How Deep Do I Plant Begonia Corms? Expert Planting Guide

Introduction
Understanding the Ideal Planting Depth
Identifying the Top of the Corm
Starting Begonias Indoors vs. Outdoors
Soil and Drainage: The Key to Success at Any Depth
Spacing Your Begonias
Watering Techniques After Planting
Adjusting for Your Climate and Zone
Summary of Planting Steps
Caring for Your Begonias as They Grow
Realistic Expectations for Your Begonia Garden
Conclusion
FAQ

Introduction

Tuberous begonias are the stars of the shade garden, offering lush foliage and vibrant flowers that brighten up darker corners of the yard. At Longfield Gardens, we enjoy how Spring Planted Bulbs for Shade can provide consistent color from midsummer all the way to the first frost. If you are ready to start your own, you might be looking at the unusual shape of the corm and wondering exactly how to get it into the soil.

This guide is designed for home gardeners who want to understand the simple mechanics of planting depth and orientation. We will explain how to set your begonias up for a successful season by getting the basics right from day one. This post covers the ideal depth for indoor starting, outdoor planting, and how to tell which side of the corm should face the sky. For shipping timing, see our shipping information. Getting the planting depth correct ensures your begonias have the support they need to grow strong stems and abundant blooms.

Understanding the Ideal Planting Depth

When you are ready to plant your begonia corms, the most important thing to remember is that they prefer to be close to the surface. For a step-by-step look at the process, see How Do You Plant Begonias?. For most varieties, the ideal planting depth is about 1 inch. This means there should be roughly one inch of soil covering the top of the corm once it is settled in its permanent home.

If you are starting your corms in shallow trays indoors—a common practice to get a head start on the season—How to Start Tuberous Begonias Indoors shows the indoor routine. In this environment, you do not need to bury the corms a full inch deep.

Fill a tray with a high-quality, damp potting mix.
Place the corms on the surface, hollow side up.
Press them gently into the soil so they are about half-submerged.
Lightly sprinkle a small amount of soil over the top so the corm is just barely covered.

This shallow placement allows you to monitor the corms for growth. Once you see green sprouts appearing, you know the plant is active. When these sprouts are 2 to 3 inches tall, you can transplant them into larger containers or the garden, placing them at the final 1-inch depth.

Why Depth Matters for Begonias

Planting at the correct depth is not just about tradition; it is about the health of the plant. Begonia corms are susceptible to rot if they stay too wet for too long. If you bury a corm 3 or 4 inches deep, the soil stays cooler and wetter, which can lead to the corm decaying before it ever has a chance to sprout.

Conversely, planting too shallowly can cause the corm to dry out. The "sweet spot" of one inch provides enough insulation to keep the corm hydrated while allowing the warmth of the sun to reach the soil and signal the plant to begin growing.

Consistency Across Varieties

Whether you are planting upright begonias with large, rose-like flowers or cascading varieties meant for hanging baskets, the depth rule remains the same. Even though the growth habits of these plants differ, the corms themselves function similarly. They all need that shallow placement to emerge quickly and develop a healthy root system.

Key Takeaway: Always aim for a depth of 1 inch. This protects the corm from drying out while ensuring it stays warm enough to sprout quickly without the risk of rot.

Identifying the Top of the Corm

Before you can worry about how deep to plant, you must know which way the corm should face. Begonia corms have a very distinct shape that makes them different from a typical round tulip or daffodil bulb.

Most begonia corms are shaped like a thick disc or a shallow bowl. One side will be rounded, and the other side will have a noticeable indentation or "hollow" spot.

The Hollow Side Faces Up

The hollow, concave side is the top of the corm. This is where the growth buds, often called "eyes," are located. These buds will eventually turn into the stems and leaves of your begonia.

The rounded, convex side is the bottom. This is where the roots will emerge to anchor the plant and draw in nutrients.

Look for the indentation: If the corm looks like a small bowl, the inside of the bowl should face the sky.
Check for "hairs": Sometimes you will see small, dried-out hairy bits on the corm. These are often old root remains and are usually found on the rounded bottom.
Search for pink buds: On healthy, viable corms, you might see tiny pink or reddish bumps in the hollow center. These are the beginning of sprouts and definitely need to face upward.

Starting Begonias Indoors vs. Outdoors

Because begonias are tropical plants, they love warmth. In most parts of the United States, the outdoor growing season is not quite long enough to see the full potential of these flowers if you wait to plant them outside until the soil is warm. Many gardeners choose to start their corms indoors to get a "jump start."

Depth for Indoor Starting Trays

If you start your corms 8 to 12 weeks before the last frost, you will likely use a shallow nursery tray or small individual pots. In this environment, you do not need to bury the corms a full inch deep.

