I received a number of requests during the first part of our dahlia sale for some tips on a things related to dahlia care. I’ve assembled this “year-in-dahlias” guide to share our practices. For most home gardeners, knowing how to plant, harvest flowers, and store tubers will be your main areas of focus. Included below is some more detailed information for folks who want to take their dahlia growing to the next level, or need help troubleshooting a specific issue. Throughout the post below, you’ll find links to some of the resources I’ve found helpful when I transitioned from a hobby grower to a professional grower.
As always, please share questions and thoughts in the comments window at the bottom of the post. I’ll get right back to you. And be sure to visit our online store for dahlia tuber shopping.
Receive purchased tubers in the mail: What to do next?
When you first receive tubers you’ll want to do one of two things - plant them or store them until you can plant them. Store tubers in a cool, dry, dark place. You should receive them packaged in some kind of material like peat moss and/or pine shavings. Those materials will help your tubers retain moisture until you can plant them. Learn more about storing tubers further down in the post. Dahlia tubers can be potted up and planted early in the season, and even in the winter for collecting cuttings, but the plants should not be planted out until their planting area is frost free. That will be earlier if you plant them in a greenhouse or tunnel and a bit later if you are field planting. We keep track of our last and first frost dates every year and carefully monitor the weather forecast when we start planting out in the spring. If you plant them and then get surprised by an upcoming frost, cover your plants to protect them with frost cloths, bed sheets, cardboard boxes - whatever will get the job done!
As mentioned above, dahlias dislike frost, so hold off on planting until the weather looks favorable. The tubers also dislike a lot of water - so plant them when you see a few clear days on the horizon on your weather forecast. Dahlias do best in sunny, well draining locations. They can be planted as close as 12 inches together. We plant ours in beds of 2 rows. The rows are approximately 14 inches apart and the dahlia plants are spaced about 12 inches apart from each other down each row. And then we leave a walking aisle on either side of the plants. To plant, dig a small, shallow grave-like hole about 3-4 inches deep and the length of the tuber. Lay the tuber on it’s side with the “eye” or “eyes” pointing slightly upward. Cover the tuber with soil. The first sprouts should start peeking up in about 10 days.
Once sprouted, dahlias do like to be watered. The amount of water you give them is going to vary on the type of soil you have and the location you’ve planted them. If they are field planted, you might be watering less if Mother Nature is helping out. If your soil has a lot of clay (like ours) you might also be watering less since the clay particles really hold on to moisture. Gardeners with more sand in their soil will likely have to water more. When it comes to watering, my general finding is that most people “kill their plants with kindness.” Plants have a natural mechanism for telling us when they are thirsty - they wilt. If you don’t know much about your soil or the plants you’re working with, be sure not to over-water them. Over-watering can lead to disease and rot. I wish there was an easier way to figure out watering, but there is no “one-way-fits-all” solution.
This section is a bit of a misnomer, because dahlias really don’t want to have too much fertilizer. In fact, you’ll get a nice big bushy green plant if you do fertilize a lot, but not many flowers. Why? Nitrogen. If you feed your dahlia plants a bunch of nitrogen through organic material or fertilizers, you will be feeding the growth of the plant, but not the flowering habit of the plant. Hold off or limit fertilizing if you can. If you are growing dahlias in a new bed and have nice, rich soil, you may not need to add anything at all. Because we are growing a lot of plants in a small space every year - yes we add some organic material (compost) to our dahlia beds.
I think a lot of dahlia guides leave this part out, maybe in hopes of not scaring potential dahlia tuber buyers away, but I find that the more information we share, the more success everyone has and the more people will love these plants. That’s the real goal, right?! Here are some problems that can afflict dahlias, what to look for, and what kind of action to take:
Dahlia Mosaic Virus - Dahlia mosaic virus is identified by a mosaic-like pattern on the leaves of the plant that can range in color from the usual healthy greens of the plant to lighter yellow-greens. Yellow can follow the veining of the plant leaves. See photos here. Dahlia mosaic virus is spread by insects like aphids and by unclean tools (your pruners as they move from plant to plant!). The disease enters through wounds in the plant, and not by plants touching each other. Aside from the plants looking unhealthy the tubers may become stunted and so may the flowers — in both size and number of blooms. Remove infected plants from your garden so the disease is not passed to healthy plants. If you want to learn more about dahlia mosaic virus, click here for an article by the National Dahlia Society of New Zealand. Note: Some dahlia varieties are asymptomatic of the virus and serve as sort of un-obvious hosts.
