Introduction
Finding a forgotten bag of dahlia tubers in the back of the garden shed can spark a mix of excitement and curiosity. You might wonder if it is too late to get these beautiful, nutrient-storing structures into the ground or if you should simply wait until next year. At Longfield Gardens, we believe that every gardening moment is an opportunity to learn and grow, even when the traditional planting window has passed. This article explores the possibilities of planting dahlias in August.
This article explores the possibilities of planting dahlia tubers in August. We will look at how your local climate and USDA zone play a major role in your success and what you can realistically expect from a late-summer start. If you need a quick reference, start with the USDA hardiness zone map. Whether you are in a warm southern region or a cooler northern state, there are ways to make the most of your tubers.
While spring is the most common time for planting, August can offer a unique second chance for color or a head start on next year’s garden. Our goal is to provide clear, practical steps to help you decide if an August planting is right for your yard.
Understanding the Dahlia Growth Cycle
To answer if you can plant in August, it helps to understand how a dahlia grows. For more on that underground start, see Dahlia Tubers: What You Need to Know. Most dahlias are considered "90-day plants." This means from the moment you put the tuber in the warm soil, it usually takes about three months to produce its first flower.
When you plant in May, your dahlias typically begin their show in July or August. If you plant a tuber on August 1st, the 90-day clock suggests you would see blooms around November 1st. For a closer look at the timeline, read How Long Does It Take for Dahlias to Grow?. For many gardeners, this timeline is the deciding factor.
Success depends heavily on when the first "killing frost" arrives in your area. A killing frost is a freeze that turns the plant's foliage black and ends the growing season. If your first frost usually happens in October, an August planting may not have enough time to bloom. However, if you live in a region where frost does not arrive until December, August is actually a fantastic time to plant.
Planting in Warm Climates (Zones 8-11)
For gardeners in the southern United States and parts of the West Coast, August is not too late at all. In fact, in places like Florida, Arizona, and Southern California, planting in August is a common strategy for what is known as "succession planting."
Succession planting is simply the practice of planting crops or flowers at different times to ensure a continuous supply of blooms. In very hot climates, dahlias planted in early spring often struggle with the intense mid-summer heat. By the time August arrives, those spring-planted dahlias might look a bit tired.
Planting a fresh round of tubers in August allows the plants to grow as the temperatures begin to dip slightly in the fall. These dahlias will often produce spectacular, vibrant flowers in October, November, and even December. Since the nights are cooler during this period, the flower colors are often more intense than those produced in the heat of July.
Benefits of Late-Summer Planting in the South
- Avoids peak heat: Young plants aren't forced to endure the harshest July sun.
- Vibrant colors: Cooler fall nights enhance the pigment in the petals.
- Extended season: You can enjoy fresh cut flowers while the rest of the country is preparing for winter.
Planting in Cooler Climates (Zones 3-7)
If you live in a northern state where the growing season is shorter, planting in August is a bit more of an experiment. In these zones, you likely will not see flowers before the first frost hits. However, there are still two very good reasons to put those tubers in the ground rather than leaving them in a bag.
Reason 1: Saving the Tuber
Dahlia tubers are living things that contain moisture and energy. If you leave a tuber in a bag from spring all the way until the following May, it will likely shrivel up and die. Planting it in August, even if it doesn't bloom, allows the tuber to wake up, grow roots, and stay hydrated in the soil.
Reason 2: Building Energy for Next Year
Even without flowers, a dahlia plant will grow foliage. Green leaves perform photosynthesis, which creates energy that is stored back in the tuber. By planting in August, you are essentially "recharging" the tuber. When you dig it up in the fall after the frost, it will be much healthier and more likely to sprout vigorously the following spring than if it had stayed in a box.
Key Takeaway: In northern zones, don't expect flowers from an August planting. Instead, view it as a rescue mission to keep your tubers healthy and strong for next year.
How to Plant Dahlias in August
The process for planting in August is very similar to spring planting, but with a few adjustments for the summer heat. For a basic walkthrough, see How to Plant Dahlias. Proper care during the first few weeks is essential because the sun is much stronger in August than it is in May.
Choose the Right Spot
Dahlias love the sun, and How Much Sun Do Dahlias Need? explains why. They need at least six to eight hours of direct light to grow strong stems. However, if you are planting in the heat of August, a spot that gets a little bit of shade in the late afternoon can help prevent the young sprouts from wilting.
Soil and Drainage
Dahlias need "well-drained soil." This means soil that lets water pass through easily rather than letting it sit in a puddle. If your soil is heavy clay, you can improve the drainage by mixing in some compost or planting in a raised bed.
