Can You Plant Dahlias in Summer?

Table of Contents

1. Introduction
2. Understanding the Summer Planting Window
3. Tubers vs. Potted Starts: Which Is Best for Summer?
4. The 90-Day Rule and Your Frost Date
5. Selecting the Right Spot for Summer Success
6. Step-by-Step Summer Planting Guide
7. Watering and Feeding During the Heat
8. Mulching: The Summer Gardener’s Best Friend
9. Managing Summer Heat Stress
10. Pinching and Staking for Stronger Plants
11. Heat-Tolerant Varieties to Consider
12. Looking Toward the Autumn Bloom
13. Conclusion
14. FAQ

Introduction

There is nothing quite like the sight of a dahlia in full bloom. These spectacular flowers come in an almost endless array of colors and shapes, from tiny pompons to dinnerplate varieties that are as large as a human face. For many of us, the sight of a neighbor's thriving dahlia garden in July or August sparks a sudden, urgent desire to get some of our own into the ground.

At Longfield Gardens, we believe that every garden has room for a few more dahlias, no matter the time of year. While spring is the traditional time for planting, many gardeners find themselves wondering if they have missed their window once the weather warms up. The short answer is that you can often plant dahlias in the summer, provided you adjust your approach to account for the heat and the shorter growing window.

This guide will walk you through everything you need to know about summer planting. Whether you found a forgotten bag of tubers or picked up a beautiful potted dahlia plant at a local nursery, we want to help you succeed. We will cover the timing requirements, heat management strategies, and care tips that ensure your late-season additions thrive.

While spring planting gives dahlias a head start, summer planting is a rewarding way to ensure your garden remains vibrant well into the autumn months.

Understanding the Summer Planting Window

When people ask if they can plant dahlias in the summer, they are usually talking about one of two things. They are either planting dormant tubers that they didn't get into the ground in May, or they are transplanting established plants from containers into the garden. Both are possible, but they require different levels of care.

The most important factor to consider is time. Most dahlia varieties need between 90 and 120 days from the moment they are planted to produce their first flowers. If you live in a region where the first frost arrives in early October, planting a tuber in late July may not give the plant enough time to bloom. However, if you are in a warmer climate or are planting a "started" dahlia that already has leaves, your chances of success are much higher.

Success with summer planting depends on your local weather and how much attention you can give the plants during their first few weeks. Dahlias are native to the high-altitude regions of Mexico and Central America. They love sunshine, but they prefer temperatures that stay between 70°F and 80°F. When you plant in the middle of a hot US summer, you are asking the plant to establish itself in conditions that are a bit more stressful than the cool, damp days of spring.

Tubers vs. Potted Starts: Which Is Best for Summer?

If you are starting your dahlia journey in June or July, you have two main options for planting. The path you choose will largely determine how much maintenance the plant needs during its first month in the ground.

Planting Dormant Tubers in Summer

It is common to find dahlia tubers on clearance in early summer, or perhaps you found a few you forgot to plant earlier in the season. You can still plant these tubers in June or even early July. Because the soil is already warm, these tubers will often sprout much faster than they would in the cold soil of April.

However, dormant tubers are vulnerable to the intense summer sun. When a tuber is first pushing up its tender green shoots, those shoots can easily be scorched by 90°F heat. You will need to monitor the soil moisture closely. While we usually tell gardeners not to water tubers until they see green growth in the spring, summer planting is different. The heat can dry out the soil completely, so a light watering to keep the soil slightly damp is often necessary to help the tuber "wake up."

Planting Potted Starts

For the best results in mid-to-late summer, we recommend using potted starts. These are dahlias that have already been growing in a container and have established root systems and foliage. Many garden centers sell these in one-gallon or three-gallon pots during the summer months.

Because these plants already have a head start, they will bloom much sooner than a bare tuber. They are also more resilient to the heat because their roots are already active and able to take up water. When you move a potted dahlia into the garden, you are simply "upsizing" its home rather than starting from scratch. This is the most reliable way to add dahlia color to your summer garden.

What to do next: Choosing your plant

• If it is before mid-June, dormant tubers are a great, budget-friendly option.
• If it is July or later, look for established potted plants at a nursery.
• Ensure any tuber you plant is still firm and has at least one visible "eye" or sprout.
• Avoid planting tubers that feel mushy or completely dried out like a brittle stick.

The 90-Day Rule and Your Frost Date

Before you dig a hole in July, you should check your local "first frost date." This is the average date when your area experience its first freezing temperatures of the autumn. Dahlias are tropical plants and will turn black and die as soon as a hard frost hits.

As we mentioned, most dahlias need about three months to reach peak blooming. If your first frost usually arrives on October 1st, you need to have your dahlias in the ground and growing by July 1st to see a significant floral display. If you live in a southern state where frost doesn't arrive until November or December, you have much more flexibility and can plant well into the summer.

If you are planting later in the season, consider choosing "early-blooming" varieties. Some smaller border dahlias or cactus-style varieties tend to flower faster than the giant dinnerplate types. This can help you beat the clock and enjoy blooms before the winter weather arrives.

