to plant.
To pre-sprout, after soaking you want to lightly cover the corms in a tray or other vessel with moist potting soil. Place the tray in a cool, dark area (around 45°F), and check on it often for signs of mold/rot. Remove any affected corms. Keep the soil moist but ensure it doesn't get waterlogged. You should see little white roots coming out of the corms in 2-3 weeks. At that point, they are ready to plant outside.
Planting Ranunculus in the Garden
Ranunculus grow best in a spot that receives full sun (6-8+ hours of sun per day). The soil should be well-draining, otherwise they are prone to rotting. As long as the soil is well-draining, ranunculus do well in raised or un-raised garden beds, as well as containers.
Ranunculus can be planted anywhere from 4 to 9 inches apart for a lush, full look. (Do note that, depending on your climate, tighter spacing might cause issues such as powdery mildew later in the season.)
They should be planted 2-3 inches deep, with the tentacles pointing down. You can either dig small holes to plant them into, or my preferred method is to arrange them on top of my planting space, and top off the bed/container with 2-3 inches of compost/soil (placed directly on top of the ranunculus corms).
How to Care for Your Ranunculus Plants
Ranunculus are sometimes called the Goldilocks of flowers because they don't like it too cold or too hot. They are also susceptible to several different kinds of pests. Read on for some common solutions to situations/problems you might run into when growing ranunculus.
Protecting Ranunculus from Cold Temperatures
Ranunculus need protection when the temperature falls below 25°F. Snow, leaves, and/or frost cloth/row cover work well to insulate the ground and protect the tender plants. (I get my Agribon Row Cover from Johnny's - please be wary of knockoffs from large e-commerce sites). A sheet also works in a pinch.
When the plants are young, you can set the frost cloth over them and leave it for several weeks. This can also help protect them from pests (more below). But as they get bigger, they need air circulation to prevent diseases so at that point it's best to only cover them when it's below 25°F.
While I haven't found it to be necessary in Seattle, many growers have success building "low tunnels" out of PVC pipes bent into hoops and covered with row cover/greenhouse plastic. This can be useful if you are getting cold temperatures later in the season when the plants are larger, and especially if there are buds/blooms as this keeps the cloth off the plants themselves. But if it gets really cold (20°F or lower), it's still recommended to add a layer of frost cloth inside the tunnel, directly on top of the plants, as added protection.
As noted above, ranunculus need air circulation, so if you are using a low tunnel, you should vent it often by opening the ends. An added bonus of the low tunnel is that you often get blooms earlier because it helps warm up the soil sooner in the season. But do be wary of it getting too hot in the tunnel as the season goes on - more on that below.
Protecting Ranunculus from Warm Temperatures
Ranunculus will go dormant when the temperatures consistently reach 70-80°. (Don't worry - a few early, sunny, 75° or even 80° spring days are not going stop your blooms.) If a hot stretch is longer than a week, there are things you can do. If your plants are looking strong and you want to extend your season, on very warm days you can ensure they are well-watered and try to provide shade from the hot afternoon sun. If you planted in a container, you can simply move it to a shadier spot. Otherwise you can use shade cloth, or fashion your own shade with a tarp or patio umbrella. Frost cloth can double as shade cloth because it does block out some of the light.
Personally, I found a great spot in my yard that I love to grow ranunculus, under the canopy of a black walnut tree. The tree loses its leaves for the winter so the plants get full sun during the darker months, and then, as the tree leafs out in May, it provides shade to the delicate blooms as the days heat up.
As noted above, the temperature inside a low tunnel can be considerably warmer than the outside air so keep that in mind in the late spring months, lest you send your corms into early dormancy.
Protecting Ranunculus from Pests
Ranunculus are susceptible to all sorts of pests including rats, rabbits and squirrels, snails, slugs, and earwigs, thrips and aphids, and powdery mildew. Yikes.
Rats and squirrels love to devour ranunculus corms, and rabbits and rats will eat the growing foliage. When storing or pre-sprouting them, ensure they are in a space that is safe from rodents. To protect the corms and foliage once planted, you can try a repellent such as a peppermint spray, a smelly product such as liquid fence, or a sprinkling of cayenne pepper. I have also read that the act of pre-sprouting makes the corms unattractive to rodents, so consider that if your rodent-pressure is high. Alternatively many people succeed with a barrier method such as covering the planting area with chicken wire/hardware cloth. I've had success stopping pests by covering my entire planting space with frost cloth, though this can invite another pest, the slug.