Planting Depth and Spacing
- Dig a hole: Make it about 4 to 6 inches deep.
- Place the tuber: Lay it horizontally (flat) at the bottom of the hole.
- Check for the "eye": If you see a small bump or sprout (the eye), point it upward. If you can't tell where the eye is, don't worry; the plant will find its way up.
- Cover with soil: Fill the hole back up, but do not pack the soil down too hard.
- Space them out: Give each plant about 18 to 24 inches of room, which works well for border dahlias. Good "airflow" (the movement of air between plants) helps prevent leaf diseases later on.
The Most Important Rule: Watering
When you plant dahlias in the spring, the general rule is to wait until the plant pokes through the soil before you start watering. This is because the soil is usually cool and moist, and too much water can cause the tuber to rot before it starts growing.
August is different. The soil in August is hot and often very dry. You should water the area immediately after planting to settle the soil and provide the tuber with the moisture it needs to "wake up." For more summer-care advice, see 8 Tips for Growing Better Dahlias.
Check the soil regularly. You want the soil to feel like a wrung-out sponge—moist but not soaking wet. If the top inch of soil feels dry, it is time to water. Deep watering once or twice a week is usually better than a light sprinkle every day because it encourages the roots to grow deep into the ground.
Managing Late-Season Pests
August is a time when many garden insects are most active. When your dahlia sprouts its first tender green leaves, it might attract some unwanted visitors.
- Slugs and Snails: They love young dahlia shoots. Check your plants in the evening or early morning. You can use simple barriers or organic baits to keep them away.
- Aphids and Thrips: These tiny bugs can sometimes cluster on the new growth. A strong spray of water from the garden hose is often enough to knock them off.
- Japanese Beetles: These are common in late summer. The easiest way to handle them is to hand-pick them and drop them into a bucket of soapy water.
Focusing on keeping the plant healthy and hydrated is the best defense against pests. A strong, vigorous plant is much less likely to be damaged by insects.
What to Do When Frost Arrives
Regardless of whether your August-planted dahlias bloomed, the first frost will signal the end of their growth. Once the leaves turn brown or black from the cold, it is time to take action.
In Zones 8-11, you may be able to leave your tubers in the ground over the winter if your soil drains well and the ground does not freeze. Many gardeners in these areas simply cut the stalks down to a few inches and cover the spot with a layer of mulch for protection.
In Zones 3-7, you must dig up the tubers to save them. Even if the plant only grew a foot tall, the tuber underground has been working. For a step-by-step storage refresher, see How Do I Store Dahlia Tubers?.
- Wait a few days after the first frost.
- Carefully dig up the tuber clump.
- Shake off the excess dirt.
- Store them in a cool, dark, dry place (like a basement or crawlspace) in a box with some peat moss or wood shavings.
Can You Grow August Dahlias in Containers?
If you are worried about the upcoming frost, planting your dahlia tubers in pots is a smart alternative. For a fuller walkthrough, see Can You Grow Dahlias in a Container?. Containers give you more control over the environment.
Using a pot that is at least 12 inches deep and 12 inches wide is ideal for most varieties. Ensure the pot has drainage holes at the bottom so the tuber does not sit in water. The beauty of container gardening in August is portability. If a surprise early frost is predicted, you can simply move the pot into a garage or onto a porch to protect the plant for a few more weeks.
This is a great way to enjoy blooms even in cooler climates. If you start a tuber in a pot in August, you can bring it indoors near a sunny window or into a greenhouse when the weather turns cold, potentially extending your bloom time into the late autumn.
What to do next:
- Identify your USDA hardiness zone to determine your first frost date.
- Check your stored tubers for any that look firm and healthy.
- Prepare a sunny spot with well-draining soil.
- Water deeply immediately after planting to combat August heat.
Realistic Expectations for August Planting
It is important to be realistic about what an August planting can achieve. Gardening is an adventure, and while we want every plant to thrive, nature sometimes has its own plans.
If you plant in August, your plants will likely be shorter than those planted in May. This is because they have less time to build a massive root system and tall stalks before the days start getting shorter. Fewer hours of daylight in the fall also tell the plant to focus more on tuber production and less on height.
However, a shorter plant can still be a beautiful plant. Many smaller dahlia varieties, such as border dahlias or ball dahlias, are perfectly happy at a shorter stature and will still produce lovely flowers if the weather stays mild.
Choosing Varieties for Late Planting
If you are buying new tubers specifically for an August planting, look for varieties that have a shorter "days to bloom" window. While big Dinnerplate dahlias are stunning, they often take the longest to reach maturity.
