Table of Contents
- Introduction
- The Ideal Planting Depth for Begonia Corms
- Understanding the "Hollow Side Up" Rule
- Why Soil Quality and Drainage Matter for Depth
- Starting Begonias Indoors for an Early Start
- Transplanting to the Garden or Final Pots
- Adjusting Depth for Different Begonia Varieties
- Watering Techniques to Protect the Tuber
- Site Selection: Sun, Shade, and Wind
- Long-Term Care and Success
- Making the Most of Your Shade Garden
- FAQ
Introduction
There is a special kind of magic in watching a shady corner of your yard transform into a lush, tropical-inspired retreat. Tuberous begonias are the stars of these spaces, offering ruffled petals and vibrant colors that thrive where other flowers might struggle. Whether you are tucking them into elegant patio pots or creating a colorful border under a favorite shade tree, the journey to a stunning summer display starts with one simple step: getting the planting depth exactly right.
At Longfield Gardens, we want to help you feel confident from the moment your begonias arrive at your door. Planting these unique corms is a straightforward process, but their unusual shape often leaves gardeners wondering which side is up and how deep they should go. A classic upright choice like Begonia Upright Double Roseform Pink is a good example. This guide will walk you through everything you need to know about planting depths for both indoor starts and outdoor displays. By understanding these basics, you can ensure your begonias have the best possible start for a long and floriferous season.
The Ideal Planting Depth for Begonia Corms
When you are ready to plant, the most important rule to remember is that begonias prefer a shallow start. For most varieties, the ideal depth is between 1/2 inch and 1 inch below the soil surface. For a step-by-step overview, see our How Deep to Plant Tuberous Begonias for Best Blooms. This shallow placement allows the developing sprouts to reach the light quickly without using up too much of the energy stored inside the corm.
If you plant a begonia too deep, the new shoots may struggle to break through the soil. This can lead to a delay in blooming or, in some cases, can cause the corm to rot before it ever sees the sun. On the other hand, planting them too shallowly—where the top of the corm is exposed—can leave the new growth vulnerable to drying out or being disturbed by wind and water.
Aim for that "sweet spot" where the corm is fully covered by about a half-inch of soil. This provides enough weight to anchor the plant as it grows while keeping the growth points close to the warmth and light they need to thrive.
Key Takeaway: The Shallow Success Rule
- Plant begonia corms 1/2" to 1" deep.
- Ensure the corm is fully covered by soil to prevent drying.
- Avoid deep planting to prevent sprout exhaustion and rot.
Understanding the "Hollow Side Up" Rule
Before you can worry about depth, you must identify which way the corm should face. Begonia corms (or tubers) have a very distinct shape that looks a bit like a small, brown, hairy bowl or a flattened potato with an indentation. One side will be rounded or "domed," and the other side will have a "dished" or "concave" center.
The indented, hollow side is the top. This is where the growth points, often called "eyes," are located. When you plant, this hollow side must face upward. The rounded, smooth side should face downward into the soil. Think of the corm as a small cup that you are setting onto the soil surface before covering it.
In some cases, you may even see small pink or white nubs beginning to emerge from the hollow area. These are the future stems of your plant. By placing the hollow side up and covering it with just a half-inch of soil, you are positioning those delicate sprouts in the perfect spot to grow straight up toward the light. For more detailed care tips, see our Tips for Growing Tuberous Begonias.
Why Soil Quality and Drainage Matter for Depth
The depth at which you plant is closely tied to the quality of the soil you use. Because begonias are tropical plants, they thrive in environments that stay consistently moist but never soggy. For the most consistent results, many gardeners prefer to grow tuberous begonias in containers where they can fully control the drainage and soil quality, especially a dramatic upright choice like Begonia Double Maxima Switzerland.
When planting in the landscape, we recommend amending your soil with organic matter like compost or peat moss. This creates a lighter, fluffier texture that allows water to move through easily. In a light, well-draining soil, a planting depth of 1 inch is perfect because it provides stability without the risk of water pooling around the corm.
If you are working with heavier clay soil and cannot amend it deeply, it is better to stay closer to the 1/2-inch depth. This keeps the corm in the upper, better-aerated layer of the soil where it is less likely to become waterlogged. For the most consistent results, many gardeners prefer to grow tuberous begonias in containers where they can fully control the drainage and soil quality.
