Growing Instructions

What to Expect

Your dahlias will arrive in tuber form (looking kind of like potatoes). Open the bag and inspect them immediately – many will be sprouting already, and they cannot sit for long periods in their packaging. Handle and open the bag gently to avoid breaking off any sprouts. Important note: Do NOT open the peat bag indoors if you are at all clumsy or if you have a fan running. 🙂

Each tuber will produce a shrubby plant 3.5-5′ tall and 3′ in diameter (multiple dahlias should be planted 18-24” apart). It will begin to bloom in mid-late July, and will continue to send up flowering stems through frost if you remove the spent blooms. Dahlias are not winter-hardy in areas where the ground freezes, but you can dig up the tubers in the fall and store them indoors for replanting in the spring.

Where to Plant

Dahlias need plenty of direct sunlight (5 hours/day minimum) to thrive. In hot climates a bit of afternoon shade is helpful. They prefer humus-rich, slightly acidic soil (around PH 6.5) but can tolerate a range of soil types. Good drainage is vital, as sitting in water will cause the tubers to rot. Note that dahlias are EXTREMELY sensitive to herbicides: any chemical that kills weeds will also kill or maim every dahlia plant in the vicinity. Such products should not be used within thirty feet of a dahlia bed. (Really best not to use them at all!)

It is possible to grow dahlias in large patio pots, but you really have to keep an eye on moisture and temperature. A dahlia’s pot should be 5 gallons minimum and light in color (dark pots can overheat and cook the tubers), with plenty of drainage holes. Dahlias grown in pots still require a stake or cage for support.

When and How to Plant

Dahlias should not be planted outdoors until frost danger has passed, conditions are reasonably dry, and the soil temperature is at least 55F. If you need to wait a few weeks to plant, keep the tubers in their bag covered with peat in a cool, dark place, but open the top of the bag to prevent condensation from building up. You can also start them indoors in pots to get a jump on the season. Plant each tuber in lightly moistened potting soil using a pot large enough to accommodate it (i.e. no part of the tuber touches the sides of the pot). You can angle the tuber downward to help it fit in the pot, but make sure the crown and eyes are facing up and just barely covered with ½ inch of soil. Place the pot in a warm place, and spritz the surface of the soil with a spray bottle if it becomes very dry, but don’t water until the plant is several inches tall (dormant tubers are vulnerable to rot). You’ll see sprouts in 1-4 weeks, at which point you should put the plant under grow lights or in a window where it will get plenty of sun. As the weather warms, you can start taking it outside during the day and moving it in at night. Transplant it into your garden once conditions are safe.

To plant tubers directly into the ground, dig a 12″ x 12″ planting hole, then backfill with loose soil to a depth of 6”. If your soil is clay-heavy or compacted, you can mix in a handful of powdered gypsum to make it easier for the roots to expand. Lay the tuber on its side in the hole with the eye facing up, and cover with 1-2″ of soil. Wait until the dahlia’s leaves have cleared ground level to fill in the rest of the hole around its stem. If heavy rains arrive before you’ve filled in the planting depression, you may need to cover it with an upside-down bucket so it doesn’t fill with water and drown the baby plant.

As it grows taller, your dahlia will need support to prevent it from toppling over and uprooting itself. A sturdy 5′ stake driven into the ground right behind the tuber crown at planting time (to avoid damaging roots later) will do the trick. As the plant grows, secure its stems to the stake using strips torn from an old t-shirt or pair of nylons. An even easier method is to center a three-ring tomato cage over the dahlia while it is small. No tying required, and in a month or so the foliage will conceal the scaffolding.

Food and Water

Dahlias need regular, thorough waterings to grow and bloom properly. Water the plant deeply every few days in the absence of rain. If you are growing multiple dahlias, it will save time to lay a length of soaker hose or drip tape along the row at the beginning of the season. Potted dahlias dry out quickly and should be checked on every day.

