Growing Dahlias

 “The glory of gardening: hands in the dirt, head in the sun, heart with nature. To nurture a garden is to feed not just the body, but the soul.“ -Alfred Austin

Dahlias are a wonderful addition to any garden.  They produce a mass of blooms from early summer until first frost.  There are so many colors, shapes, and sizes of dahlias available, there is truly something for everyone. 

Dahlias do best in well draining, loose, slightly acidic soil. Raised beds are a great option if you live in wetter climates, or have heavy, clay soil. Pots can also be used, provided they are big enough to accommodate tuber growth.  Dahlias require partial to full sun. They prefer warm weather and are not frost tolerant.  Native to Mexico and portions of South America, dahlias are a tender perennial in much of the US, but can be hardy in zones 7-10 given the proper conditions.

When shopping for dahlias, it’s important to buy from reputable growers rather than getting imported tubers from wholesale catalogs, certain online sites, or big box stores.  Reputable growers will have good quality stock that is free from disease.  Unfortunately, many imported tubers are mislabeled, mishandled, and can potentially contain diseases which can infect your soil and other plants. 

When starting your dahlia collection, you’ll quickly realize there are thousands of named and unnamed options out there to choose from. With so many varieties and classifications, it can get confusing very quickly. If possible, find local growers and groups who may be selling stock or hosting trades.  This way, you’ll be getting varieties that you already know will do well in your local environment, and you’ll make contacts with others who share your interest.  Trading is a great way to increase your stock without draining your wallet.

Once you’ve narrowed your search and have gotten your tubers, you’ll want to wait to plant your them until your soil has reached 55-60 degrees, and all threat of frost has passed.  You can amend your planting area with some peat or sand to help with drainage and to keep the soil loose.  Your plants will also benefit from some bone meal mixed into to each planting hole.  Depending on variety height, you want to leave about 2’ in between each plant.  

If you have varieties that require staking (which most do), make sure you get those supports in place before you plant.  Otherwise you run the risk of driving a stake through your tuber.  Many factors will determine what method you choose to support your dahlias.  Are you growing a single plant?  A border along a fence? Rows of plants for cuttings? Netting, individual stakes, or even tomato cages can be good options.   

Once you have your soil prepped and your support system in place, you are ready to plant your tubers.  Place each tuber or clump 4-6” deep, with viable eyes facing up. Cover with loosened soil.  Do not water your tubers until you see sprouts beginning to grow above the surface. 

Slugs and snails are enemy number one for dahlias.  Those slippery little pests will devour your sprouts in a single evening if left unprotected.  The best option is to bait them early and often, so that your tender young plants can have a chance at getting themselves established.  We use beer traps made from old yogurt containers as well as an organic iron phosphate product. Once the plants are well established, the slugs and snails aren’t as interested and don’t cause as much damage. 

Once the sprouts are above the surface, you can begin regular watering as needed.  Dahlias need a deep watering 3-4 times a week.  Keep in mind, plants grown in pots will require more frequent waterings than those grown in the ground.  

As hard as it is to make yourself do, pinching back your plants will make them bushier and stronger, with longer stems and more flowers later on. Once your plants are at least a foot tall and have 3 or 4 sets of leaves on the center stalk, take a clean snipper and cut the upper most sprout just above a set of leaves.  By removing the plant’s terminal bud, you encourage it to produce more lateral buds, which will grow into more stems. While this sets you back a week or two on getting blooms, you’ll eventually be rewarded with lots of flowers on more usable stems.  

A month after you plant your tubers, you should begin your fertilization schedule.  When choosing a fertilizer for your plants, keep in mind that too much nitrogen will encourage green growth but produce less blooms. A good rule of thumb is to choose a product with half as much nitrogen as it has potassium and phosphorous. (Ex a 5-10-10)  Apply once monthly to plants growing in the ground, and every 2-3 weeks for plants grown in pots.  Always follow the application with a deep watering.   

After all that prepping, planting, guarding, and pinching, your plants are finally blooming!  All that hard work is paying off!  The best time of day to harvest your flowers is during the cool of the morning.  Choose blooms that are 1/2 to 2/3 open and make deep cuts, even though that may include taking some unopened buds.  By taking long stems you encourage your plant to continue producing. 

For longest vase life, place stems immediately into metal buckets with hot water (160-180 degrees) and leave them until the water has cooled.  Then you can trim stems to the desired length and place in new, cool water.  As with any cut flower, changing the water in the vase daily will help keep them fresh.   

Deadheading your dahlias will encourage the plants to produce more blooms, and the spent flowers are a great addition to your compost pile.  Since dahlias are sensitive to herbicides and chemicals, a natural approach is best for weed and pest control.  Japanese beetles love to feast on dahlia blooms, but can be picked off by hand and dropped into a jar of soapy water.  Weeds are best dealt with by hand, and can be minimized by the use of an organic mulch, such as straw.  Mulches are best placed after the sprouts have established themselves, and care should be taken that it doesn’t interfere with the water intake of the plants.