Botrytis blight - Botrytis is caused by a fungus often associated with a lack of air flow between plants and/or too much humidity. To avoid Botrytis, make sure you have spaced your dahlias so that they can get some good air between the rows, if not between the plants themselves. Make sure your tunnels have proper ventilation. And strip away and remove any suspect leaves or rotting plant material from your growing space. If left to run its course botrytis in dahlia plants will produce significantly fewer flowers.
Potato Leaf Hopper - tiny (teeny tiny!) potato leafhopper bugs jump from plant to plant and suck the juices from the underside of the plants’ leaves, usually early in the season. If un-cared for, dahlias can be stunted in their growth for the rest of the season, but usually not completely killed by these buggers. Identify the bugs by tapping the plants and watching for tiny little jumping insects to hop off. Exclusion of potato leafhoppers with the use of insect netting is one way to keep them at bay, if you start early. You can also spray plants with an organic pesticide like Neem oil mixed with water (always follow the product’s use recommendations).
Tarnished Plant Bug (TPB) - Tarnished plant bugs are the bane of my existence. They come on strong when your flowers do, and they can do a lot of damage. These bugs take one little nibble from a developing dahlia bud — just one cell sometimes. The flower will be completely deformed from that damage. Dahlia flowers that look like half flowers or have a sort of strange u-shaped center have likely been affected by Tarnished Plant Bug. Neem can help with them, too. Just remember that anything you spray on your plants will be washed off them if they are field grown. Any sort of pesticide (even the organic ones like we sometimes use) should be applied in the early morning or evening to protect the good pollinators and should be applied when the weather looks clear for several days. We try not to spray our plants - instead we cover buds with white organza bags to exclude TPB. You can purchase the bags here from Uline.
Grasshoppers - I didn’t notice issues with grasshoppers on our dahlias until we started growing dahlias in a tunnel. They are happy to munch down leaves and petals. The organza bags will also protect your dahlias from grasshoppers.
Cucumber beetles - We are fortunate (knock on wood) to not have problems with cucumber beetles in our dahlias. I’ve found them on the coreopsis - and I battle them there, but not on the dahlias yet. Cucumber beetles are leaf and petal munchers. I know they are a problem for many growers. Your local extension agent might have the best advice for controlling them in your area. For more information on cucumber beetles, including images, click here.
Supporting dahlia plants:
Dahlia plants can grow quite tall depending on the variety and growing conditions. Some of the plants in our tunnel get to be 6ft tall and taller. We really have to reach to harvest from them! If you are growing dahlias in rows, you should provide some kind of support for the plants. They will need it, especially as they start to develop those huge showy blooms. We use a simple “stake and corral” method. Wooden stakes are placed every 4 feet or so down each side of the bed and bailing twine is used to connect the stakes and hold the plants upright in their bed so that they don’t fall out into the aisles. Plants can by tied directly to individual stakes in smaller home gardens. Other farmers like to use horizontal netting - I find horizontal netting too difficult to harvest through and too annoying to deal with at the end of the season (for dahlias).
Harvesting dahlia flowers:
Now, harvest time! (The best part about growing dahlias is being able to share them with others!) Cut dahlias during the coolest part of the day and place them immediately into buckets of hot (not boiling) water. Allow the buckets with flowers to cool on their own in a shady space. Then put the buckets into a flower cooler (or a cool air-conditioned room). To encourage longer stems and more of them, cut each flower long - even if it means you are sacrificing buds. (My mom always winces when she visits and wants to help with harvesting, and I have to tell her to give me long stems!) This action tells the plant to hurry up and grow more long stems with flowers on them. Note: Dahlia buds do not open after cutting. Dahlias should be harvested when they are about 3/4 open and the back petals are not yet wilting. Sometimes I hear customers at the farmers’ market say they are choosing one bouquet over another because of the “extra buds.” I have to let them down gently by informing them that while they are beautiful, those buds unfortunately won’t open and bloom.
End of season care for dahlias:
After exposure to the first significant frosts your dahlia plants will start to blacken and die back. Let them. This event tells the plants to invest energy in their tuberous roots. After a few weeks, cut back your plants to the base of the stems and gently dig up the tubers.
