Table of Contents
- Introduction
- Why Grow Dahlias in a Greenhouse?
- Choosing the Best Dahlias for Greenhouse Growing
- Setting Up Your Greenhouse for Success
- How to Start Dahlia Tubers in the Greenhouse
- Watering and Feeding Your Greenhouse Dahlias
- Pinching for a Bushier Plant
- Managing Pests and Diseases in the Greenhouse
- Transitioning Dahlias to the Outdoor Garden
- Growing Dahlias Year-Round in a Greenhouse
- Storing Your Tubers for Next Year
- Conclusion
- FAQ
Introduction
There is nothing quite like the joyful moment when the first dahlia of the season opens its petals. These spectacular flowers are the crown jewels of the late-summer garden, offering an incredible variety of colors, shapes, and sizes. Whether you are drawn to the massive, pillowy blooms of dinnerplate varieties or the intricate, geometric patterns of pompons, dahlias bring a sense of wonder to any landscape.
For many home gardeners, the biggest challenge is the wait. Because these plants love warmth, they often don't start blooming until late summer, leaving only a few weeks of color before the first frost arrives. This is where a greenhouse becomes a gardener's best friend. At Longfield Gardens, we have found that using a greenhouse is one of the most effective ways to get a head start on the growing season and enjoy those stunning dahlias much earlier than usual.
In this guide, we will explore everything you need to know about growing dahlias in a greenhouse. We will cover the benefits of starting your tubers under glass, the ideal environmental conditions they need to thrive, and how to transition your plants from the greenhouse to your outdoor garden beds. This article is designed for home gardeners who want to maximize their bloom time and grow healthier, more robust plants. For a quick reference on flower forms and sizes, see our Planning Guide for Dahlias.
By using a greenhouse to start your tubers in early spring, you can gain several weeks of extra growth and ensure your dahlias are ready to burst into bloom the moment the summer weather arrives.
Why Grow Dahlias in a Greenhouse?
Growing dahlias in a greenhouse offers several distinct advantages that can transform your gardening experience. While dahlias are traditionally planted directly into the ground once the soil has warmed up, the controlled environment of a greenhouse provides a level of protection and consistency that nature often lacks in early spring.
The primary reason to use a greenhouse is to "wake up" your tubers earlier than the outdoor weather allows. In many parts of the country, the soil doesn't reach the ideal planting temperature of 60°F until late May or June. By starting your tubers in a greenhouse in March or April, you provide them with the warmth they need to begin sprouting while the ground outside is still cold and damp. This head start often results in plants that begin blooming in early July rather than late August.
Another benefit is protection from unpredictable spring weather. Late frosts, heavy spring rains, and fluctuating temperatures can be hard on young dahlia shoots. In a greenhouse, you can manage the temperature and moisture levels, ensuring that your young plants grow strong and steady without the stress of the elements. This extra care in the early stages leads to sturdier stems and a more prolific root system.
Finally, a greenhouse allows you to extend the season on the back end as well. If you have a heated greenhouse, you can keep your dahlias blooming well into the autumn, long after the first frost has claimed the rest of the garden. This ability to manipulate the growing cycle makes the greenhouse an invaluable tool for any dahlia enthusiast.
Choosing the Best Dahlias for Greenhouse Growing
One of the most exciting parts of gardening is choosing which varieties to grow. Because a greenhouse provides such a supportive environment, almost any dahlia variety will respond well to being started indoors. However, understanding the different types can help you plan your space more effectively.
Dinnerplate Dahlias
These are the showstoppers of the dahlia world. Varieties like Café au Lait or 'Emory Paul' produce massive flowers that can reach 10 to 12 inches in diameter. Because these plants grow quite large and produce such heavy blooms, starting them in a greenhouse helps them develop the strong, thick stems necessary to support the weight of the flowers later in the summer.
Decorative and Cactus Dahlias
Decorative dahlias, such as the bicolor 'Duet', are known for their classic, petal-packed look and consistent performance. Cactus dahlias have unique, spiky petals that add wonderful texture to floral arrangements. Both types are prolific bloomers and benefit greatly from the early start a greenhouse provides, as it allows them to reach their full flowering potential before the season ends.
