When To Plant Dahlias in Zone 5 For The Best Results

Patience and grit, commitment and optimism. These are the must-have qualities of a zone 5 gardener. Winter does not pass quickly in these parts. It takes its sweet, snowy time. For dahlia lovers, anxious to bring their favorite dahlia tubers up from the basement and get some whimsy back in the yard, spring can feel like an eternity.  

There are some plants that zone 5 gardeners can get into the ground earlier. But, dahlias need a little more time in order to be out of potential freeze territory and put their beautiful colors on display. They do not like the cold, and will not survive a cold snap without plenty of help.

In this article, you’ll learn all about planting dahlias in zone 5. You’ll learn exactly when to plant them in order to get the best results, along with a few tips that will help you get better dahlia blooms. Ready to learn a bit more about planting these gorgeous flowers in a colder climate? Let’s jump in!

Planting Dahlias in Zone 5

We’ve been told the safe date for planting dahlias in zone 5 is typically May 14. But we also know this date is based on an average and is not a guarantee. The National Centers for Environmental Information (a division of NOAA) arrives at this recommendation using 30 years of recorded dates. These dates in this spread have at least a 50% chance of freezing temperatures.  

However, while freeze ‘normals’ are based on this data, they do not account for severity. With no distinction made between mild, moderate, or severe freeze dates in history, there is certainly a lot of room for variability. And there’s no accounting for a multitude of microclimates within our zone.

Sometimes, due to differences in geography and infrastructure, the temperature on one side of the street can be 5-10 degrees lower than temps on the other. I live near Lake Michigan, which throws off chilly air long into the growing season. I can tell you first hand that magnolias bloom near the lake almost a full two weeks after they start popping a mile or two inland.  

Anyone who’s lived in zone 5 long enough (and worn a down jacket to a baseball game in early summer) will tell you that mild days come in fits and spurts. We know that planting anything outside before Memorial Day is a major garden gamble and a common rookie mistake. So we’ve adopted this bookend holiday as the ‘real’ date to plant outside safely.  

This is particularly true for all dahlia species, as these heat-loving, sun mongers do not take well to frosty nights. As tender tubers with roots that are vulnerable to freezing, dahlias require a soil temperature of 60 degrees before they can be safely planted outside. They also take at least a hundred days to bloom. So, it’s important to get shoots going as early as possible. Fortunately, there are a couple of ways to make that happen. 

Potting Up  

In zone 5, dahlia growth can be started in pots sometime in late April. Hopefully, you’ve saved your gallon-sized nursery pots from the previous season and can use these to get things going. If not, any gallon pot with good drainage will do the trick.  

Fill containers halfway with potting mix that contains a slow-release, general fertilizer, and place tubers with their ‘eyes’ facing up. Found above a root’s neck, the eyes are the location from which sprouts will grow. They may even be off to the races by now, depending on how warm your tubers were in storage. Cover loosely with about five more inches of growing medium. Then let them get acclimated for a day or two before watering lightly.  

Pots may be covered loosely with plastic wrap or a plastic dome if you’re looking for immediate gratification. However, this step isn’t necessary and shoots will grow when they’re ready. Keep pots in a bright location on a tray to collect drainage. Then water sparingly until they’re ready for their time in the sun.  

Starting From Seed 

While dahlias will grow more quickly and easily from an established tuber, you can start them in trays if that’s what floats your boat. Some time in mid-April, fill seed trays with a seed starting mix and place seeds on top.

Sprinkle a bit more medium over the seeds (just enough to cover lightly), then move trays to a bright location that will remain consistently warm (65-70 degrees) or place under a grow light. Seedlings should sprout within a week or so. Keep moist using a spray bottle or mister, but do not overwater. 

Hardening Off 

If you just can’t wait until June 1 to bring your babies outside, you may begin a process called ‘hardening off,’ in late May. This involves bringing trays for a few hours on day one, a little longer on day two, and so on. It’s not an exact science but it will help your budding beauties to adapt more naturally to their new outdoor homes.  

Outside Planting 

After using your gut and/or your calendar to determine the right day for planting, prepare your garden beds to receive their new residents and get them in the ground. Follow each cultivar’s direction for proper depth and spacing, and don’t forget to plant their stakes at the same time. Inserting them later might damage your dahlias’ root systems. Do not soak plants, but you should keep them evenly watered since wet soil retains more heat than dry soil. 

Extra Protection 

If the risk of frost has passed, you’ve selected a location that gets at least 6 hours of direct sunlight, and your dahlias are not exposed to high winds or freezing microclimates, your chances of successful growth are very good at this point. But what if you jumped the gun, or if the weather forecast has you wringing your dirty hands with worry?  

There are a couple of things you can do to protect your dahlias from being damaged or killed by a rogue frost. If you know a cold snap is coming, you can tent young growth with a blanket or tarp, using stakes or ties to make sure fragile stems are not damaged.

This works particularly well for large beds of dahlias, but maybe not so much for smaller groupings. The bottom of a severed water bottle, milk jug, or orange juice carton turned upside down over each individual sprout might be a bit time consuming, but it will usually do the trick.  

Dealing With Cold Snaps

For quick cold snaps, a lower maintenance solution might be piling up some mulch around plant stems for a night or two. Just make sure you return promptly to pull the mulch back after the frigid air has moved on. If left in place, mulch may burn young tissue or inhibit growth.  

If your attempts to mitigate the damage from frost should fail and your dahlia shoots are damaged or discolored, don’t fret. Unless cold temperatures prevailed for more than a week, your soil will likely have retained enough warmth to keep your tubers alive below ground.

Cut stems down to the surface and you will most likely get some new green shoots when temps warm up. If nothing happens, you’ll have learned a valuable lesson about dahlias; they do not like to be cold! Be sure to deadhead any dahlia blooms that have been bitten by frost, once you are sure they won’t come back.