Dahlias are the showstoppers of the late summer garden, bursting onto the scene just as other plants start to fade. With their vibrant colors and stunning variety of shapes and sizes, dahlias can turn any garden into a head-turning display that lasts well into fall.
While these beauties may need a bit of extra care in some climates, the spectacular results are well worth the effort. This article will cover everything you need to know about dahlias, how to grow them, and tips for overwintering.
Important Things To Know About Dahlias
Dahlias are versatile flowers that come in an array of options for gardeners. From small border plants to towering specimens, there’s a dahlia to suit every garden style and space.
Buying Dahlias
Dahlias are typically sold as tubers, which resemble small, elongated potatoes. Garden centers may also offer potted dahlias in spring for those who prefer to start with established plants. For the widest selection of dahlia varieties, local dahlia societies are excellent resources. Another tip for securing the best plants is to check online nurseries, which often carry a more extensive and sometimes exclusive inventory.
Cold Hardiness
Dahlias are perennials in USDA zones 8–11, where they can remain in the ground year-round. In cooler regions, gardeners have two options: treat dahlias as annuals or dig up the tubers in fall for winter storage. When purchasing dahlias, check the variety’s cold hardiness, which can vary greatly. If you live in an area with harsh winters, measures like heavy mulching can offer some protection, but digging up the tubers to protect them during the cold is the generally recommended tactic.
Size and Variety
Dahlias range from compact 1-foot-tall varieties perfect for containers to impressive 6–8 foot giants that make stunning focal points. Flower forms are equally varied, including cactus, decorative, ball, and collarette types, among others. Some varieties offer unique qualities such as bicolor petals or particularly long bloom times, expanding the palette for gardeners looking to make a vivid statement.
Dahlia Growing Conditions
Dahlias need specific growing conditions to thrive and produce an abundance of blooms. Provide the right environment for healthy plants and get spectacular flowers in return.
Sunlight Requirements
Dahlias love the sun and perform best with 6–8 hours of direct sunlight daily. Some afternoon shade protects the plants from heat stress in particularly hot climates. Give your plants enough sunlight so they produce vibrant and plentiful blooms.
Soil Preferences
The key to successful dahlia growth lies in the soil. These plants prefer loose, fertile, well-draining soil with a slightly acidic pH. Dahlias develop an extensive network of fine feeder roots, so light soil that retains moisture without becoming waterlogged is ideal. Amend heavy clay soils with compost, sand, or humus to improve drainage and texture. If you’re unsure about your soil’s composition, conducting a soil test can provide valuable insights that help you make precise changes and get better results.
Watering Needs
Dahlias need consistent moisture, but they won’t tolerate soggy conditions. Water deeply and regularly, especially during dry spells, and use well-draining soil to prevent rot. A good rule of thumb is to provide about an inch of water per week, adjusting for rainfall. Installing a drip irrigation system provides consistent watering while reducing the risk of fungal issues caused by water splashing onto foliage.
Fertilization
Dahlias are heavy feeders, particularly when in full bloom. Start with rich soil and supplement with a balanced organic fertilizer throughout the growing season. A fertilizer with a ratio lower in nitrogen and higher in phosphorus and potassium (such as 5-10-10) promotes strong blooms. Over-fertilizing can lead to lush foliage at the expense of flower production, so follow the recommended feeding schedule. Additionally, incorporating slow-release fertilizers provides continuous nourishment without the risk of overdoing it.
How To Plant Dahlias (5 Steps)
Proper planting sets the stage for healthy, productive dahlia plants. By following these steps, you can give your dahlias the best start possible.
When To Plant
Timing is extremely important when you’re planting dahlias. Wait until all danger of frost has passed and the soil has warmed in spring. If the ground is still soggy from spring rains, wait until it dries out a bit to prevent rot. Soil temperatures should ideally be at least 60°F for optimal root development and disease resistance.
Soil Preparation
Before planting, prepare the soil by tilling to a depth of about 12 inches. Work in plenty of well-rotted compost or other organic matter to improve soil structure and fertility. Avoid using fresh manure or compost, as these can be too high in nitrogen and may encourage leafy growth at the expense of flowers. Adding a bit of bone meal during soil preparation can provide additional phosphorus, which is beneficial for root and flower development.