Ball and Pompon Dahlias
If you enjoy cutting flowers for bouquets, ball dahlias are essential. Their perfectly round, symmetrical shapes and long vase life make them favorites for floral design. These varieties tend to be a bit more compact, making them very easy to manage in a greenhouse setting where space might be at a premium.
Key Takeaway: Using a greenhouse allows you to grow a wider variety of dahlias, including long-season types that might not have enough time to bloom fully if planted directly outdoors in cooler climates.
Setting Up Your Greenhouse for Success
To get the most out of your dahlias, your greenhouse needs to provide the right balance of light, heat, and air. Dahlias are native to Mexico, so they are programmed to thrive in warm, sunny conditions.
Lighting Requirements
Light is perhaps the most critical factor when growing dahlias under glass. Dahlias are "long-day" plants, meaning they need about 12 to 14 hours of light per day to grow actively and eventually produce flowers. In the early spring, natural daylight hours may be too short.
If your greenhouse doesn't receive full, direct sun for the majority of the day, you might notice your plants becoming "leggy"—tall, thin, and weak as they stretch toward the light. To prevent this, many gardeners use supplemental grow lights. Keeping the light source close to the tops of the plants helps them stay stocky and strong.
Temperature Control
Dahlias prefer a daytime temperature between 65°F and 75°F. At night, they appreciate a slightly cooler environment, but the temperature should never drop below freezing. If your greenhouse is unheated, you must be vigilant during cold snaps. Using heat mats under your seedling trays or pots can provide the consistent soil warmth (around 60-70°F) that encourages tubers to "eye up" and sprout.
Airflow and Humidity
While dahlias love warmth, they do not like stagnant, humid air. High humidity in a greenhouse can lead to problems like powdery mildew or gray mold (botrytis). To keep your plants healthy, ensure your greenhouse has good ventilation. Opening vents on warm days and using a small fan to keep air moving will go a long way in preventing disease.
How to Start Dahlia Tubers in the Greenhouse
Starting your tubers is a straightforward process, but a few simple steps will ensure the best results. For a complete walkthrough, see our How to Plant Dahlias.
Preparing the Tubers
Before planting, inspect your tubers. A healthy tuber should feel firm, like a fresh potato. If you see any soft, mushy spots, it is best to trim those away. Look for the "eye"—a small bump or sprout located at the neck of the tuber where it joins the old stem. This is where the new growth will emerge. For a deeper dive, read Dahlia Tubers: What You Need to Know.
Choosing Containers and Soil
You can start tubers in individual pots (at least 6 to 8 inches wide) or in large, shallow trays. At Longfield Gardens, we suggest using a high-quality, well-draining potting mix. Avoid heavy garden soil, which can pack down too tightly and hold too much moisture, potentially causing the tubers to rot. A mix based on peat moss or coconut coir is usually ideal because it stays light and airy.
Planting Depth and Spacing
When planting in the greenhouse, you don't need to bury the tubers deeply. Lay the tuber horizontally in the soil with the eye facing upward. Cover it with about 1 to 2 inches of soil. If you are using trays, you can space the tubers close together, as they will only stay in the tray until they have sprouted and grown a few inches tall.
What to do next:
- Check your tubers for healthy "eyes" before potting.
- Use a light, sterile potting mix to encourage drainage.
- Place containers on a heat mat if the greenhouse is cool.
- Keep the soil slightly moist but never soggy.
Watering and Feeding Your Greenhouse Dahlias
Proper hydration and nutrition are the engines that drive dahlia growth. However, the needs of the plant change as it moves through its life cycle.
The "Dry Start" Method
One of the most common mistakes is overwatering tubers before they have sprouted. A tuber that is sitting in cold, wet soil is very likely to rot. When you first pot up your tubers, give the soil a light drink to settle it, and then wait. Do not water again until you see green shoots poking through the soil. The tuber contains all the energy and moisture the plant needs to get started.
