Planting Dahlias: When, Where, and How Deep

While they may not be my favorite plants, dahlias are certainly pretty high on the list, right behind daffodils, lilies, and chamillias. They’re undeniably beautiful, they’re great for sharing, and they come in a rainbow of colors - of course, with the exception of blue, like tulips. They’re also fairly easy to care for, making them an excellent addition for any spring garden.

How to Plant Dahlia Tubers

We think of dahlias as grown from tubers, like potatoes, but they’re not really. They’re actually grown from tuberous roots. Unlike tubers, which are made up of stem tissue, tuberous roots are made up of actual root tissue that is slightly larger so they can store more nutrients.

Many of the vendors who sell dahlias in this country ship out a single tuberous root with an eye where it was attached to the old stem. These are fine, and still develop a plant, but they typically don’t develop as many blooms and bloom stalks. Instead, we send out whole clusters of tuberous roots so you can have a really neat plant with a lot of blooms to enjoy all season long, and planting them is simple.

When to Plant Dahlias

Dahlias are very frost-sensitive and cannot take freezing temperatures. So, in order to avoid any accidental nips of cold in the southeast, we recommend waiting until the soil warms up above 60℉. This gives them plenty of wiggle room with the weather to ensure they are still cool enough without accidentally taking a polar plunge.

In the cooler climates to the north, we recommend giving your dahlias a 2-3 week head start inside. Simply place your dahlia roots on a seedling heat mat at 70℉ and transfer outside once the temperatures are optimal for planting.

Where to Plant Dahlias

Dahlias are sun-lovers and should be planted in full sun in a good, coarse mix that drains well, like a composted media or a bark and peat mix. Most people don’t realize that plant leaves are solar collectors that charge the batteries of the plant to keep them alive. Although there are some plants the good Lord created that have bigger leaves that will get more light in shade, most - including dahlias - need a regular recharge of sun to keep them going. In the hottest parts of the country, though, some part shade may be good to protect them from the harsh afternoon heat.

How Deep to Plant Dahlias

While many bulbs are planted at a regular depth of 3-4 inches deep, dahlias should only be buried deep enough that the old stem is right at soil level. The tuberous root cluster should be buried, but the old stem should be sticking out - as that is where it will sprout from.

Caring for Dahlias

Dahlias bloom best when the nights are cool in May and June - and the blooms we get then are absolutely glorious. But by the time the night gets hot in July, the blooming tapers off and they don’t bloom nearly as much.

We keep them watered throughout the summer with drip irrigation, but not too much. They’re plants that like adequate moisture, but don’t like to be left standing in old water. However, if they’re well-kept through to the end of August, by September you’ll notice really nice, new growth once again. As the nights cool off, their glorious blooms return once more and will last until frost - so we’re always hopeful for a late frost.

A very important thing to note is that dahlias are “cut and come again” plants. The more you take off the old blooms - in particular, those that are spent - the longer they will bloom! If you let them get pollinated and they go to seed, they will stop blooming, so deadheading your old dahlias is key.

Are Dahlias Perennial Plants?

Dahlias are what we refer to as “temperennials” - or temporary perennials - for most climates. Being from the mountains of the southwest and Mexico, they come from somewhat warmer environments but grow in the cooler temperatures of the high mountains. So while they can tolerate the brisk weather of the spring and thrive in it, they are still too tender to last the winter in the garden in zones below 7 - and even those in zone 7 will need a good protective layer of mulch. However, if overwintered indoors, dahlias can easily be saved to grow again next season.

Digging Up Dahlias for Winter?

After about a week in the frost, which browns the foliage, go out and gently lift the tuberous dahlia roots with a gardening fork or spade, leaving plenty of room beside them. Once lifted, shake some of the soil off and let them dry in the sun for an hour or so and then in the shade with a fan on them.

One thing you never want to do is wash them. Unless they’re handled extremely gently, washing the roots introduces fungi and bruises that does them in. Instead, store them in sand or wood shavings with a little soil on them. Soil is filled with wonderful bacteria and fungi that take care of any bad guys coming for your dahlias, and the light media will keep them from drying out too much.

Dahlias are a captivating treat that dazzle in any garden from spring until fall, but they also make fantastic cut flowers. They’re long-lasting in an arrangement, so they’re great for making bouquets. I make a lot of arrangements with these and every time I take them to my dinner hostess, we get invited back the next week or so. They just make people smile, so spread the joy - not only in your garden, but in your life as well!