Dahlia dividing and storage:
There are probably as many different dahlia dividing and storage regimens as there are flower farms. Here is what we do. After digging, dahlia tuber clumps are washed with a nice strong jet of hose water and left to drip dry for a few hours in bulb crates (sort of like big milk crates). The washing is an annoying and sometimes very cold job for us here in northern New York, so we take turns and only say really nice things to whomever is tasked with the job! The crated tubers are either pulled out and packed in a combination of peat moss and wood shavings in plastic bins or divided and then packed in peat moss and wood shavings in plastic bins. Tubers that aren’t divided right away will be divided over the next several days. I have helpers work with me on the these jobs because of our scale. I personally take some late nights to do extra dividing, too. But when I started out and just had a hobby garden, I did everything by myself. Please don’t let the details of this post discourage you! It really is not too difficult.
We do all of our dividing in the fall, and we carefully inventory each variety as we’re dividing so that we have accurate information for our winter tuber sale.
The dahlia bins are stacked in our farmhouse cellar for the winter. The temperature is about 50-55 degrees. (This is also where we grow tulips!) It is really important to have a cold, unfreezing place to keep your tubers. They prefer 40-45 degrees. But mine do fine in our space.
We use all sorts of lidded plastic bins, and we don’t drill ventilation holes in them any more. We find that the peat and wood shavings manage a balance in moisture that is just right in our storage space without any extra ventilation. Occasionally, I see condensation building up in a bin and I open the lid in a panic and find that everybody is doing just fine in there. I’ve never found a bin of rotten tubers. Any tuber rot has always been on tubers that dried out too much initially and then were left exposed to air.
To divide dahlias, use a sharp, clean pair of pruners or a knife to split the tubers apart. This job takes a little practice, but there are a TON of YouTube videos (like this one from Swan Island Dahlias) out there to show you what to do. Essentially - each tuber clump can be split into pieces to start other plants. As long as each tuber has at least one “eye” it will grow a completely new plant for you. Discard any tubers that break off or are accidentally cut off without an eye. And especially do not share or sell any possibly “eye-less” tubers with friends or customers. Those tubers will not grow. What do the “eyes” look like? Sometimes they are more pronounced and you can already see a sort of rupture of white, dark red, or light green plant material trying to start. Other times - they are so tiny— just a little dot. When in doubt, leave more eyes on each tuber or allow some tubers to remain joined as a little mini clump that you feel more confident will sprout. All of our plants are started new each season from just one tuber.
Winter tuber care:
Peek in on your dahlia tubers every couple of weeks to check for tuber rot and mold. If you find one that looks questionable, just throw it away. That one rotten tuber could cause problems for the others in your bins. Don’t risk it. If any tubers seem to be drying out and getting wrinkly— make sure you completely cover them in peat in their storage bins. I’ve been able to “re-inflate” less wrinkly tubers that way if caught early enough.
If you want to grow your stock of dahlias before the season starts you can do so by taking cuttings in the winter time. You’ll need a warm, sunny or well lit place to keep potted up tubers while they are developing sprouts. You will have to water these inside tubers a little to get them going. It takes about a month to generate the first series of sprouts from a tuber in the winter, so you’ll want to start this process in January in order to collect a few cutting from each tuber before the spring. When a sprout has reached about 3-4 inches tall, cut it cleanly at the base of the stem. Dip the stem into a rooting hormone powder and place the cutting into a small pot or cell pack filled with damp play sand - the kind you get from a hardware store. Poke a little hole in the sand so you don’t inadvertently remove the rooting hormone powder. Close a clear plastic bag around the cuttings and hold them in a warm room or on a warm seed starting mat. In about 10 days, your little cuttings should have roots! Keep looking after your “mother tubers” and they will produce more sprouts for you to propagate as cuttings in a few weeks. Nurture your cuttings and pot them up as needed until it is warm enough to plant them outside. For A+ video tutorials see @santacruzdahlias Instagram posts from December 23, 2018.
And that brings us back to spring and the top of the post! Thank you for reading. I hope this information was helpful. Please know that these are our practices for caring for dahlias. Everyone is going to have a slightly different routine and methods that work well or don’t for their situation. I always recommend trying different techniques to find the best ones for you. There’s nothing more fun than an experiment! To try out some of our favorite dahlia varieties, hop on over to the shop.