Deep Watering for Established Plants
Once the plants are several inches tall and have developed a root system, you can begin watering more regularly. The goal is to keep the soil consistently moist but not saturated. A good rule of thumb is to let the top inch of soil dry out before watering again. This "deep then dry" approach encourages the roots to grow deeper into the pot in search of moisture.
Fertilizing for Blooms
Dahlias are heavy feeders, but they are sensitive to too much nitrogen. High nitrogen fertilizers can produce lush, green leaves but very few flowers. Instead, look for a balanced fertilizer or one that is slightly higher in phosphorus and potassium (the second and third numbers on the label). A 5-10-10 or 10-20-20 ratio is often a great choice. Start feeding your plants about two weeks after they have sprouted and continue once a month.
Pinching for a Bushier Plant
If you want a dahlia plant that is full of flowers rather than one single, tall stalk, "pinching" is a technique you should master. It might feel counterintuitive to cut off part of a healthy plant, but it is one of the best things you can do for its long-term performance.
When your dahlia is about 10 to 12 inches tall and has at least three or four sets of leaves, snip off the very top of the main center stem. You only need to remove about an inch or two. This simple action signals the plant to stop putting all its energy into growing upward and instead start growing outward. For more details, see How to Pinch and Stake Dahlias.
By pinching the center lead, you encourage the "axillary buds" (the spots where the leaves meet the stem) to grow into new branches. The result is a bushier plant with more stems, which ultimately means more flowers for you to enjoy.
Key Takeaway: Pinching your dahlias in the greenhouse phase creates a stronger, more branched structure that will support a heavier bloom load once the plant is moved outdoors.
Managing Pests and Diseases in the Greenhouse
A greenhouse is a sanctuary for plants, but it can also be an inviting environment for a few common pests. Monitoring your plants regularly is the best way to keep them healthy.
Common Greenhouse Pests
- Aphids: These tiny, soft-bodied insects like to cluster on new growth. They can be easily managed by wiping them off with a damp cloth or using a gentle spray of water.
- Spider Mites: These thrive in warm, dry conditions. If you see tiny webbing on the underside of leaves, you may have mites. Increasing the humidity slightly or using a misting bottle can help discourage them.
- Whiteflies: These look like tiny white moths and often fly up when the plant is disturbed. Good yellow sticky traps can help you monitor and control their population.
Preventing Disease
As mentioned earlier, airflow is your best defense against disease. If you notice any leaves with white, powdery spots (powdery mildew), remove those leaves immediately and improve the ventilation in your greenhouse. Keeping the foliage dry when you water—aiming the watering can at the soil rather than the leaves—is another simple way to keep your plants in top shape. For more detail, see Common Dahlia Pests and Diseases.
Transitioning Dahlias to the Outdoor Garden
Once the weather warms up and the danger of frost has passed, it is time to move your dahlias to their permanent summer home. However, moving a plant directly from a cozy greenhouse to the unpredictable outdoors can cause "transplant shock." To prevent this, you should "harden off" your plants.
The Hardening Off Process
Think of this as a slow introduction to the real world. Start by placing your potted dahlias outdoors in a sheltered, shady spot for just two or three hours during the warmest part of the day. Bring them back into the greenhouse at night.
Over the course of 7 to 10 days, gradually increase the amount of time they spend outside and the amount of sunlight they receive. By the end of the week, they should be ready to stay out all night and handle full sun.
Planting in the Garden
When you are ready to plant, choose a spot with at least 6 to 8 hours of direct sunlight and well-draining soil. Because your greenhouse dahlias already have a head start, they will likely be quite large. Be sure to provide them with stakes or a support system at the time of planting so you don't damage the roots later on.
Steps for a smooth transition:
- Check that the outdoor soil temperature is at least 60°F.
- Start the hardening-off process 10 days before planting.
- Prepare your garden beds with compost to provide extra nutrients.
- Install stakes or cages immediately after planting to support future growth. If you're unsure about your zone, check the Hardiness Zone Map.
