Introduction
There is nothing quite like the excitement of seeing the first dahlia buds unfurl into a kaleidoscope of color. These vibrant, architectural flowers are the crown jewels of the summer garden, offering everything from tiny, intricate pompoms to massive dinnerplate blooms. For many of us, the only downside to growing dahlias is the wait. Because they are tender perennials that love warmth, they often don’t hit their peak until late summer, leaving a relatively short window of beauty before the first frost arrives.
If you have a greenhouse, you may be wondering if you can use that protected space to get a head start or even grow these stunning plants all year long. At Longfield Gardens, we believe that understanding the needs of your plants is the fastest way to gardening success. Using a greenhouse for your dahlias can significantly extend your enjoyment of these flowers, provided you get a few environmental basics right.
This guide will walk you through the practical steps of growing dahlias in a greenhouse environment. Whether you want to "wake up" your tubers early for a mid-summer show or keep them blooming in a temperature-controlled space, we have the tips you need to succeed. Using a greenhouse gives you more control over the growing season and helps you make the most of every tuber you plant.
The Benefits of Greenhouse Growing
Growing dahlias in a greenhouse offers several distinct advantages over starting them directly in the ground. The most common reason gardeners use a greenhouse is to "jump-start" the tubers in late winter or early spring. Because dahlias are native to the warm climates of Mexico and Central America, they are sensitive to cold soil. In many parts of the United States, the ground doesn't reach the ideal 60°F temperature until late May or June.
By starting your tubers in a greenhouse in March or April, you can have established plants that are 12 to 15 inches tall by the time the weather is safe enough to move them outside. This jump-start can result in flowers appearing four to six weeks earlier than plants started in the ground. Instead of waiting until late August for a full display, you could be cutting fresh bouquets by early July.
Beyond timing, a greenhouse provides a controlled environment that protects young, vulnerable shoots from unpredictable spring weather, such as heavy rain or late-season cold snaps. It also keeps your plants away from early-season pests like slugs and snails that often feast on tender new growth in the garden. For those with a heated greenhouse, it is even possible to keep dahlias growing and blooming well into the late fall or early winter.
Waking Up Tubers in Early Spring
If your goal is to get flowers as early as possible, the "waking up" process is the first step. This typically begins about four to six weeks before your last expected spring frost. You will need a greenhouse that can stay consistently above 50°F at night, as these plants do not like to be chilled.
Start by inspecting your tubers. They should feel firm, similar to a fresh potato. If you see small, green "eyes" or sprouts beginning to form at the neck of the tuber, the plant is already signaling that it is ready to grow. If you don't see sprouts yet, the warmth of the greenhouse will soon encourage them.
To wake them up, place the tubers in shallow trays or individual pots. You don't need deep soil at this stage; you just need to provide a moist, warm environment. Lay the tubers horizontally and cover them with about an inch or two of potting mix or damp peat moss. Keep the soil lightly moist but never soggy. In a greenhouse, the combination of sun and ambient warmth acts as a wake-up call, and you should see green shoots emerging within 10 to 14 days.
Key Takeaway: Starting tubers in a greenhouse 4–6 weeks before the last frost can result in blooms appearing up to a month earlier than traditional outdoor planting.
Choosing the Right Containers
When growing dahlias in a greenhouse, container choice matters. If you plan to eventually transplant them into the garden, use 1-gallon nursery pots or similar containers. These provide enough room for the roots to develop without the plant becoming root-bound too quickly.
For gardeners who intend to keep the dahlias in the greenhouse for their entire life cycle, larger pots are necessary. A 3-to-5-gallon container is usually sufficient for most varieties. Smaller "mignon" or bedding dahlias can thrive in slightly smaller pots, but the large "dinnerplate" varieties need the extra space and weight of a larger pot to keep the plant stable as it grows tall.