All plants require nitrogen, phosphorus, and potassium (macronutrients) to build their tissues, as well as trace amounts of various micronutrients. However, care is needed in applying fertilizer as it is possible for some of these nutrients (looking at you, phosphorus!) to build up in the soil faster than your plants can utilize them, causing toxicity. In addition to this, soil PH (acidity/alkalinity) affects the availability of nutrients to your plants’ roots and may need to be adjusted. Before beginning a fertilizer program, it’s important to submit a soil sample to your local agricultural extension or university soil test lab so you know which nutrients need adjusting. These services are available cheaply through the mail, and the test results will come back with targeted recommendations.

Pinching

When your dahlia has developed 3-5 leaf pairs (around 8″ tall), gently pinch out the central growing stem just above the top leaf pair, being careful not to damage the growth buds at the junction of stem and leaf. Do not neglect this step — it helps the plant to develop a sturdy growth habit and greatly increases flowering.

Digging

Your dahlia’s foliage will turn brown and shrivel after the first fall frost. This is natural — do not panic! The tubers are still alive beneath the soil. Cut the plant’s stem back to 6″ above the ground and allow the tubers to cure in the ground for a week before digging. (Note: if either heavy rains or a hard freeze is predicted, skip this part and evacuate all your tubers immediately.) 

Over the course of the growing season, dahlias form a clump of new tubers surrounding the central stem. To dig up the tuber clump, insert a digging fork about a foot away from the stem and gently loosen the soil. Repeat, moving around the stem in a circle, until you are able to easily lift the tuber clump by pulling up on the stem while holding the fork underneath for stability. 

Dividing

Dahlia clumps should be divided each season to keep the roots from getting cramped. This is also how your herd multiplies! Detailed instructions are beyond the scope of this guide, but there are a number of free pictorial/video resources available on YouTube. General tips:

1) Each tuber or divided group of tubers must have at least one eye in order to sprout. The eyes are located around the crown of the tuber — the raised area where it attaches to the plant’s main stem. They look like little bumps and are easiest to see when they begin to swell in the spring. 

2) Tubers with broken necks won’t sprout. Be careful to support the tubers from underneath when handling a clump; once removed from the supporting soil, gravity can be enough to snap their necks.

3) If you have to leave part of the stem attached to a tuber (sometimes you do), scrape out as much of the pith as possible. This is the part most likely to mold/rot in storage.

4) Dahlias are subject to nasty plant viruses which can be spread by cutting tools. When dividing multiple plants, always sanitize your tools between clumps. I recommend having two sets of tools on hand, along with a container of 10% bleach solution for dipping and a solution of Dawn dish soap (4 tsp. mixed into 5 c. water) for soaking. When you finish a clump, dip the tools you used in the bleach solution and then place them in the Dawn solution to soak. Remove the tools you used on the previous clump from the Dawn soak, and keep this rotation going as you work.

5) Zenport Deluxe scissors, recommended by Connie Thompson of Connie’s Dahlias, are the best tool I have tried for dividing dahlias. It’s also helpful to have some retractable utility blades for fine cuts and a pair of curved pruners to deal with woody stems. 

Winter Storage

Tuber clumps can be divided in either the fall or spring. If dividing in the fall, start by rinsing the tuber clump off with a garden hose. (Go easy on the water pressure; too much can blast off the tubers’ skin!) Divide the clump following the guidelines above. Lay the tubers out on a towel or newspaper and allow their surface to dry completely, flipping them over partway through. Drying usually takes about a day. Check them every 6-8 hours to make sure they don’t over-dry and start to shrivel from exposure to the air.

Once your tubers are dry, find an appropriately-sized storage container (cardboard box, lidded plastic bucket or bin, etc.) and cover the bottom with an inch or two of peat or vermiculite. Lay the tubers in the container, cover completely with the storage medium, and close the lid. Store them between 38F and 50F. (Note: freezing temperatures, even for one night, will definitely kill your tubers. Room temperature can work in a pinch, but keep them away from light/heaters/vents and inspect periodically to make sure they aren’t starting to sprout.)

For spring division, don’t bother rinsing the clump — simply brush off as much soil as you can and trim the stem down to a few inches. Wrap the tubers loosely with a plastic bag, leaving the stem exposed to dry for a week or so. Once the stem is crispy, trim it back, remove the bag, and pack the entire clump in your storage medium.