Smaller varieties, like Mignon or PomPon dahlias, often bloom faster. These are excellent choices for a late-summer start because they maximize the remaining weeks of the season. At Longfield Gardens, we offer a wide variety of shapes and sizes, and our dahlia collections can make it easier to find a faster option even with a late start.
Troubleshooting Common August Issues
Because August is different from spring, you might run into a few unique challenges. Here is how to handle them with ease.
Wilting New Growth
If your new sprouts look limp during the heat of the day, they are likely just thirsty or stressed by the sun. Check the soil moisture. If it's wet, the plant is just reacting to the heat and will perk up in the evening. If it's dry, give it a deep drink.
Slow Sprouting
In the spring, it can take 2 to 4 weeks for a dahlia to sprout. In August, it might actually happen faster because the soil is so warm. If you don't see anything after three weeks, gently scratch the soil away to see if the tuber is firm. If it is firm, it is just taking its time. If it is mushy, it has rotted, usually due to poor drainage.
Yellow Leaves
Yellow leaves at the bottom of the plant can be a sign of too much water. Make sure your soil isn't staying "soggy." If the yellowing is at the top, the plant might need a light feeding with a balanced, water-soluble fertilizer. Follow the instructions on the label and avoid over-fertilizing late in the season.
The Joy of the "Second Season"
There is something special about flowers that bloom when the rest of the garden is starting to fade. Dahlias are the champions of the autumn garden. Their colors—deep burgundies, burnt oranges, and golden yellows—perfectly match the changing leaves of the trees.
Planting in August is a way to lean into that "second season." While other gardeners are putting away their tools, you are starting a new project. It keeps the joy of gardening alive well into the fall months. Even if your experiment only results in a few blooms or a healthier tuber for next year, the time spent in the dirt is always worth it.
Summary of Late-Season Care
Success with August planting comes down to a few simple steps. By focusing on moisture and protecting young plants from extreme heat, you can overcome the challenges of a late start.
- Water right away: Unlike spring planting, August tubers need immediate moisture.
- Mulch the soil: A 2-inch layer of straw or shredded bark helps keep the roots cool and holds moisture in the soil.
- Watch the weather: Be ready to protect your plants if an early cold snap is predicted.
- Keep it simple: Don't worry about complex pruning or "pinching" for August plantings; just let the plant grow naturally to save time.
"August planting isn't about following the rules—it's about making the most of the season you have left. Whether you get a handful of blooms or just a stronger tuber for next spring, it's a win for your garden."
FAQ
Will my dahlias bloom if I plant them in August?
This depends entirely on your climate. Dahlias usually need about 90 days to bloom. If your first frost is at least 90 days away from your planting date, you have a great chance of seeing flowers. If your frost comes sooner, the plant will likely focus on growing foliage and strengthening the tuber instead of blooming.
Is it better to plant a forgotten tuber in August or wait until next spring?
It is almost always better to plant it in August. A dahlia tuber is a living structure that can dry out and die if left in storage for too long. Planting it now allows it to hydrate and store energy, making it much more likely to survive until next year than if it stayed in a bag.
Do I need to fertilize dahlias planted in August?
You can use a balanced, water-soluble fertilizer once the plant is about 6 inches tall, but don't overdo it. In late summer, you want to avoid high-nitrogen fertilizers that encourage too much soft green growth, as this can be easily damaged by the coming frost. For more feeding and care ideas, see How to Best Grow Dahlias. A light, general-purpose feeding is usually sufficient.
How deep should I plant dahlias in the summer heat?
You should still plant them about 4 to 6 inches deep. This depth protects the tuber from the fluctuating temperatures at the soil surface and helps keep the roots cool. Adding a layer of mulch on top of the soil is also a very helpful step for August plantings to prevent the ground from drying out too quickly.
Conclusion
Planting dahlia tubers in August is a wonderful way to extend your gardening season or rescue tubers that might otherwise be lost. While the timing is later than usual, gardeners in warm climates can enjoy a spectacular "second act" of autumn blooms, while those in cooler areas can ensure their tubers remain healthy and ready for a vigorous start next spring.
Gardening is rarely about perfection; it is about working with nature and enjoying the process. At Longfield Gardens, we love seeing how dahlias can surprise us with their resilience and beauty, no matter when they are planted.
- August planting is ideal for succession color in Zones 8-11.
- In northern zones, August planting saves the tuber from drying out.
- Consistent watering is the most important factor for summer success.
- The 90-day growth cycle determines if you will see flowers before frost.
We encourage you to get those tubers into the ground and see what happens. You might just end up with the most beautiful surprise of the season. To find more tips on growing a vibrant garden, visit our website for planning guides and expert advice.