Selecting the Right Spot for Summer Success

In the spring, we usually tell gardeners to plant dahlias in the sunniest spot available. In the summer, the rules change slightly. While dahlias still need at least six hours of sunlight to produce strong stems and plenty of flowers, the intense afternoon sun in July and August can be punishing.

The Value of Afternoon Shade

If you are planting in the heat of summer, look for a spot that gets bright morning sun but is shielded from the direct, scorching rays of the late afternoon. A location on the east side of a fence, building, or large shrub is often ideal. This allows the plant to get the light it needs for photosynthesis while staying a few degrees cooler during the hottest part of the day.

If you don't have a spot with natural shade, you can use a temporary shade cloth. Propping up a piece of lattice or a light-colored fabric over your new plants for the first two weeks can prevent transplant shock. Once the roots have settled in, the plant will be better equipped to handle the full sun.

Drainage is Essential

"Drainage" refers to how quickly water moves through your soil. Dahlias have tuberous roots that act like sponges. If they sit in soggy, heavy clay soil for too long, they will rot. This is especially important in the summer when you might be watering more frequently to combat the heat.

To check your drainage, dig a small hole and fill it with water. If the water is still sitting there an hour later, your drainage is poor. You can improve this by mixing in compost or planting in raised beds. Raised beds are a favorite for us because they allow the soil to drain freely and stay aerated, which dahlia roots love.

Step-by-Step Summer Planting Guide

Planting in summer requires a bit more preparation than spring planting. Follow these steps to give your plants the best possible start.

1. Prepare the Soil

Start by digging a hole that is twice as wide as the pot or tuber you are planting. Mix in a shovel or two of high-quality compost. This adds nutrients and helps the soil hold onto just the right amount of moisture without becoming waterlogged.

2. Planting Depth

If you are planting a dormant tuber, place it horizontally in the hole about 4 to 6 inches deep. The "eye" or sprout should be facing upward. If you are planting a potted start, plant it at the same depth it was sitting in the pot. Do not bury the green stem deeper than it was previously growing, as this can lead to stem rot.

3. Space Them Out

Dahlias need plenty of "air circulation." This means there should be enough room between plants for a breeze to pass through. When plants are crowded, the leaves stay wet longer after a rain, which can lead to diseases like powdery mildew. Space your dahlias 12 to 18 inches apart depending on the variety.

4. Water Immediately

As soon as the plant is in the ground, give it a deep drink of water. This helps settle the soil around the roots and removes any large air pockets. In the summer, this initial watering is critical to prevent the plant from wilting in the heat.

Key Takeaway: The Summer Planting Secret Planting in summer is all about managing moisture. Unlike spring planting, where the soil stays naturally damp, summer soil can dry out in hours. Check your new plants daily for the first two weeks until they are established.

Watering and Feeding During the Heat

Watering is the most important part of dahlia care, especially for summer-planted crops. Because dahlias have shallow "feeder roots" that stay near the surface, they are very sensitive to the soil drying out.

How to Water Correctly

The goal is to keep the soil consistently moist but never swampy. Instead of giving your plants a light sprinkle every day, try to give them a deep soak two or three times a week. This encourages the roots to grow deeper into the ground where the soil stays cooler and wetter.

Always water at the base of the plant. Avoid getting water on the leaves, especially in the evening. Wet leaves are an invitation for fungus and mold. If you have a large dahlia patch, a drip irrigation system or a soaker hose is a great investment. These tools deliver water directly to the soil level, which keeps the foliage dry and healthy. For more general growing advice, see 8 Tips for Growing Better Dahlias.

Nutrients for Fast Growth

Dahlias are heavy feeders. They need plenty of energy to grow from a small tuber to a four-foot bush in just a few months. When planting in summer, we recommend using a balanced fertilizer. Look for a product with a label like "5-10-10" or "10-10-10."

Avoid fertilizers that are very high in nitrogen (the first number on the package). High nitrogen will give you a giant green bush with very few flowers. Phosphorus and potassium (the second and third numbers) are what the plant needs to produce strong roots and beautiful blooms.

Mulching: The Summer Gardener’s Best Friend

If you only do one extra thing for your summer-planted dahlias, let it be mulching. Mulch is a layer of organic material—like straw, shredded leaves, or wood chips—placed on top of the soil.

In the summer, mulch serves three vital purposes:

1. Temperature Control: It acts like insulation, keeping the soil (and the roots) significantly cooler than the air temperature.
2. Moisture Retention: It prevents water from evaporating out of the soil, meaning you don't have to water as often.
3. Weed Suppression: It stops weed seeds from germinating. Weeds compete with your dahlias for water and nutrients, so keeping them away is a major win.

Apply a 2-to-3-inch layer of mulch around the base of your dahlias, but keep it an inch or two away from the actual stem to prevent rot.

Managing Summer Heat Stress

Sometimes, despite our best efforts, the temperature climbs above 90°F for several days in a row. When this happens, dahlias may enter a "survival mode." You might notice that they stop producing new buds or that the flowers they do produce are smaller or slightly misshapen.

