How to Plant Begonia Corms for Summer Color

Table of Contents

1. Introduction
2. Choosing Your Begonia Varieties
3. When to Plant Begonia Corms
4. Selecting the Right Containers and Soil
5. Step-by-Step: How to Plant the Corms
6. Caring for Begonias Indoors
7. Moving Your Begonias Outdoors
8. Summer Care for Maximum Blooms
9. Troubleshooting Common Issues
10. Overwintering for Next Year
11. Conclusion
12. FAQ

Introduction

Finding bright, beautiful flowers that thrive in the shade can sometimes feel like a challenge, but tuberous begonias make it easy. These garden favorites are known for their oversized, rose-like blooms and lush foliage that can transform a dim corner into a vibrant oasis. Whether you want to fill hanging baskets with cascading colors or add texture to a shady border, learning how to plant begonia corms—technically referred to as tubers—is the first step toward a stunning summer display.

At Longfield Gardens, we enjoy helping gardeners discover how simple it is to grow these tropical treasures. These plants are exceptionally rewarding because a single corm can produce a full, bushy plant covered in flowers from midsummer right until the first frost. This guide is designed for home gardeners who want clear, practical steps to get their begonias off to the best possible start.

We will walk you through everything from selecting the right pots to the specific way you should tuck your corms into the soil. You do not need years of experience to succeed with begonias. By following a few basic principles regarding timing, light, and water, you can enjoy professional-looking results in your own backyard.

Choosing Your Begonia Varieties

Before you dig in, it helps to understand the different types of tuberous begonias available. While they all grow from similar-looking corms, their growth habits vary significantly. Knowing which one you have will dictate how you plant it and where it will look best.

Upright Begonias

Upright varieties grow on sturdy, vertical stems. These are perfect for garden beds, borders, and traditional patio containers. They often feature massive "dinnerplate" blooms that face outward, making them a focal point in any shade garden. If you are planting in the ground, these are usually the best choice because they hold their flowers high above the foliage.

Trailing or Pendula Begonias

As the name suggests, these begonias have a cascading habit. Their stems grow out and then drape downward, covered in smaller but more numerous flowers. These are the gold standard for hanging baskets and window boxes. When planting these, you may want to space them slightly closer together to create a full, overflowing look.

Scented Varieties

While many begonias are grown primarily for their looks, some varieties offer a delicate, sweet fragrance. These are wonderful choices for porch containers or seating areas where you can enjoy the scent on a warm summer evening. Regardless of the type you choose, the planting process remains largely the same.

Key Takeaway: Match the begonia's growth habit to your container. Use upright varieties for pots and garden beds, and trailing varieties for hanging baskets.

When to Plant Begonia Corms

Timing is one of the most important factors when growing begonias. Because these plants are native to tropical regions, they have no tolerance for frost. They love warmth and humidity, so you must plan your planting schedule around your local weather.

Starting Indoors for an Early Show

Most gardeners prefer to start their begonia corms indoors about 8 to 12 weeks before the last expected frost date. Begonias can be slow to wake up, sometimes taking several weeks just to show their first sprouts. Starting them inside gives them the "head start" they need to begin blooming in early summer rather than late summer.

If you live in a cold climate, starting indoors is almost a necessity. It allows the plant to develop a strong root system and several sets of leaves in a protected environment. By the time the weather is warm enough to move them outside, you will have a well-established plant ready to take off.

Planting Directly Outdoors

You can plant begonia corms directly into the garden once the soil has warmed up and all danger of frost has passed. However, keep in mind that begonias planted directly in the ground in late spring may not start blooming until much later in the season. For the best results, we recommend the indoor start method for almost all growing zones.

Understanding Your Zone

We ship our begonia corms according to your USDA hardiness zone so they arrive at the ideal time for indoor starting. If you are unsure of your last frost date, you can check with a local university extension office. As a general rule, do not move begonias outside until nighttime temperatures consistently stay above 60°F.