Growing Dahlias Year-Round in a Greenhouse
For those who have a heated greenhouse and live in a climate with cold winters, it is possible to grow dahlias year-round. This is a more advanced technique, but it can be incredibly rewarding.
To grow dahlias through the winter, you must replicate the conditions of a summer day. This means maintaining a temperature above 60°F and providing high-intensity grow lights to make up for the short winter days. Without enough light, the plants will go dormant and focus their energy on the tubers rather than the flowers.
Even in a year-round setup, dahlias benefit from a rest period. Most growers find that allowing the tubers to go dormant for at least two months in the late fall or early winter results in much more vigorous growth the following spring. You can achieve this by withholding water and letting the foliage die back naturally, then keeping the tubers in a cool (but not freezing) part of the greenhouse until you are ready to start them again.
Storing Your Tubers for Next Year
When the growing season finally comes to an end and the first frost hits, your greenhouse can serve one last purpose: a staging area for winter storage.
After the foliage has turned black from the frost, carefully dig up your dahlia clumps. Bring them into the greenhouse or a garage to dry out for a day or two. This helps prevent rot during storage. Once the soil is dry enough to be brushed off, you can trim the stems down to about 4 inches.
Store your tubers in a cool, dark, and dry place (between 40°F and 50°F). Many gardeners find success packing them in boxes with slightly damp peat moss, vermiculite, or wood shavings. At Longfield Gardens, we always remind our customers to check their stored tubers once or twice during the winter to ensure they aren't getting too dry or showing signs of mold.
Conclusion
Growing dahlias in a greenhouse is a wonderful way to deepen your connection with your garden and ensure a spectacular display of blooms. By providing a warm, protected start, you give your plants the best possible foundation for a successful season. Whether you are aiming for early July flowers or simply want to protect your favorite varieties from a fickle spring, the greenhouse offers the control and consistency needed to grow world-class dahlias.
The process of watching a dormant tuber transform into a lush, budding plant under the glass of a greenhouse is one of the most rewarding experiences in gardening. It reminds us that with a little bit of care and the right environment, nature is capable of incredible things.
Growing dahlias doesn't have to be a race against the calendar. With a greenhouse, you set the pace, allowing you to enjoy bigger, bolder, and more beautiful blooms for the longest possible season.
We encourage you to try starting a few of your favorite varieties in a greenhouse this year. Our team at Longfield Gardens is here to support you with high-quality tubers and the practical advice you need to succeed.
FAQ
Can I grow dahlias in a greenhouse that isn't heated?
Yes, you can certainly use an unheated greenhouse to start dahlias, but you must wait until the threat of extreme freezing has passed. The greenhouse will still be significantly warmer than the outdoors during the day, providing a great head start. Just be prepared to cover your pots with frost blankets or move them to a warmer spot if a late-season cold snap is predicted.
Why are my greenhouse dahlia stems so thin and weak?
Thin, spindly stems are usually a sign that the plants aren't getting enough light. Because light levels in early spring are lower, the plants stretch upward to find more sun. To fix this, move your plants to the sunniest part of the greenhouse or add supplemental grow lights. Pinching the tops of the plants when they are 12 inches tall will also help them grow thicker, stronger stems.
How often should I water my dahlias while they are in the greenhouse?
Before the tubers sprout, you should water very little—only enough to keep the soil from becoming bone-dry. Once you see green growth, you can start watering whenever the top inch of soil feels dry to the touch. In a greenhouse, pots can dry out faster than garden soil, so check them daily during sunny weather, but always avoid keeping the soil constantly soggy.
Do dahlias need a special type of fertilizer in the greenhouse?
Dahlias in the greenhouse benefit from a balanced or bloom-boosting fertilizer. Avoid fertilizers with very high nitrogen (the first number), as this leads to lots of leaves but fewer flowers. A formula like 5-10-10 is excellent. Wait until the plant has a few sets of true leaves before you start a regular feeding schedule, and always follow the instructions on the product label.