Soil and Drainage Basics
Dahlias require soil with excellent drainage. Drainage is simply a measure of how fast water leaves the soil; you want the water to move through the pot freely so the tubers don't sit in a puddle. Use a high-quality, peat-based or coir-based potting mix designed for containers. Avoid using garden soil in greenhouse pots, as it is often too heavy and can harbor pests or diseases that thrive in a confined space.
Managing the Greenhouse Environment
The secret to success with greenhouse dahlias lies in managing three main factors: light, temperature, and air circulation. Because a greenhouse is an enclosed space, these factors can change rapidly, and staying on top of them will ensure your plants stay healthy and strong.
Light Requirements
Dahlias are "long-day" plants, which means they need plenty of light to trigger flower production. In a greenhouse, they should receive at least 12 to 14 hours of light per day. During the early spring, when days are still short, you may need to supplement the natural sunlight with grow lights.
Without enough light, dahlia stems will become "leggy." This means the stems grow very tall, thin, and weak as the plant reaches for a light source. Strong, direct light produces stocky, robust plants that can support the weight of their own flowers. If you notice your plants leaning toward one side of the greenhouse, rotate the pots every few days to encourage even growth.
Temperature Controls
While dahlias love warmth, they don't enjoy extreme heat. Ideally, your greenhouse should stay between 65°F and 75°F during the day. If the temperature climbs above 85°F, the plants may become stressed, and flower production could slow down.
On sunny days, even in the spring, greenhouses can overheat quickly. Use vents, fans, or open the doors to keep the air moving and the temperature stable. At night, try to keep the temperature above 55°F. While dahlias can survive cooler nights, their growth will slow down significantly if they are consistently exposed to temperatures in the 40s.
Air Circulation and Humidity
A common challenge in greenhouses is high humidity, which can lead to fungal issues. Dahlias are particularly susceptible to powdery mildew—a white, flour-like dusting on the leaves—and botrytis, which can cause buds to rot.
To prevent these issues, ensure there is plenty of space between your pots. Crowded plants prevent air from moving around the foliage, creating pockets of stagnant, moist air. Running a small oscillating fan in the greenhouse can make a world of difference in keeping the leaves dry and the plants healthy.
What to do next:
- Set up an oscillating fan to keep air moving around your plants.
- Check the daily temperature to ensure the greenhouse stays between 65°F and 75°F.
- Ensure your plants are getting at least 12 hours of light, using supplemental lamps if necessary.
- Space pots several inches apart to allow for maximum air circulation.
Watering and Feeding Greenhouse Dahlias
Watering in a greenhouse is different than watering in the open garden. Because there is no wind and the environment is controlled, the soil may stay damp for longer than you expect.
The "Deep and Dry" Method
The best way to water dahlias is to water deeply until it runs out the bottom of the pot, and then wait until the top inch of soil feels dry before watering again. Overwatering is the most common mistake with dahlias, especially when they are first starting to grow. Before the plant has several sets of leaves, it doesn't have a large root system to drink up the water. If the tuber sits in wet soil for too long, it can rot before it even has a chance to bloom.
As the plant grows larger and the weather warms up, you will find you need to water more frequently. Large dahlias with lots of foliage can be quite thirsty on a warm afternoon. Always check the soil with your finger before reaching for the watering can.
Choosing the Right Fertilizer
Dahlias are heavy feeders, but they need the right kind of "food" to produce flowers rather than just green leaves. When you first plant the tubers, you can use a balanced, general-purpose fertilizer. However, once the plant is about a foot tall, it's best to switch to a fertilizer that is lower in nitrogen and higher in phosphorus and potassium.
Look for a fertilizer with a ratio like 5-10-10.
- Nitrogen (the first number) encourages green, leafy growth. Too much nitrogen can lead to weak stems and very few flowers.
- Phosphorus (the middle number) promotes strong root development and big blooms.
- Potassium (the third number) supports the overall health and "sturdiness" of the plant.
Feed your greenhouse dahlias about once every two to three weeks during the growing season. Always follow the instructions on the product label to avoid over-fertilizing, which can burn the roots.