What to do next:

• Look up the average last frost date for your area.
• Count back 8 to 12 weeks to find your indoor planting window.
• Prepare a warm, bright indoor space for your starter pots.

Selecting the Right Containers and Soil

Begonias are not overly picky, but they do have specific needs regarding "drainage," which is simply how fast water leaves the soil. If the soil stays too wet, the corm can rot before it even has a chance to grow.

The Best Soil Mix

Always use a high-quality potting mix rather than garden soil for starting begonias. Garden soil is often too heavy and may contain pests or diseases. A good potting mix is "loamy," meaning it is light, fluffy, and holds just enough moisture without becoming soggy. Look for mixes that contain peat moss, perlite, or vermiculite. These ingredients help the soil breathe, which is vital for developing roots.

Choosing a Pot

For starting individual corms, small 3-inch or 4-inch plastic pots work perfectly. They are easy to manage on a windowsill or under grow lights. If you prefer to start several at once, you can use a shallow nursery tray.

When it comes to the final decorative container, make sure it has plenty of drainage holes. Begonias have relatively shallow root systems, so they don't need incredibly deep pots, but they do need space to spread out.

• 6-inch pot: Ideal for 1 corm.
• 8-inch pot: Fits 2 corms comfortably.
• 12-inch pot: Perfect for a full look with 3 corms.

Step-by-Step: How to Plant the Corms

Now that you have your supplies and your timing right, it is time to plant. This process is simple, but getting the orientation of the corm right is the most important part.

Step 1: Identify the Top

Begonia corms look like small, brown, lumpy discs. If you look closely, one side is usually "convex" (rounded like a bowl's bottom) and the other side is "concave" (indented or hollowed out like a bowl's interior).

• The indented side is the top. This is where the sprouts will emerge.
• The rounded side is the bottom. This is where the roots will grow.

If you see tiny pink or white buds in the hollow part, those are the "eyes" or sprouts. If you truly cannot tell which side is which, you can plant the corm on its side, and the plant will naturally find its way up.

Step 2: Prepare the Soil

Fill your pots or trays with your potting mix. Water the soil lightly before planting so it is moist but not dripping wet. This "pre-moistening" ensures the corm makes good contact with the soil immediately.

Step 3: Planting Depth and Spacing

For indoor starting, you don't need to bury the corms deep. In fact, many gardeners simply "nestle" them into the soil. Place the corm with the hollow side facing up. Press it down so the top of the corm is just level with the soil surface or covered by a very thin layer (no more than half an inch) of soil.

If you are planting multiple corms in a tray, space them about 2 to 4 inches apart. If planting directly into a large outdoor container or garden bed, space them 8 to 12 inches apart to allow for mature growth.

Step 4: Initial Watering

Water the corm gently after planting. This settles the soil around the tuber and removes any large air pockets. Be careful not to pour water directly into the "hollow" of the corm, as sitting water in that indentation can cause rot. Instead, water the soil around the edges of the corm.

Key Takeaway: Always plant begonia corms with the hollow side facing up and only cover them with a very thin layer of soil to prevent rot.

Caring for Begonias Indoors

Once your corms are tucked into their pots, they need the right environment to "wake up." This phase requires patience, as begonias are not the fastest starters in the garden world.

Light and Warmth

Place your pots in a warm spot that stays between 65°F and 75°F. They do not need much light until the first sprouts appear, but once you see green growth, they need plenty of indirect sunlight. A bright windowsill or a spot under grow lights is perfect. Avoid harsh, direct midday sun, which can scorch the tender new leaves.

Humidity and Moisture

Begonias love humidity. If your home is very dry, you can loosely cover the pots with a clear plastic bag or a plastic dome to create a mini-greenhouse effect. This keeps the moisture levels consistent.