Don't worry—this is normal. The plants are simply focused on keeping their leaves hydrated rather than making flowers. Once the weather cools down in late August or September, they will start blooming vigorously again.

To help them through a heatwave:

• Mist the foliage: In the early morning, you can lightly mist the leaves with a hose to cool the air around the plant.
• Increase watering: You may need to water every other day during extreme heat.
• Don't over-fertilize: Avoid feeding your plants during a heatwave. Wait until the weather breaks so you don't encourage new growth that the plant can't support.

Pinching and Staking for Stronger Plants

Two simple chores will make your dahlias look like they were grown by a professional: pinching and staking.

The Art of Pinching

"Pinching" sounds like you are hurting the plant, but it is actually one of the kindest things you can do. When your dahlia is about 12 to 18 inches tall, use a pair of clean garden snips to cut off the very top of the center stem.

This tells the plant to stop growing "up" and start growing "out." It encourages the dahlia to send out side branches, resulting in a bushier plant with many more flowers. A pinched dahlia is much more stable and produces a better floral display than a plant left to its own devices.

Support Your Stems

Dahlia stems are hollow and can be quite brittle. As the flowers grow, they become very top-heavy. A sudden summer thunderstorm or a gust of wind can easily snap a beautiful branch.

We recommend staking your dahlias at the time of planting. If you wait until the plant is large, you risk driving the stake through the underground tubers. Use a sturdy bamboo pole, a wooden stake, or a tomato cage. As the plant grows, loosely tie the stems to the stake using soft garden twine or strips of old fabric.

Heat-Tolerant Varieties to Consider

While all dahlias can grow in the summer, some varieties are known for being particularly "tough" when the mercury rises. If you are shopping for late-season additions, look for these reliable performers:

• Thomas Edison: A classic deep purple decorative dahlia that is famous for its vigor.
• Kelvin Floodlight: A massive yellow dinnerplate variety that handles sunshine beautifully.
• Cornel: A bright red ball dahlia with very sturdy stems and excellent heat resistance.
• Cafe au Lait: This creamy, blush-colored favorite is surprisingly resilient and continues to bloom even as the season shifts.
• Bishop of Llandaff: Known for its dark, almost black foliage, this variety is very heat-tolerant and provides a stunning contrast in the garden.

Looking Toward the Autumn Bloom

The best part of planting dahlias in summer is the reward you get in September and October. While many other garden flowers are starting to fade or look tired, dahlias are just hitting their stride. The cooler nights of early autumn bring out the most intense colors and clearest patterns in the petals.

Because you planted in summer, your plants will be fresh and energetic just as the rest of the garden is winding down. This "second act" for your garden is one of the many reasons we at Longfield Gardens love these plants so much.

If you find that your summer-planted dahlias are still blooming when the first frost is predicted, you can extend their life by a few days by covering them with a light frost blanket or an old bedsheet at night. This can often buy you an extra week or two of beautiful bouquets.

Conclusion

Planting dahlias in the summer is not only possible but can be a fantastic way to refresh your landscape and ensure you have plenty of flowers for cutting late in the season. By choosing potted starts for late-season planting, providing afternoon shade, and being diligent with water and mulch, you can overcome the challenges of the heat.

At Longfield Gardens, we want every gardener to feel confident in their ability to grow a beautiful yard, and our 100% Quality Guarantee helps make that easier. Gardening should be a source of joy and relaxation, not stress. If you missed the spring planting window, don't let that stop you from enjoying the magic of dahlias this year. With a little extra care during the first few weeks, your summer-planted dahlias will reward you with a spectacular show that lasts until the very first frost.

Final Action Steps for Summer Success

• Check your first frost date to ensure you have at least 90 days of growing time.
• Prepare your soil with compost to ensure excellent drainage.
• Apply a thick layer of mulch to keep the roots cool and moist.
• Pinch your plants when they are 12 inches tall to encourage more blooms.

FAQ

Can I plant dahlia tubers in July?

Yes, you can plant tubers in July, but it depends on your climate. If you live in a region with a long growing season (Zones 7–10), you will likely have enough time for the plants to bloom before the first frost. For a quick climate check, use the hardiness zone map.

How often should I water dahlias planted in the summer?

New dahlias planted in the summer heat need more frequent watering than those planted in spring. Check the soil daily. If the top inch of soil feels dry, give the plant a deep soak. Once the plant is established, watering two to three times a week is usually sufficient, provided you have used mulch to help retain moisture.

Do dahlias need full sun in hot climates?

While dahlias generally love sun, they can struggle in temperatures consistently above 90°F. In very hot climates, it is best to provide your dahlias with morning sun and some afternoon shade. This prevents the foliage from scorching and helps the plant conserve energy for flower production.

What should I do if my summer-planted dahlias aren't blooming?

If your plants are healthy and green but have no flowers, it is likely due to either high heat or too much nitrogen in the soil. Dahlias often "stall" during heatwaves and will naturally start blooming again once temperatures cool down. Ensure you are using a fertilizer low in nitrogen and high in phosphorus to encourage bud development.