There are many reasons to plant dahlias in your garden. First, the plants are easy to grow and offer months of gorgeous blooms in a wide assortment of flower shapes, sizes, and colors. You can enjoy those blooms in the garden or cut them for beautiful bouquets and arrangements. Healthy, vigorous plants offer the greatest display of flowers and the best way to get a strong start to the growing season is knowing when to plant dahlia bulbs. Below I’ll walk you through all the options of when to plant dahlia bulbs.
What are dahlias?
Dahlias are tender perennials and originate from parts of Mexico, Guatemala and South America. There are over 10,000 dahlia cultivars with flowers that come in a wide assortment of shapes, sizes, and colors. The plants flower from mid-summer until frost and dahlias have become very popular with gardeners who want to grow their own cut flowers. The diversity of blooms as well as the long straight stems and 5 to 7 day vase life make dahlias a perfect cut flower. Plus, they’re easy to cultivate when given the right growing conditions.
While many gardeners refer to dahlia tubers as ‘bulbs’, botanically they are tubers. For simplicity’s sake, I’ll use bulb and tuber interchangeably in this article. Unlike spring flowering bulbs, dahlias are only winter hardy in USDA zones 8 to 11. Gardeners in colder climates, like mine, treat them like annuals and dig up the tubers in autumn to winter over indoors. Those in warmer parts of the country can leave dahlias in the ground to return the following spring.
Why it’s important to know when to plant dahlia bulbs
Dahlias are tender tubers and if they’re planted too early in the spring, the plants may be damaged by frost and cold temperatures. Wait too long to plant and they won’t have time to flower before autumn arrives. When you understand timing, you can plant outdoors at the right time or encourage extra-early flowers by starting the tubers indoors.
Where to buy dahlia tubers
Dahlia tubers are available at garden centres, nurseries, and online bulb suppliers. I place my orders the previous autumn or early winter with the tubers shipped the following spring. If you wish to buy dahlias locally, they typically start to appear in garden centers a month or two before you can plant them outdoors. Online orders also arrive weeks before the planting date. Store tubers in a cool, dark place until you’re ready to plant. It’s a good idea to check them periodically to ensure they’re not sprouting. If they have begun to grow, remove the tubers from their packaging and pot them up.
The best site for growing dahlias
Dahlias thrive when planted in a site that offers at least 8 hours of direct sunlight each day. They need rich soil, but also well-drained soil as too much water can cause the tubers to rot. To ensure the soil is fertile, I dig in organic matter like compost or rotted manure before planting. I also add a granular organic flower fertilizer to provide a steady source of nutrients like nitrogen, phosphorous, and potassium.
No space? Plant dahlias in pots. Look for a container that is at least 12 to 15 inches in diameter with several drainage holes on the bottom. Fill it with a blend of potting mix and compost, and add a flower fertilizer to provide nutrients. Learn more about growing dahlias in containers.
When to plant dahlias: 3 easy options
There are several possibilities for when to plant dahlia bulbs. Which you choose depends on the length of your growing season and whether you wish to speed up the flowering process. Here are the 3 options for planting dahlia tubers:
- Planting dahlia tubers directly in the garden
- Starting the tubers indoors 4 to 6 weeks before the last frost date
- Transplanting potted dahlias outdoors
Let’s look more closely at each of these options.
Option 1 – When to plant dahlia bulbs directly in the garden
As noted above, dahlias are heat-loving plants and not tolerant of frost or cold weather. Plant the clumps of tubers in the garden or large pots when the soil and air temperature are reliably 60 degrees F (15 degrees C). It’s important to know when to plant dahlia bulbs outdoors as the tubers won’t grow if the soil is still cold. In fact, cold, wet soil can cause tubers to rot. It’s best to wait to plant until the soil has warmed up and dried out from spring rains.
Planting time for dahlia tubers is typically late spring from mid-April through mid-June, depending on your climate. Plant after the danger of frost has passed, around the same time you’d plant tomatoes and peppers outdoors. When the time is right I prepare the bed and then insert a sturdy stake for each plant. Putting the stake in the ground before you plant prevents damage to the clumps of tubers. Using a transplanting spade or hand trowel, dig a hole 5 to 6 inches deep and wide and place the tuber with the stem end and eyes facing up. Carefully replace the soil, firming slightly. The tuber should be covered with about 2 inches of soil. Space dahlias 18 to 24 inches apart. Unless there is a prolonged drought, there’s no need to water until the shoots emerge through the soil a week or two after planting.
Option 2 – When to start dahlia bulbs indoors
Because I live in a cold climate, I like to start my dahlia tubers indoors under grow lights. This gives me a strong start to the growing season and good-sized plants when the time is right to move them outdoors. About 4 to 6 weeks before the last frost date I gather my supplies and pot up the tubers. Here is a step by step guide to starting dahlia bulbs indoors:
- Step 1 –Gather your supplies. I use 1 to 2 gallon nursery pots, a high quality potting mix, labels, a waterproof marker, and of course the dahlia tubers.
- Step 2 – Add enough pre-moistened growing medium to the pots so that you’ll have room to accommodate the tubers and cover them with 2 inches of soil. Gently firm the growing medium to ensure there are no air pockets around the tubers.
- Step 3 – Place the pots beneath a grow light, in a heated greenhouse, or in a sunny window.
Keep the soil slightly moist and watch for the green sprouts to emerge. Once the shoots have emerged from the growing medium, you can start to water as needed. I also fertilize the plants with a liquid organic fertilizer when the shoots are several inches tall.
Option 3 – When to transplant dahlias outdoors
If you started dahlia tubers indoors, it’s time to transplant them into the garden 4 to 6 weeks later. At this point, most of the tubers will have sprouted and many may be a foot tall or larger. Start the hardening off process a week before you intend to move the plants to the garden. Hardening off acclimatizes the young plants to outdoor growing conditions and is done the same way tomato seedlings are hardened off.
Transplant the dahlia plants into prepared beds after the last frost date and when the temperatures are reliably above 60 degrees F (15 degrees C). If the temperature dips after transplanting, cover the dahlia bed with a row cover. Space the plants 18 to 24 inches apart. Be sure to insert a label or tag with the dahlia name to keep track of your varieties.
Caring for dahlias
Dahlias are easily cultivated and staying on top of the three tasks below helps ensure vigorous plants:
- Toilet – Dahlias need lots of moisture to produce a good show of flowers. Wait until the foliage has emerged from the soil and then start watering weekly if there has been no rain. In summer, I deep water twice a week and mulch the plants with straw to hold soil moisture. You can also run a soaker hose or set up drip irrigation for easy watering.
- pinch – Pinching dahlias results in bushier plants and plenty of dahlia flowers. Using my fingers, I pinch when the plants are 10 to 12 inches tall by removing the top of the main stem.
- Fertilize – Dahlias require plenty of nutrients to produce months of show-stopping flowers. As mentioned above, I add a granular organic fertilizer at planting time. I follow this up with a liquid organic flower fertilizer every 3 to 4 weeks during the growing season.
There are various pests that affect the flowers and foliage of dahlia plants. Common culprits include aphids, earwigs, and snails and slugs. Monitor your plants for pests, taking action when necessary. I handpick or set traps for earwigs, use diatomaceous earth for slugs and snails, and knock aphids off the flowers with a jet of water from my hose.
For more information on growing dahlias and other summer flowering plants, be sure to check out these articles:
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