Be very careful with your watering during this stage. The corm does not have a root system yet, so it cannot drink much water. Keep the soil "damp" but never "soggy." A common mistake is overwatering before the plant has leaves; if the soil feels wet, wait another day or two before watering again.

Transplanting to Larger Pots

Once your begonias have two or three leaves and are several inches tall, they may outgrow their starter pots. If you see roots coming out of the drainage holes, it is time to move them to their "forever home" for the summer. Be very gentle when moving them, as the stems are succulent and can snap easily.

What to do next:

• Check your pots every few days for signs of sprouts.
• Maintain a steady temperature above 65°F.
• Use a spray bottle for gentle watering if the soil surface looks dry.

Moving Your Begonias Outdoors

Transitioning your plants from a cozy indoor environment to the garden is a process called hardening off. Doing this gradually ensures your begonias don't go into shock from the change in temperature and light.

The Hardening Off Process

When the weather is consistently warm (nighttime temperatures above 60°F), start by placing your pots in a shaded, protected spot outside for just an hour or two. Each day, increase the amount of time they spend outdoors. After a week of this, they should be ready to stay out all night.

Finding the Perfect Spot

Begonias are famous shade lovers, but "shade" can mean different things.

• Partial Shade: This is usually 2 to 4 hours of sun, ideally in the morning.
• Full Shade: This means no direct sun, but plenty of bright, indirect light.

In hotter climates, begonias need more shade to prevent the leaves from wilting. In cooler, northern climates, they can handle a bit more morning sun. Avoid windy spots, as the heavy blooms and brittle stems can be damaged by strong gusts.

Soil Preparation for Garden Beds

If you are moving your begonias from pots into the ground, take a moment to improve the soil. We recommend mixing in some compost or well-rotted manure. This adds nutrients and helps the soil hold the right amount of moisture. Space the plants about 10 inches apart to ensure good air circulation, which helps prevent powdery mildew.

Summer Care for Maximum Blooms

Once your begonias are established in their summer home, they are relatively low-maintenance. A few simple habits will keep them blooming until autumn.

Watering Correctly

The "goldilocks" rule applies here: not too dry, not too wet. Feel the soil with your finger. If the top inch feels dry, it is time to water. Always water at the base of the plant rather than over the leaves. Wet foliage in the shade can lead to fungal issues. If your begonias are in hanging baskets, they will dry out much faster than those in the ground, so check them daily during heatwaves.

Feeding Your Plants

Begonias are heavy feeders because they are constantly producing new flowers. Use a balanced, water-soluble fertilizer every two weeks. Look for a formula where the three numbers on the label (nitrogen, phosphorus, and potassium) are roughly equal, or one specifically designed for flowering plants. This provides the energy the plant needs to sustain those big, beautiful blooms.

Deadheading and Grooming

Deadheading is the practice of removing faded flowers. For begonias, this isn't strictly necessary for the plant to keep blooming, but it keeps the display looking tidy. When a flower starts to wilt or turn brown, simply pinch it off at the base of its small stem. Also, remove any yellowing leaves to keep the plant's energy focused on new growth.

To Pinch or Not to Pinch?

Some gardeners choose to "pinch back" their begonias when they are young. This involves snipping off the very tip of the main stem to encourage the plant to grow more side branches. This results in a bushier plant with more flowers. If you want one massive, show-stopping bloom, you can leave the plant alone or even remove the smaller side buds to let the central flower get as large as possible.

Key Takeaway: Consistent watering at the base and regular feeding every two weeks are the secrets to a lush, long-blooming begonia display.

Troubleshooting Common Issues

Even with the best care, you might run into a few minor hurdles. Most begonia problems are easily solved by adjusting your care routine.

No Blooms

If your plant looks healthy and green but isn't flowering, it might not be getting enough light. While they love shade, they still need "bright" indirect light to produce buds. Try moving the pot to a slightly brighter location. Another cause can be too much nitrogen in your fertilizer, which encourages leaves but discourages flowers. Switch to a fertilizer with a higher middle number (phosphorus).

