One of the riskiest aspects of growing dahlias is timing bulb planting correctly. You want them planted early to enjoy their beautiful blooms every day possible, but you don’t want cool temperatures and wet soils to rot away your investment. Finding that planting time sweet spot is critical to getting the most enjoyment from your bulbs.
Before getting started, keep in mind these planting times are highly dependent on your local climate and if you overwinter bulbs in the ground or dig them up and store them. The dates for warmer areas are a guideline for planting new bulbs; the dates for colder areas are planting new bulbs or replanting bulbs from the previous season.
When Should I Plant Dahlia Tubers Outside?
The general guideline for getting dahlias in the ground is after the spring frost-free day once soil temperatures climb above 60°F. Exact dates vary from one growing zone to the next and are always subject to change depending on the individual season. After all, all areas are prone to random weather episodes once in a while.
Determining Your Local Frost Free Day
Most people are familiar with their USDA hardiness zone or growing zone but don’t know their frost-free date. It is an average date based on past climate data and tells approximately when the last light freeze (29° to 32°F) occurs in spring.
Keep in mind frost-free dates are not set in stone. They are only estimates.
General Timelines For Planting Dahlias Outside
The best way to time planting is by checking your soil temperatures every couple of days. Once the soil is warm enough, plant your tubers. If you don’t want to measure soil temps, find your local frost-free date and add at least two weeks. This method typically puts you past a light freeze, but the soil temps may be low.
Considering average frost-free dates, this is the earliest you should plant dahlias in your growing zone. Zones 10 and higher typically don’t freeze.
- Zone 3: Plant mid to late-May or early June
- Zone 4: Plant early to late-May
- Zone 5: Plant late-April to mid-May
- Zone 6: Plant mid-April to early-May
- Zone 7: Plant early to mid-April
- Zone 8: Plant late-March to mid-April
- Zone 9: Plant late-February to mid-March
How Late Is Too Late To Plant Dahlias?
Now that we’ve talked about the earliest you can plant, let’s touch on when it becomes too late to plant. Dahlias need at least 120 days in the ground before they go dormant or are dug up for the winter. To determine when it’s too late to plant, figure out your first autumn frost date and count backward three months.
Successfully Growing Dahlias In Your Growing Zone
Dahlia plants are classified as “tender perennials.” They are perennials that will come back year after year, but they aren’t winter hardy. Their lifecycle depends on your hardiness zone. In warmer climates, they grow as perennials without extra care. In the coolest climates, they grow as annuals, or you must dig them out of the ground for winter.
To explain a little further:
- Zone 10 & Warmer: You can grow dahlias as perennials, and they don’t need any winter protection. The tubers survive in the ground through the cold temperatures.
- Zones 8 & 9: You can grow dahlias as perennials, which can overwinter in the ground. They do need winter protection, though. Cover the planting bed with straw or mulch until spring after the foliage dies back following a hard autumn frost.
- Zones 7 & Colder: You can grow dahlias as perennials only if you dig the tubers and overwinter them between 40 and 50°F, replanting them in spring. If you leave them in the ground, they will not survive the freezing temperatures and rebloom the following year.
Determining Your USDA Plant Hardiness Zone?
If you aren’t sure what plant hardiness zone you live in and want to grow dahlias, it’s easy to find out using the USDA Hardiness Zone Finder. The continental US is broken up into zones based on their annual low winter temperatures. These zones are separated into 10°F regions from 2 to 10 and help determine how to grow dahlias.