Planting Process
Plant the dahlia tubers as follows:
- Dig holes about 6 inches deep and space them 2 feet apart for most varieties (larger types may need more room).
- Place the tuber horizontally in the hole with the eye (the point where the stem will emerge) facing up.
- Cover with soil and water lightly.
- For tall varieties, insert a sturdy stake at planting time to avoid damaging the tuber later.
- Consider placing a small marker to identify the variety and flowering time, which helps with future garden planning.
Mulching and Labeling
After planting, apply 2–3 inches of mulch in a layer around the plants to help retain moisture and suppress weeds. Mulch also moderates soil temperature, reducing stress on plants during heat waves and retaining warmth during cooler periods. Label each plant with its variety name, as this will be helpful if you need to dig up the tubers in the fall.
5 Dahlia Growing Tips
With the basics of planting covered, here are some tips to help dahlias thrive throughout the growing season:
1. Fertilizing Schedule
Feed dahlias monthly with a balanced organic fertilizer low in nitrogen and higher in phosphorus and potassium. A 5-10-10 ratio works well. Dahlias may not need additional fertilizer in rich soil, so observe plant growth and adjust accordingly. If using liquid fertilizers, avoid over-application, which can lead to nutrient imbalances and plant stress.
2. Watering Practices
Aim for about an inch of water per week, including rainfall. Deep, infrequent watering encourages strong root development. Use a rain gauge to track natural precipitation and supplement as needed. To avoid fungal issues, water at the base of the plant rather than overhead, and water early in the day to allow foliage to dry before evening.
3. Deadheading and Disbudding
Regular deadheading (removing spent blooms) encourages continuous flowering. For larger, showier blooms, some gardeners practice disbudding. This involves removing the two side buds in a cluster of three, allowing the center bud to develop into a larger flower. This technique can be particularly useful for those planning to use dahlias as cut flowers or preparing them for exhibitions.
4. Support and Pinching
Tall dahlia varieties benefit from staking to prevent wind damage. Install stakes at planting time to avoid root disturbance later. For bushier plants with more blooms, pinch back the growing tips when plants are about 12 inches tall. This encourages branching, resulting in more flowering stems and a fuller appearance.
5. Pest and Disease Management
Dahlias can be susceptible to pests and diseases such as aphids and powdery mildew. Monitor plants regularly and address issues as soon as you notice them. Organic pest control methods and proper spacing for air circulation help prevent many common problems. Introducing beneficial insects like ladybugs can help control aphid populations naturally. Additionally, maintaining good garden hygiene by removing debris and old foliage reduces the risk of disease.
Overwintering Dahlias
In colder regions, dahlias need special care to survive the winter. Gardeners have two main options: in-ground protection or digging and storing tubers.
Protecting Dahlias in the Ground
In zones where dahlias are marginally hardy, they can sometimes be overwintered in the ground with extra protection. Here’s how:
- After the first frost, cut back the foliage to about 6 inches aboveground.
- Apply a thick layer of mulch (8–12 inches) over the plant crowns.
- In spring, remove the mulch gradually as the weather warms.
- If your region experiences particularly harsh winters, consider adding a layer of straw or leaves under the mulch for added insulation.
Digging and Storing Tubers
For reliable overwintering in cold climates, digging up tubers is the safest method:
- After the first frost, cut back foliage and carefully dig up the tuber clumps.
- Clean off excess soil and allow tubers to dry for a day or two.
- Store in a cool, dark place in slightly damp peat moss or vermiculite.
- Label each variety before storage for proper identification and organization for the next planting season.
- Check tubers periodically over winter and remove any that show signs of rot.
Using Dahlias as Cut Flowers
Dahlias make excellent cut flowers, bringing their vibrant beauty indoors. Here’s how to use dahlias as cut flowers:
- Cut flowers in the early morning when they’re fully open.
- Place stems immediately in warm water.
- Remove lower leaves that would be below the waterline in the vase.
- Change the water every few days to prolong freshness.
- Adding a floral preservative to the water can help extend the life of the blooms.
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