When to Plant Dahlias

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Dahlia Cafe Au Lait blooming in garden
Grow your own deliciously romantic Cafe au Lait dahlias.

When to plant dahlias; that’s a commonly asked question. The best time for planting these cold averse beauties depends on where you garden. Here’s how to gauge the ideal timing for your region.

When to Plant Dahlias Outdoors

Plant your dahlias outdoors when the air is consistently at least 60-65 degrees overnight. This generally means the soil in your garden is about that same temperature and that’s perfect.

Vegetable gardeners, use your timing for planting tomatoes as a guide. When do you plant dahlia bulbs? Whenever it’s safe – from late frosts and cold nights – to set out your tomatoes. Planting too soon, in chilly, damp spring soil can result in dahlia tubers that just sit there. Or worse, they may begin to rot.

The first wild tomatoes originated in the Andes Mountain in South America. Dahlias are native to high plateau regions in Mexico. Both regions offered warm days and cooler nights, the temperature combination that encourages flower production in dahlias and tomatoes. Think of these two as seasonal sisters in the garden.

When to Start Dahlias Indoors

Another common question: when to plant dahlias indoors. Dahlia tubers can be started indoors to get a jump on the season and to extend the number of weeks your plants will flower.  We do this every season because more flowers are better, right?  Yes!

Dahlias sprouting in plastic pots
Those first little dahlia sprouts are so exciting!

Start your dahlias inside about 4-6 weeks before they can be transplanted outdoors. You want to give the tubers time to wake up and sprout, but not so much time that they grow too large before the weather outdoors is suitable for transplanting. Resist the urge to start them extra early.

If you’re thinking “I don’t really have a super sunny window for starting the plants inside”, don’t worry. Early on, dark is okay.

For most of that 4-6 week period the dahlias will be under the soil in your pots, in total dark. Just waking up. It’s only after the sprouts appear that sunlight is a consideration.

Also, do consider temperature, as that’s a key factor for dahlia growth. Warmth nudges the tubers to wake up. Dahlia clumps started in a 75 degree room sprout sooner than those grown in a 60-65 degree room. Temperatures much below 60 degrees encourage your dahlias to keep sleeping. Zzzzzz.

Choosing a Pot 

Use the size of the tubers (bulbs) or tuber clumps to guide pot selection. Some dahlia cultivars produce huge clumps while others are always much smaller. Don’t worry, tuber or clump size has little to do with vigor or with the final size of the plant.

Choose a container that’s large enough to allow the tuber or clump to be placed with the collar area, where the tubers attach to last season’s stem, to sit 3″ deep in the soil.

We prefer plastic pots for starting dahlias. Soil moisture doesn’t evaporate through the sides of plastic containers as it does with terracotta and fiber pots. Limiting the evaporation helps avoid puzzling over “hmm, the sides are dry, does that mean the soil in the middle of my pot, where the tubers are, is dry too?” That raises the question of whether to water or not. Non-porous containers are easier to manage.

Make sure whatever material you choose for starter containers, there is at least one drainage hole in the bottom. If not, add one or choose new pots. Pooled water will rot tubers and that’s never good.

For starting single dahlia tubers, we use light weight 1 gallon nursery pots. They are 6.5″ at the top, 5″ at the base and 7″ tall. For dahlia clumps, which are both larger and often oddly shaped, we use 2 gallon nursery pots which measure 9″ x 9″ x 9″. Occasionally, it’s necessary to hunt for a larger container for really a large or weirdly formed tuber clump.

When it’s time to transplant the sprouted dahlias outdoors, these pots are washed, stacked in the shed and stored for next year’s use.

Watering Dahlia Tubers

Plant your tubers, water once to settle the soil and then try to refrain from watering again until you see sprouts. If no sprouts are visible after 3-4 weeks, add just a little water.

Wet soil encourages dahlia tuber rot; very lightly moist soil is the goal. At this stage little moisture is needed because the plants don’t yet have any stems or leaves. You’re just trying to provide a little “hey-wake-up” nudge, not a big drink.

Flowering Ivanetti ball dahlia
Wine colored dahlia Ivanetti adds depth to summer bouquets.

Waiting for Sprouts: Everyone Hates to Wait

If you’ve grown dahlias before you know that they sprout when they’re good and ready. Some plants produce that first ground breaking sprout in under two weeks. We’ve also had perfectly healthy dahlia roots that took almost 6 weeks to show themselves.

The slow pokes tend to catch up by mid summer so don’t throw in the towel if you have a plant or two that seems to be doing nothing.

If you simply can’t stand waiting, here’s a little way to cheat. Gently, carefully, sweep away the soil in the pot where the sleepy dahlia is planted. Use the tip of your finger, take your time. The last thing you want to do now is tear off the top of a sprout that’s just under the soil surface, getting ready to emerge.

Gradually, dig down to where the tuber or clump is and look for signs of sprout development. Found a little shoot? Yea! Cover it back up with soil (we’ll never tell) and wait for it to work it’s way to the surface. No sprout yet? Give it another week or two.

Now that your know when to plant dahlias in your part of the country, need help with how to plant them? Find all you need to know here: Dahlia Planting Guide

Ready to choose dahlias for your garden?  Find lots of delicious choices here: Buy Dahlias

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