Starting Begonias Indoors for an Early Start
In most parts of the United States, the growing season isn't quite long enough for begonias to reach their full potential if they are started directly in the ground. To get those big, showy blooms by early summer, many gardeners choose to "wake up" their corms indoors about 8 to 12 weeks before the last frost date.
Choosing Your Containers
You don't need deep pots for this initial phase. Shallow trays or small 4-inch pots work perfectly for a compact option like Begonia Upright Double Picotee White. Since the goal is just to get the roots and sprouts started, a few inches of soil is more than enough.
The Indoor Planting Process
- Fill your tray or pot with a high-quality, pre-moistened potting mix.
- Press the corm into the soil with the hollow side facing up.
- Cover the corm with just enough soil so it is barely hidden (about 1/2 inch).
- Place the container in a warm spot (around 70°F) with indirect light.
During this stage, be very careful with water. The soil should be slightly damp but never wet. Until the corm develops leaves, it doesn't have a way to "use" much water, so overwatering at a 1-inch depth can lead to rot very quickly.
What to Do Next: Starting Indoors
- Start 8–12 weeks before the last expected frost.
- Use shallow trays with drainage holes.
- Keep soil warm (70°F) to encourage sprouting.
- Water sparingly until the first green leaves appear.
Transplanting to the Garden or Final Pots
Once the weather warms up and the danger of frost has passed, it is time to move your started begonias to their permanent summer home. By this point, your plants likely have several leaves and a healthy root system.
When transplanting, you should maintain the same depth the plant had in its starter pot. Avoid burying the stems deeper than they were originally growing. If you are moving a begonia from a small pot to a large decorative container, dig a hole that allows the top of the root ball to sit just below the new soil surface. For more spacing guidance, see our How Far Apart Do You Plant Begonias?.
If you are planting dormant corms directly into the garden because you live in a warm climate, follow the same 1/2-inch to 1-inch depth rule. Space the corms about 8 to 12 inches apart to allow for the lush foliage and flowers to fill in without crowding each other. This spacing ensures good air circulation, which helps keep the plants healthy and prevents mildew.
Adjusting Depth for Different Begonia Varieties
Not all begonias are the same, and the specific variety you choose might influence how you space and plant them. We offer several beautiful types that suit different garden roles.
Upright Varieties
Large-flowered varieties, like the stunning Upright Double Begonias, grow upright and can become quite top-heavy once they are in full bloom. For these plants, a slightly deeper planting of 1 inch can provide a bit more physical support for the heavy stems. As they grow, you might even find that a small bamboo stake helps keep the massive blooms from bending the stalks.
Hanging Basket and Trailing Varieties
Trailing begonias are designed to spill over the edges of pots and baskets. Because these plants are often grown in smaller volumes of soil in a hanging container, staying at the 1/2-inch depth is usually best. Hanging Begonias are often the right choice here. This leaves more room for the roots to grow downward into the pot. When planting a 12-inch hanging basket, you can fit about three corms to create a full, lush look that overflows with color.
Scented and Specialty Types
Varieties like the Scentiment series offer the added bonus of fragrance. These should be treated much like the standard upright types. Planting them at a 1-inch depth in containers near a seating area or doorway allows you to enjoy their perfume all summer long.
Watering Techniques to Protect the Tuber
Once your begonias are planted at the correct depth, how you water them becomes the next priority. The hollow side of the corm, which faces up, can act like a tiny bowl that collects water. If water sits in this indentation for too long, it can lead to stem rot.
To avoid this, try to water the soil around the base of the plant rather than pouring water directly over the center of the corm. This is especially important early in the season before the foliage has grown large enough to act as a natural umbrella. Using a watering can with a long, narrow spout makes it easy to reach the soil surface without getting the crown of the plant wet.
As the plants grow and the weather gets hotter, they will need more frequent watering. The goal is to keep the soil consistently moist, like a wrung-out sponge. If you have used a well-draining potting mix and followed the 1/2-inch depth rule, the soil should breathe well enough to keep the roots happy while providing the hydration the plant needs to produce those spectacular flowers.