Fill a tray with a high-quality, damp potting mix.
Place the corms on the surface, hollow side up.
Press them gently into the soil so they are about half-submerged.
Lightly sprinkle a small amount of soil over the top so the corm is just barely covered.

Depth for Direct Outdoor Planting

If you live in a warm climate or choose to wait until the weather is consistently above 60°F at night, you can plant directly into the ground. In this scenario, go straight to the 1-inch depth.

Dig a shallow hole about 1 to 2 inches deep.
Place the corm hollow side up.
Cover with 1 inch of loose, friable soil.
Gently pat the soil down, but do not pack it tightly.

What to do next:

Check your local frost dates to decide when to start.

Gather shallow trays and a bag of well-draining potting soil.

Inspect your corms for the hollow "top" side before planting.

Soil and Drainage: The Key to Success at Any Depth

No matter how perfectly you measure your planting depth, your begonias will struggle if the soil is not right. "Drainage" is a word gardeners use to describe how fast water leaves the soil. Begonias need excellent drainage because their corms act like sponges; if they sit in a puddle, they will absorb too much water and rot.

Choosing the Right Potting Mix

For containers and indoor starting, we recommend using a peat-based potting mix or a professional "soilless" mix. These mixes are designed to hold onto the right amount of moisture while allowing excess water to flow out of the bottom of the pot. Avoid using "garden soil" or "topsoil" in pots, as these are too heavy and will compact around the corm, cutting off oxygen.

Amending Garden Beds

If you are planting your begonias in the ground, take a look at your soil texture.

Sandy soil: Water moves through it very quickly. You may need to add some compost to help hold just enough moisture for the roots.
Clay soil: This soil stays wet and can feel sticky. It is very dangerous for begonia corms. If you have heavy clay, we suggest planting in raised beds or containers instead. For more container guidance, see Can You Plant Begonias in a Pot?

Spacing Your Begonias

While depth is a vertical measurement, spacing is a horizontal one, and it is just as important. Begonias need room for their roots to spread and, more importantly, room for air to circulate around their fleshy stems and leaves.

Spacing in Containers

In a standard 12-inch pot, you can typically plant three begonia corms. For a ready-made option for a shady porch or deck, see the Bold Container Collection for Shade. This gives them enough room to grow into a full, lush display without being so crowded that they compete for nutrients.

Space the corms about 4 to 6 inches apart in a triangle pattern.
Keep them away from the very edge of the pot to ensure the roots have soil on all sides for insulation.

Spacing in Garden Borders

In a landscape setting, begonias can grow quite large over the course of a summer.

Space upright varieties 10 to 12 inches apart.
Space cascading varieties 8 to 10 inches apart if you want them to knit together into a solid carpet of color.

Proper spacing prevents a common begonia issue called powdery mildew. This is a white, dusty-looking fungus that grows on leaves when the air is still and humid. By giving your plants enough elbow room, you allow breezes to move through the foliage, keeping the leaves dry and healthy.

Watering Techniques After Planting

The moment you finish covering your corms with an inch of soil, it is time for the first watering. This "watering-in" process is vital because it settles the soil particles around the corm, removing any large air pockets that could cause the roots to dry out.

The First Soak

Use a watering can with a "rose" attachment (the nozzle with many small holes) to provide a gentle rain. You want to moisten the soil thoroughly without washing away the 1 inch of soil you just placed on top. Water until you see it running out of the drainage holes of the pot.

Ongoing Moisture Needs

After the initial watering, your goal is to keep the soil "damp, but not wet." A good way to visualize this is a wrung-out sponge. It should feel cool and moist to the touch, but if you squeezed a handful of soil, water shouldn't drip out of it.

Wait to water again: Don't water every single day. Stick your finger into the soil. If the top half-inch feels dry, it is time to water. If it still feels moist, wait another day or two.
Avoid the "crown": Try to water the soil around the plant rather than pouring water directly into the hollow center of the corm. Water sitting in that "bowl" can encourage rot.

Adjusting for Your Climate and Zone

At Longfield Gardens, we ship begonia corms at the appropriate time for your USDA hardiness zone. However, the weather in your own backyard is the ultimate guide for when those corms should move from their shallow indoor trays to their final 1-inch deep outdoor spots.

Temperature is Everything

Begonias are not just frost-tender; they are "chill-tender." This means they can be damaged even if it doesn't freeze.

Wait for 60°F: Do not move your begonias outside permanently until the nighttime temperatures are consistently above 60°F.
Warm Soil: If you are planting directly into the ground, wait until the soil feels warm to the touch. Planting a corm 1 inch deep into cold, spring-rain-soaked soil is a recipe for a slow start.

If a surprise cold snap is predicted after you have planted your begonias, you can protect them by temporarily placing an upside-down bucket over them or bringing containers into a garage or porch for the night.