Snails and slugs love to eat the tender new shoots from your corms, while earwigs love to nestle inside the beautiful blooms. To combat these three pests, I recommend liberally applying a product called Sluggo Plus every couple of weeks during the growing season. The active ingredient in this product is Spinosad which is an incredible substance that is detrimental to the critters we hate, yet totally organic and non-toxic to humans, kids, pets, and most beneficial insects (when it's in a spray form, it can be harmful to bees, but in this case the product is pellets so the bees remain unharmed). You can usually purchase this product at your local Costco starting in January into the spring - stock up! (Not sponsored, I just love a good deal. Our Costco price is $28.99 for the 5lb bottle which is a huge discount from elsewhere).
Sucking insects such as aphids and thrips can be a problem once the flowers are in bloom. They can weaken the plants over time, as well as cause aesthetic damage to the blooms. Many people recommend neem oil which is an organic substance that targets bad bugs and also works for powdery mildew. However, some studies suggest that it is harmful to beneficials like ladybugs and their larvae so I prefer to avoid using it. Instead, I recommend strategic interplating, as well as using good bugs to fight the bad bugs. Aphids detest aromatic plants such as alliums, garlic, onions, chives, marigolds, and other aromatic herbs, so generously planting those in your planting space should help keep them away. Many of those plants, as well as fast-growing flowers such as sweet alyssum, nasturtiums, and calendula will also encourage beneficial insects to stick around in your garden and reproduce. You can also periodically release beneficial insects such as ladybugs, lacewings, and minute pirate bugs. These bugs do no harm to your plants, but they and their offspring will help them tremendously by eating up the bad bugs. You can often find them at your local garden center, or there are a few places that will overnight them to you such as NaturesGoodGuys.
Ranunculus are susceptible to powdery mildew, especially towards the end of the growing season, or in cases of poor air circulation. Several studies have shown that whole cow's milk is extremely effective at combating this fungal disease. It's best to spray it preventively. Mix 1 part milk with 4 parts water and spray it liberally over the foliage (I generally try to avoid the buds/blooms to prevent aesthetic damage.) Depending on your risk of powdery mildew, you can do this every 1-2 weeks.
One of the best ways to combat pest pressure is to ensure your ranunculus are as healthy and happy as they can be. You can do this by providing them with great soil and sufficient watering so as not to stress them out. Stressed plants invite pests and disease.
Watering Your Ranunculus Plants
If you are in an area like Seattle that gets sufficient rainfall during the fall, winter, and spring, you might not have to water them particularly often. I hardly do unless there is a stretch of several warm days without rain. Remember that the corms are prone to rotting so it's better to underwater than overwater.
Fertilizing Your Ranunculus Plants
The best way to provide essential nutrients to your budding plants is to give them healthy soil to grow in. If you are growing in a container, use high-quality potting mix. If you are growing in the ground, or in raised beds, I recommend topping your beds off with several inches of high-quality compost every year. (Some local places to buy in bulk: Dirt Exchange, Pacific Topsoils, Cedar Grove, Burien Bark, ZooDoo from the Woodland Park Zoo). If you do this, your plants can thrive with minimal added fertilizers.
If you think your plants need an added boost of nutrition, you can mix a balanced, organic, slow-release fertilizer into the soil at planting time (Floret recommends Nature’s Intent, 7-2-4). Throughout the growing season, you can use a liquid fertilizer such as Neptune's Harvest Fish and Seaweed, every couple of weeks.
Saving the Ranunculus Corms for Future Growing
There are patents in place for some ranunculus varieties such as Cloni and growers are legally required to dispose of corms at the end of each growing season.
However, if your corms are not protected by a patent, then you can reuse them again the following year - and you may have found that they have profusely multiplied!
One way to do this is to simply leave the corms in the ground over the summer and look for new growth in the fall. I do this under my black walnut tree because I don't plant anything else there over the summer. Be patient - they often need some rainfall/water to wake them from their dormant state.
If you prefer to dig them up, you should wait until the foliage has completely died back so it can feed the corms for next year. Store in a cool, dry place until planting time later in the year.
While this was a lot of information, don't be intimidated. I tried to cover all the problems that growers might run into but it's incredibly unlikely that you would run into ALL of these problems in the same year. In any case, let me know if you have any other questions and happy growing!