How to Grow Lobelia

Every fall, I get excited when I see lobelia at the nursery. And almost every fall, I buy it too early and lose a few plants before the weather cools down.

Lobelia (Lobelia erinus) might be my favorite flower. I love the intense blue, especially when it spills over the edges of my raised beds and containers. But it’s a cool-season flower and waiting to plant is the biggest lesson lobelia has taught me in my Arizona garden. Here’s how I grow lobelia from seed or transplants, when to plant it, and how to keep it blooming as long as possible.

1. Grow Lobelia From Seed or Transplants

You can grow lobelia from seed, but transplants are usually easier. Lobelia seeds are tiny, and seedlings take time to develop.

If you’re starting lobelia from seed:

• Start seeds indoors about 8 to 10 weeks before you plan to transplant them outside.
• Gently press seeds onto the surface of moist seed-starting mix. Don’t cover them because they need light to germinate.
• Keep the soil surface consistently moist while the seeds germinate.
• Give seedlings plenty of light once they emerge.

Lobelia is one of those flowers I recommend starting indoors rather than direct sowing. The tiny seedlings are easier to care for in a controlled environment. Learn more about how to start seeds indoors.

If you’re buying transplants:

Look for compact plants with healthy green leaves and just a few flowers. Avoid plants that are yellowing, leggy, or already struggling.

In hot climates like Arizona, wait until temperatures cool before planting, even if the nursery has transplants available. Before you put them in your garden, learn how to plant transplants.

Lobelia Varieties I Grow

Cascade of Color: A mix of colors with a trailing habit that’s especially nice in hanging baskets and pots.

Crystal Palace: Compact plants with deep blue flowers.

Blue Heaven (Windowbox): Light blue flowers that work well in containers.

2. Plant Lobelia at the Right Time

Timing matters when growing lobelia, especially in hot climates.

Lobelia prefers cool weather. It can bloom for months when temperatures are mild, but flowering slows or stops when it gets hot.

When to plant lobelia in Arizona’s low desert:

• Start seeds indoors: July through October
• Plant transplants outside: Late September through December

I usually wait until daytime temperatures have cooled down before planting. If we’re still having days in the upper 90s°F, it’s better to wait a little longer.

Once established, lobelia grows well through our mild winters and into spring. It often begins declining as temperatures rise in late spring or early summer.

Plant in a spot with morning sun and afternoon shade to help it last longer.

When to plant lobelia in cooler climates:

Plant lobelia outdoors in spring once the danger of hard frost has passed. In areas with cooler summers, it may continue blooming through much of the growing season.

If you’re gardening in Arizona, my September planting guide and December planting guide can help you plan what to plant alongside lobelia.

If you want an easy reference for planting times, spacing, and flower varieties, my Flower Planting Guide can help you plan your garden.

3. Give Lobelia the Right Growing Conditions

Lobelia grows best with the right combination of sunlight, moisture, and well-draining soil.

Soil: Plant lobelia in rich, well-draining soil. Adding compost before planting helps improve soil structure and provides nutrients.

Sunlight: In cooler climates, lobelia grows well in full sun or partial shade. Here in Arizona, I prefer morning sun with afternoon shade. Afternoon shade makes a difference as temperatures warm up.

Mulch: Add a layer of mulch around the plants to help retain moisture and moderate soil temperatures.

Fertilizer: Lobelia benefits from fertile soil, but you don’t necessarily need to fertilize every time you plant. I often use compost and worm castings, and they work well. If plants need extra nutrients, especially in containers, use a flower fertilizer as directed on the label.

Temporary shade: If the weather is still warm when you transplant lobelia, provide temporary shade for the first few days to reduce transplant stress.

4. Care for Lobelia as It Grows

Once lobelia is established, it doesn’t need a lot of complicated care. The most important thing is keeping the soil consistently moist.

Water regularly. Lobelia doesn’t tolerate dry soil well. Check the soil often, especially in containers, and water before it dries out completely. Avoid keeping the soil soggy, though, because too much moisture can lead to root and stem rot.

Trim plants when needed. Lobelia doesn’t require constant deadheading. If plants begin looking leggy or flowering slows, give them a light trim. When temperatures are still cool, this can encourage fresh growth and more flowers.

Watch for signs of stress. Wilting may mean plants need water, while yellowing or collapsing stems can signal overly wet soil or root problems. If your lobelia begins declining as temperatures rise, heat may be the cause.

Allow some flowers to set seed. Lobelia can sometimes self-seed when conditions are right. Leave a few flowers in place at the end of the season if you’d like to see whether new seedlings appear.

Keep pets and children in mind. Lobelia contains compounds that can be toxic if eaten. Keep plants out of reach of pets and young children who might ingest them.

What to Plant with Lobelia

One of my favorite things about lobelia is how well it fits into small spaces. It stays low-growing, usually around 6 to 9 inches tall, although trailing varieties can spread much farther.

I especially like planting lobelia along the edges of raised beds and containers. The flowers soften the edges and spill over the sides as the plants grow.

Lobelia also looks beautiful paired with other cool-season flowers. Some of my favorite combinations include:

• Alyssum: Both are low-growing and look beautiful spilling over container edges. Learn more in my guide to growing alyssum.
• Violas: Their larger flowers pair nicely with lobelia’s tiny blooms. For more information, read how to grow violas.
• Blue salvia: I love combining these two flowers in the garden.
• Spring bulbs: Lobelia helps fill in the space around bulbs as they grow and bloom.

When I plant bulbs in containers, I tuck lobelia or alyssum around the edges for color while the bulbs grow. It’s a simple way to make the container look good before the bulbs bloom. If you’d like to try this, see my guide to growing bulbs in containers in hot climates.

For more cool-season flower ideas, check out my 10 easiest fall flowers and Arizona annual flowers visual guide.

Frequently Asked Questions About Growing Lobelia