Another reason to avoid chemicals is that both the tuber and the petals of dahlias are edible! While the tubers are actually the most nutritious part of the plant, and can be eaten raw or cooked, the flavor can vary depending on the soil and growing conditions.  Personally, I’ve actually never tried eating a tuber, since they’re far too valuable to me as a future plant.  Luckily the petals are also edible, and add a colorful twist to a salad or on a cake.

End of the Season

Given the proper conditions, your dahlias should produce blooms right up until the first frost, upon which the plants will turn black and die off. This killing frost sends a signal to the plant that the growing season has ended, and the new tubers should prepare themselves for the winter.  

Depending on your location, climate, soil type, etc you may decide to leave your tubers in the ground over the winter, or you may want to lift them for storage indoors.  Here in the soggy PNW our tubers are very likely to rot if left in the ground. Freezing temperatures in cooler zones can also kill them.  If in doubt, it’s always safest to dig up your tubers in the fall and store them in a safe environment.  

As we dig our tuber clumps up, we carefully hose them off and make sure they are labeled properly.  We then leave the whole clump to dry for several days in our shop.  The original tuber we planted in the spring has used up it’s energy stores and will no longer benefit the plant. Only the new tubers produced over the summer will be viable for next year.  The clumps can either be divided in the fall when you dig them, or they can wait until you have time during the winter or early spring. If you leave the clump whole, it does take up more storage space. On the other hand, dividing in the spring can be helpful as you can better see the eyes on the tubers as they begin developing.  It’s really just a personal preference, and can often just depend on how busy your schedule is.   

Splitting your tubers is probably the most intimidating part for those new to dahlias.  While there are many How To videos on social media, blogs, and grower websites, the best recommendation I have for new growers is to find a local gardener, and spend an afternoon watching them divide clumps.  They may even be willing to trade tubers for help dividing once you’ve gotten the hang of it.  Nothing beats hands on experience and in person learning.

Also very important, remember to label your tubers!! After the outer skin has dried a bit, a permanent marker is a great way to mark your tubers. Alternatively, if you are storing as a clump, you can use a marker tag wrapped around a tuber (we use these).

Storing your tubers correctly can be tricky depending on your circumstances. Too much moisture can cause rot over the winter, but keeping them too dry will leave you with shriveled, sad tubers come spring.  Some people store their dahlias in sand, peat moss, vermiculite, or shavings.  Others wrap individual tubers in plastic wrap or newspaper.  We are able to store ours nestled into pine shavings in large totes; placed in our unheated but insulated garage.  Other people I know use their basements or walk in coolers.  It should be cool, dark, and dry, without major temperature swings.  Whatever storage method you use, check in on them every month or so.  Remove any rotting pieces so they don’t ruin their neighbors too.  

Tubers vs Seeds

Dahlias reproduce themselves in two ways at once.  While tubers are the most common method for growing dahlias, spent blooms that have been pollinated and left to properly ripen will also produce seeds you can save. While both of these methods will produce flowering plants the following year, there is a big difference between them.

Seeds collected from a bloom will not produce flowers true to the mother plant.  The cross pollination from different blooms will create all kinds of hybrids. Unless the pollination is very controlled (such as by covering blooms with mesh bags and hand pollinating), the results will be a mystery. In fact, different seeds from each seed-head can produce radically different blooms.  Sometimes these hybrids create an amazing plant, sometimes not.  If you have the time and space in your garden, it can be a fun project to play around with.  

Tubers, on the other hand, are always true to the mother plant.  When you split a clump at the end of the season, every tuber with a viable eye will produce the exact same plant the following year. This is the most common way dahlias are propagated and sold.  

Classifications

In your dahlia research and shopping, you’ve probably come across lots of terms and codes you weren’t sure about.  One can easily get lost in the descriptions and classes.  According to the American Dahlia Society, dahlias are generally classified based on their size, form, and color. In order to understand the ADS Classification Numbers, you must first speak the code:

The sizes of dahlias are:

The six main forms include:

There are 15 color classes that are used to classify dahlias:

Those considered “Other Types”:

You’ve probably come across many of these descriptions as you’ve shopped tubers or walked around an exhibition show, or maybe noticed a four digit code under the variety name. This number is found by listing the corresponding number for size, then type, and lastly, the color code.   

For example, Diva (a personal favorite!) would be considered a size small at 4-6” (BB), is a Formal Decorative (FD), and is purple (PR). The corresponding American Dahlia Society code for that tuber would then be 3009. 

Since “Other Types” don’t use the size code numbers, Day Dreamer would have 73 as the first two numbers as it’s a waterlily, and since the main color is yellow the last two digits would be 02; making the four digit code 7302. 

Is all this important to know? If you’re just wanting to enjoy some backyard blooms, probably not.  But, it helps to understand the lingo when you’re shopping so you know exactly what you’re getting.  And who knows! You may enjoy growing dahlias so much, you’ll want to start creating your own varieties, or even enter some of your blooms in a show.  But be warned, once you start collecting dahlia varieties, you’ll never want to stop!