Wilting or Drooping

If the soil is wet and the plant is wilting, you are likely overwatering. Let the soil dry out completely before watering again. If the soil is bone-dry and the leaves look limp, give them a deep, thorough drink. They should perk up within a few hours.

Brittle Stems or Breaking

Begonia stems are full of water and can be quite heavy. If your upright begonias are leaning or look top-heavy, you can use a small bamboo stake and a loose tie to give them extra support. This is especially helpful for large-flowered varieties.

Toxicity Note

It is important to remember that begonia corms and plants are toxic to pets if ingested. If you have curious dogs or cats, it is best to keep your pots on high tables or in hanging baskets where they are out of reach.

Overwintering for Next Year

One of the best things about tuberous begonias is that they are not a one-year investment. You can save the corms and replant them next spring.

Entering Dormancy

In the fall, as the weather cools, your begonias will naturally start to turn yellow and die back. This is the plant's way of sending energy back into the corm for the winter. Stop fertilizing in late summer and reduce watering as the leaves begin to fade.

Storing the Corms

Before the first hard frost, dig up the corms (or bring the pots inside). Cut the stems back to about an inch above the corm. Let the corms sit in a dry, shady spot for a few days until the remaining stem stub dries up and falls off easily.

Shake off the excess soil and store the corm in a cool, dark, dry place. We recommend placing them in a paper bag or a box filled with dry peat moss or sawdust. Check on them once a month during the winter to make sure they aren't shriveling up or rotting. In the spring, you can start the whole process over again.

What to do next:

• Watch for yellowing leaves in late September or October.
• Prepare a storage box with peat moss before the first frost.
• Label your corms by color or variety so you know what is what next year.

Conclusion

Planting begonia corms is one of the most rewarding ways to bring life and color to the shaded parts of your landscape. By starting them indoors, paying attention to the orientation of the corm, and providing consistent moisture and food, you can create a professional-grade garden display that lasts all summer long. Gardening should be an enjoyable, creative process, and begonias are the perfect partner for achieving a beautiful yard with minimal stress.

• Start corms indoors 8-12 weeks before the last frost for early blooms.
• Plant with the hollow side facing up and tucked just below the soil surface.
• Use a light potting mix and ensure your containers have good drainage.
• Provide bright, indirect light and feed every two weeks during the summer.

We invite you to explore the many colors and forms of begonias at Longfield Gardens. Whether you choose the massive blooms of the upright varieties or the elegant drape of the trailing types, we are here to support your gardening journey with quality plants and practical advice. Happy planting!

Tuberous begonias are the "shade-garden champions," offering more color and texture per square inch than almost any other plant. With a little warmth and a bit of patience, those small brown corms will soon become the highlight of your summer garden.

FAQ

Which side of the begonia corm should face up when planting?

The side with the hollow or indented "bowl" shape should always face upward. This is where the stems and leaves will emerge, while the rounded bottom side will produce the root system. If you see tiny pink buds or sprouts, those should also be pointing toward the sky.

How deep should I plant begonia corms?

Begonia corms should be planted very shallowly. Tucking them into the soil so the top of the corm is either level with the soil surface or covered by no more than half an inch of potting mix is ideal. Planting them too deep can lead to rot and may prevent the sprouts from reaching the surface.

Do begonia corms need to be started indoors?

While it isn't strictly required, starting them indoors 8 to 12 weeks before the last frost is highly recommended. Begonias are slow to grow initially, and starting them inside ensures they are large enough to begin blooming by early summer. If you wait to plant them outdoors, you may only get a few weeks of flowers before the fall frost arrives.

How often should I water my newly planted begonia corms?

Before the corms have sprouted leaves, they should be watered very sparingly—only when the soil surface feels dry. Once the plant has established leaves and is growing actively, keep the soil consistently damp but never soggy. Always water the soil at the base of the plant to keep the corm and foliage dry.