When To Plant Dahlias In Pennsylvania

If you’re a gardener in Pennsylvania, you know timing is everything. Knowing exactly when to plant dahlias in Pennsylvania is the key to a spectacular summer and fall display of blooms.

These stunning flowers are not frost-hardy, so getting the date right protects your investment. Planting too early risks losing the tubers to a late cold snap. Planting too late gives you a shorter blooming season. Let’s break down the optimal schedule so you can plan with confidence.

For most of Pennsylvania, the safe window for planting dahlia tubers directly in the ground is from mid-May to early June. The single most important rule is to wait until after the danger of the last spring frost has passed.

Dahlia tubers will rot in cold, wet soil and new growth is instantly damaged by frost. A good benchmark is to plant around the same time you would put out tomato plants.

Understanding Your Pennsylvania Growing Zone

Pennsylvania spans USDA Plant Hardiness Zones 5a to 7b. This variation means your specific location greatly influences your planting date.

Zones 6 & 7 (Southeastern PA, Philadelphia area, parts of the Lehigh Valley): Your last frost date is typically between April 15th and April 30th. Aim to plant in early to mid-May.
Zone 6 (Central PA, including State College and Harrisburg areas): Your last frost is usually around May 1st to May 10th. Target mid-May for planting.
Zones 5 & 6 (Western PA, Pittsburgh area, and the higher elevations/Northern Tier): Frost can linger until mid-May. It’s safest to wait until late May or even early June to plant.

Always check a local frost date calendar for your exact town. When in doubt, wait another week. Dahlias planted in warm soil will catch up quickly.

Getting a Head Start: Pre-Sprouting Indoors

To enjoy blooms several weeks earlier, many Pennsylvania gardeners pre-sprout (or “wake up”) their tubers indoors. This process gives the plants a crucial head start.

You begin about 4 to 6 weeks before your intended outdoor planting date. Here’s how to do it:

Gather your dahlia tubers, some shallow trays or pots, and slightly moist potting mix or peat moss.
Place the tubers in the tray with the eye (the growth point, often near the old stem) facing upward. Cover them lightly with your potting medium.
Put the tray in a warm spot (60-70°F) with bright, indirect light.
Keep the medium barely moist, not wet. In a few weeks, you’ll see sturdy sprouts emerge.

Once sprouts are a few inches tall and outdoor soil is warm, you can carefully transplant them. This method is especially useful in shorter-season areas of the state.

Step-by-Step Guide to Planting Dahlias

When your soil is warm and it’s time to plant, follow these steps for success.

1. Choose and Prepare the Planting Site

Dahlias need full sun—at least 6 to 8 hours of direct sunlight daily. They thrive in rich, well-draining soil. Work in several inches of compost or well-rotted manure a week or two before planting. Good drainage is non-negotiable to prevent tuber rot.

2. Dig and Amend the Planting Hole

Dig a hole about 6-8 inches deep. For larger, dinnerplate varieties, go a bit deeper. Mix a handful of bone meal or a low-nitrogen fertilizer into the bottom soil. Nitrogen promotes leaves at the expense of flowers.

3. Plant the Tuber Correctly

Place the tuber horizontally in the hole with the eye (or sprout) pointing up. Cover with 2-3 inches of soil. Do not fill the hole completely yet. As the stem grows, you will gradually fill in the hole, which supports the plant. This is called “hilling up.”

4. Spacing and Staking

Space tubers according to their mature size:

Small bedding dahlias: 12-18 inches apart.
Medium to large dahlias: 18-24 inches apart.
Giant dinnerplate dahlias: 24-30 inches apart.

Insert a sturdy stake at planting time for taller varieties to avoid damaging the tuber later. Tie the stem loosely to the stake as it grows.

5. Water and Label

Water the planted tuber gently but thoroughly. Use a plant label to remember the variety name. It’s easy to forget what you planted where!

Caring for Dahlias Through the Season

Consistent care after planting ensures healthy plants and an abundance of blooms.

Watering: Provide about 1-2 inches of water per week. Deep, infrequent watering is better than daily sprinkles. A soaker hose is ideal. They are quite drought tolerant once established, but bloom best with consistent moisture.
Fertilizing: Use a low-nitrogen, high-phosphorus fertilizer (like a 5-10-10 or 10-20-20) every 4-6 weeks after the plant is 12 inches tall. Stop fertilizing by late August.
Pinching: When the plant has 3-4 sets of leaves, pinch out the center growing tip. This encourages bushier growth and more flowering stems.
Deadheading: Regularly remove spent flowers to encourage the plant to produce more buds right up until frost.

The Fall Routine: Digging and Storing Tubers

In Pennsylvania, dahlias cannot survive winter in the ground. After the first fall frost blackens the foliage, it’s time to dig them up.

Cut the stems back to about 6 inches.
Carefully dig around and lift the clump of tubers with a garden fork.
Gently shake off excess soil and let the clumps dry upside down in a frost-free, airy place for a day or two.
Store the clumps in a cool (40-50°F), dark place in boxes filled with slightly moist peat moss, vermiculite, or wood shavings. Check them occasionally through winter for shriveling or rot.

Proper storage means you can replant the same beloved varieties year after year.

Common Dahlia Problems in Pennsylvania

Watch for a few typical issues. Slugs and snails love young dahlia shoots; use organic bait or traps. Earwigs can chew on petals; trap them in rolled-up newspaper. Powdery mildew can appear in late summer; ensure good air circulation and avoid overhead watering. Good garden hygiene helps prevent many fungal diseases.

FAQ: Dahlia Planting in Pennsylvania

Can I plant potted dahlias earlier?

Yes, you can plant potted, pre-grown dahlia plants from a nursery a little earlier than tubers, as they are more established. But you must still be ready to cover them if a late frost is forcasted.

What if I miss the spring planting window?

You can plant dahlias as late as early July in Pennsylvania and still get a good show of blooms in the fall before frost. The plants may be slightly smaller, but they will flower.

Can I just leave dahlias in the ground over winter?

We do not recommend this in Pennsylvania. Winter soil is too cold and wet, and the tubers will almost certainly rot. Digging and storing is the reliable method.

When do dahlias typically bloom in PA?

With a mid-May planting, expect your first blooms to begin in mid to late July. Flowering will peak in August and September, continuing untill the first hard frost in October.

Following this timing guide takes the guesswork out of growing dahlias. With a little planning for your specific region of the state, you can enjoy a vibrant, colorful garden filled with these magnificent flowers all season long. The effort is well worth the stunning results.