Site Selection: Sun, Shade, and Wind
While depth and soil are critical, the location of your begonias will determine how well they perform. These plants are famous for their love of shade, but "shade" can mean many things. For help choosing the right spot, see our Where Do I Plant Begonias?
The Right Light
Most tuberous begonias prefer filtered light or dappled shade. Imagine the light under a tall tree or on a porch that gets a few hours of gentle morning sun but is protected from the harsh afternoon heat. If begonias get too much direct sun, their leaves can scorch. If they get too little light, they may become "leggy" and produce fewer flowers.
Wind Protection
Because begonia stems are succulent and filled with water, they can be somewhat brittle. If you live in a windy area, place your pots in a sheltered spot. A 1-inch planting depth provides some stability, but a strong gust can still snap a heavy stem. Keeping them near a wall, fence, or among other sturdy plants can provide the protection they need.
Long-Term Care and Success
One of the best things about tuberous begonias is that they are not a "one and done" plant. With a little bit of care, you can enjoy the same corms year after year.
As the summer winds down and the first frost approaches, the plants will naturally begin to yellow. This is a sign that they are moving energy back into the corm for winter. Once the foliage has died back, you can lift the corms from the soil, let them dry in a frost-free place, and store them in a cool, dry spot like a basement. To check your growing zone, use the Hardiness Zone Map.
The following spring, you can start the process all over again. You might even find that your corms have grown larger over the summer, which means even more blooms the following year!
Making the Most of Your Shade Garden
Gardening is a journey of discovery, and begonias are some of the most rewarding companions you can choose for that journey. They take a bit of patience to start, but the payoff is a summer-long display of color that rivals any sun-loving flower.
By focusing on the simple basics—planting 1/2 inch to 1 inch deep, ensuring the hollow side is up, and providing well-drained soil—you are setting yourself up for success. We are here to support you every step of the way with high-quality corms and the practical advice you need to see them flourish, backed by our 100% Quality Guarantee.
Summary Checklist for Begonia Planting
- Identify the top: Look for the hollow, indented side.
- Check the depth: Aim for 1/2" to 1" of soil coverage.
- Select the spot: Choose a location with dappled shade and wind protection.
- Monitor moisture: Keep soil moist but avoid pouring water into the center of the corm.
- Start early: Consider an indoor start 8–12 weeks before the last frost.
At Longfield Gardens, we believe that every gardener can achieve a beautiful yard with the right plants and a few simple rules. Tuberous begonias are an achievable way to bring high-impact color to the quiet, shaded corners of your home. Whether you are planting your very first corm or you are a seasoned pro, the joy of seeing those first ruffled petals emerge never fades.
FAQ
Can I plant begonia corms on their side if I can't tell which side is the top?
If the corm is very smooth or you truly cannot identify the indented side, planting it on its side is a safe alternative. The plant is naturally programmed to grow toward the light and will eventually send its sprouts upward and its roots downward. While planting "hollow side up" is the ideal method for the fastest growth, planting on the side is a great way to ensure the plant still succeeds without the risk of planting it completely upside down. For a fuller walkthrough, see our How Do You Plant Tuberous Begonias?
Will begonias grow if I plant them deeper than 1 inch?
They may still grow, but you might notice a few issues. Deeply planted corms take much longer to emerge from the soil, which can delay your bloom season. Additionally, the deeper the corm is buried, the more likely it is to stay in contact with cold, wet soil for extended periods, which increases the risk of rot. Stick to the 1/2-inch to 1-inch range for the most vigorous and healthy plants.
Do I need to peel the brown "skin" off the corm before planting?
No, there is no need to peel or clean the corm. The brown, hairy outer layer is a natural part of the tuber and provides a bit of protection. Removing it could accidentally damage the delicate growth points or "eyes" located in the center of the hollow side. Simply plant the corm as it arrives, and the sprouts will push through that outer layer on their own.
Should I water the corms immediately after planting them?
Yes, you should give them a light watering to help settle the soil around the corm and eliminate large air pockets. However, the key is to be very conservative. Until the corm has sprouts and leaves, it cannot process much water. The soil should be moist like a wrung-out sponge, but not saturated. If you are starting them indoors in a humid environment, you may only need to water them once every week or two until you see green growth.