Pinching and Supporting Your Plants
To get the most beautiful shape and the highest number of flowers, you should "pinch" your dahlias. This sounds like it might hurt the plant, but it actually encourages it to grow bushier and produce more flower-bearing side branches.
When your dahlia is about 12 inches tall and has three to four sets of leaves, use your fingers or a clean pair of snips to remove the very top of the center stem. This signals the plant to stop putting all its energy into growing one single tall stalk and instead start growing outward. You will end up with a fuller plant and significantly more blooms throughout the season.
Support is also vital in a greenhouse. Even without wind, dahlia stems can be brittle and may snap under the weight of their own flowers. For pot-grown dahlias, insert a sturdy bamboo stake or a decorative metal support into the pot at the time of planting. As the plant grows, use soft garden twine to gently tie the stems to the support.
Transitioning Dahlias to the Garden
If you have used your greenhouse to start your plants early with the intention of moving them outside, you must go through a process called "hardening off." This is a vital step that allows the plant to adjust to the intensity of direct sunlight and the fluctuations of outdoor temperatures.
Plants grown in a greenhouse are a bit "soft" because they haven't had to deal with wind or harsh UV rays. If you move them directly into the full sun, the leaves may burn or the plant may wilt from the shock.
About 7 to 10 days before you plan to plant them in the garden, start moving the pots outside to a sheltered, shady spot for a few hours in the afternoon. Each day, 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 able to handle a full day of outdoor conditions. Always bring them back into the greenhouse if there is any threat of frost during this transition period.
Growing Dahlias Year-Round
For those lucky enough to have a heated and well-lit greenhouse, it is possible to grow dahlias even when snow is on the ground. This requires a significant commitment to maintaining a tropical environment. You will need powerful grow lights to mimic the long days of summer and a consistent heat source to keep the space above 60°F.
While year-round growth is possible, most dahlia tubers benefit from a period of dormancy. In nature, the tubers use the winter to rest and store energy for the following year. If you choose to grow them year-round, you may find that the plants eventually become less vigorous. Many greenhouse growers prefer to have two "crops"—starting one set of tubers in the winter for spring blooms and another set in late summer for fall and winter color—allowing each set of tubers a rest period in between.
Managing Pests and Diseases
The greenhouse environment is a haven for plants, but it can also be a haven for pests if you aren't careful. Regularly inspect the undersides of leaves and the tender new tips of the plants.
- Aphids: These tiny, pear-shaped insects like to suck the sap from new growth. They can usually be washed off with a sharp stream of water or treated with insecticidal soap.
- Spider Mites: These thrive in warm, dry greenhouse conditions. If you see tiny webbing or speckled leaves, you likely have mites. Increasing the humidity slightly and ensuring good air movement can help prevent them.
- Thrips: These are very small, fast-moving insects that can distort flowers and leaves.
If you notice any signs of disease, such as the white powdery mildew mentioned earlier, act quickly. Remove any affected leaves and increase the air circulation in the room. At Longfield Gardens, we recommend keeping your greenhouse clean—removing fallen leaves and old potting soil—to prevent pests and diseases from taking hold in the first place.
Realistic Expectations for Greenhouse Success
While a greenhouse is a powerful tool, it’s important to remember that nature still plays a role. The success of your dahlias will depend on your specific microclimate, the quality of your light, and how much time you can dedicate to monitoring the environment.
Greenhouse-grown dahlias may sometimes have slightly different colors than those grown in full outdoor sun. UV light plays a role in pigment development, so if your greenhouse glass or plastic has a strong UV filter, you might notice subtle shifts in hue. Additionally, because there are no natural pollinators in a closed greenhouse, you won't see seeds forming unless you hand-pollinate or leave the doors open for bees to enter.
Gardening is a journey of observation. Every greenhouse is a little bit different, and you will learn more about your specific space with each passing season. The joy of seeing a dahlia bloom in the "off-season" is well worth the effort of learning these basics.