Summary of Planting Steps

To ensure you get the depth and placement exactly right, follow this simple checklist when you are ready to plant.

Step 1: Select a container with drainage holes or a garden spot with loose, well-draining soil.
Step 2: Fill the container or loosen the garden soil.
Step 3: Identify the hollow side of the begonia corm.
Step 4: Place the corm hollow side up on the soil surface.
Step 5: Cover the corm with 1 inch of soil.
Step 6: Water gently to settle the soil.
Step 7: Place in a spot with filtered light or partial shade.

Final Planting Checklist:

Depth: 1 inch of soil over the top.

Orientation: Hollow side facing up.

Spacing: 8-12 inches apart.

Water: Thoroughly once, then only when the surface feels dry.

Caring for Your Begonias as They Grow

Once your corms have sprouted and are growing at their 1-inch depth, they will require very little maintenance. Tuberous begonias are efficient plants that know how to put on a show. For a broader overview, read All About Begonias.

Feeding for Better Blooms

Because begonias bloom so prolifically, they appreciate a little extra food. Once the plants have several sets of leaves, you can begin using a balanced, water-soluble fertilizer. Follow the instructions on the label, typically applying the fertilizer every 2 to 3 weeks. This provides the nutrients the plant needs to keep producing those large, heavy flowers.

Deadheading

"Deadheading" is the process of removing faded flowers. For begonias, this is mostly for appearance. When a flower begins to wilt or turn brown, simply pinch the stem off about an inch behind the flower head. This keeps the plant looking tidy and prevents old petals from falling onto the leaves, which can sometimes lead to rot in humid weather.

Watching for Pests

Begonias are generally quite hardy, but keep an eye out for slugs and snails, especially if your garden is very shady and damp. These pests love the juicy stems of begonias. If you see ragged holes in the leaves, a simple non-toxic slug bait or a copper barrier can help keep your plants safe.

Realistic Expectations for Your Begonia Garden

While we provide these guidelines to give you the best possible start, it is important to remember that gardening is a partnership with nature. Your local weather, the amount of humidity in your air, and the specific microclimate of your porch or garden bed will all influence how fast your begonias grow.

Some corms may sprout in two weeks, while others might take four or five weeks to show their first green leaves. This is normal. As long as the corm is firm to the touch and planted at that 1-inch depth in well-draining soil, it is likely just taking its time to build a strong root system before it sends up its first leaves.

Conclusion

Planting begonia corms is a rewarding way to bring bright, tropical color to the shaded areas of your home and garden. By focusing on the simple rule of planting 1 inch deep with the hollow side facing up, you remove the biggest hurdles to success. We find that when you provide the right foundation—good drainage, proper spacing, and the correct depth—these plants will reward you with stunning blooms for months on end.

Plant corms 1 inch deep in loose, well-drained soil.
Ensure the hollow side of the corm faces the sky.
Space plants 8 to 12 inches apart to allow for airflow.
Wait for warm nighttime temperatures before moving plants outdoors.

Gardening is a journey that starts with a single step into the soil. At Longfield Gardens, we believe that by getting the basics like planting depth right, you set the stage for a season filled with beauty and joy.

If you haven't yet chosen your varieties for the year, now is the perfect time to explore the different colors and shapes available in our 40% Off Pinks Purples & Whites.

FAQ

What happens if I plant my begonia corm upside down?

If a begonia corm is planted with the hollow side down, the sprouts will still try to grow, but they will have to grow around the side of the corm to reach the light. This uses up a lot of the plant's energy and results in a much slower start. If you realize you have planted one upside down, it is usually safe to gently dig it up and flip it over as long as the sprouts haven't grown too long.

Should I cover my begonias with mulch after planting?

While mulch is great for many plants, we generally recommend against putting a thick layer of mulch directly over the spot where begonia corms are planted. Mulch holds onto moisture and can keep the soil too wet and cool for the corm, which increases the risk of rot. It is better to leave the soil surface bare or use a very thin layer of very fine compost.

Can I plant begonia corms deeper to protect them from winter?

No, planting them deeper will not protect them from freezing temperatures. Tuberous begonias are not cold-hardy and will die if the ground freezes. Regardless of how deep they are planted, they must be dug up and stored indoors for the winter in cold climates (zones 8 and lower). Planting them deeper than 1 inch will only lead to poor growth or rot.

How many begonia corms can I plant in a 10-inch hanging basket?

For a 10-inch hanging basket, we recommend planting 2 or 3 corms. This ensures the basket looks full and lush as the plants grow and trail over the sides. Be sure to space them evenly around the pot, keeping them about 1 inch below the soil surface to allow room for watering at the top.

How Deep Do I Plant Begonia Corms?

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