Conclusion
Growing dahlias in a greenhouse is a rewarding way to extend the life of these spectacular flowers. Whether you are using the space to get a jump-start on the spring season or attempting to keep the blooms going into the winter, the key is consistency. By providing ample light, maintaining steady temperatures, and ensuring your soil has great drainage, you can enjoy healthy, vibrant plants that produce an abundance of flowers.
At Longfield Gardens, our goal is to make gardening feel accessible and fun for everyone. Dahlias are remarkably resilient plants, and with the added protection of a greenhouse, they can truly thrive.
- Start early: Begin the waking-up process 4–6 weeks before the last frost.
- Prioritize light: Ensure 12–14 hours of light to prevent leggy, weak stems.
- Water wisely: Use the "deep and dry" method to protect the tubers from rot.
- Provide support: Stake your plants early to keep them upright as they grow.
"The greenhouse is more than just a structure; it’s a tool that allows you to partner with nature to create a longer, more vibrant season of color in your life."
Ready to start your dahlia journey? Explore our selection of premium tubers and find the perfect varieties to fill your greenhouse and garden with beauty this year. For shipping details, see our shipping information.
FAQ
Can I grow dahlias in an unheated greenhouse?
Yes, you can use an unheated greenhouse to start dahlias, but you must wait until the risk of a hard freeze has passed. The greenhouse will stay warmer than the outside air during the day, which helps the soil warm up faster, but it may still drop to near-freezing at night. If a frost is predicted, you should cover your trays with a frost blanket or bring the tubers inside temporarily to protect the tender new growth.
Why are my greenhouse dahlias growing tall but not blooming?
This is usually caused by either a lack of light or too much nitrogen in the fertilizer. Dahlias need 12–14 hours of bright light to trigger flower production; without it, they put all their energy into stretching toward the sun. Additionally, ensure you are using a fertilizer higher in phosphorus (the middle number) to encourage blooms rather than just green leaves.
Do I need to pollinate dahlias if they are in a greenhouse?
If you only want to enjoy the flowers for cutting or display, you do not need to worry about pollination. However, if you are interested in collecting seeds to grow new varieties, you will need to allow bees and butterflies into the greenhouse or use a small paintbrush to transfer pollen from one flower to another. Most gardeners grow dahlias from tubers, which are clones of the parent plant, so pollination is not necessary for standard growth.
How do I prevent my dahlia tubers from rotting in the greenhouse?
Rot is almost always caused by overwatering or poor drainage. Ensure you are using a light, airy potting mix and that your containers have large drainage holes. Only water the tubers when the top inch of soil feels dry to the touch, especially in the early stages before the plant has developed a full root system. Improving air circulation with a small fan can also help keep the soil surface from staying too wet. For more planting basics, see Dahlia Tubers: What You Need to Know.
How should I start dahlias in pots before moving them outside?
For a strong early start, use a light potting mix, keep the tuber barely covered, and place the pot in a warm spot with good light. A step-by-step reference is 8 Tips for Growing Better Dahlias, which covers timing, planting depth, and early care.
How do I keep greenhouse dahlias compact and flowering?
Pinching the growing tip when plants are still young encourages branching and more blooms. For a fuller walkthrough, see How to Pinch and Stake Dahlias or All About Dahlias.
What if I want to choose dahlias that are especially good for big blooms?
Dinnerplate types are a strong fit if you want oversized flowers for cutting and display. Browse the Dahlia Dinnerplate Daybreak Collection or the Dahlia Dinnerplate Cafe Au Lait & Mero Star collection to compare bloom style and color.
How do I know which hardiness zone I garden in?
You can check your zone with Longfield Gardens’ Hardiness Zone Map, which helps match plant choices to your climate.
What should I do before ordering dahlias for spring?
Before ordering, review shipping information so you know when your tubers are expected to arrive and how to handle them once they do.
Where can I find more general dahlia growing help?
If you want a broader overview, What Is the Best Way to Grow Dahlias? is a helpful next stop for planning a successful season.
