Starting seeds may seem intimidating to a beginner, but it's a reasonably straightforward process. This guide discusses everything you need to know to start Nasturtium seeds. It walks you through all of the supplies you’ll need, teaches you how to prepare for planting, gives you step-by-step instructions for sowing, explains how to care for your new seedlings, and helps troubleshoot some of the most common problems.

Direct Sowing Seeds vs. Starting Seeds Indoors

Nasturtiums are vibrant, easy-to-grow annual flowers known for their bright orange, red, and yellow blooms, as well as their edible leaves and flowers that add a peppery flavor to salads and garnishes. These versatile plants can be grown in garden beds, along fences, or in containers, and they make excellent companions for vegetables because they attract pollinators and can help deter certain pests. Nasturtiums are particularly favored for their low-maintenance growth, rapid flowering, and ability to thrive in poor soil conditions without heavy fertilization.

Nasturtium seeds can be direct sown outdoors after the last frost or started indoors for an earlier start.

When to Start Nasturtium Seeds

When direct sowing seeds:

In most regions, outdoor sowing of nasturtiums is best done in late spring once the threat of frost has passed. 

When starting seeds indoors:

Nasturtiums can also be started indoors 2-4 weeks before the last frost to get a head start on the season. Seeds generally germinate within 7-14 days under the right conditions.

Presoaking Seeds Before Planting

Soaking seed before planting is a common way to improve germination. Seeds naturally have a hard outer layer to protect themselves from insects, diseases, and unfavorable germinating conditions. Soaking them in water softens the hard seed coat, exposing the embryo to moisture and kickstarting growth. 

Before planting, soaking Nasturtium seeds in warm water for 4-8 hrs. can help soften the seed coat and improve germination rates.

Direct Sowing Seeds Outdoors

Prepping the Soil

Prepare soil for nasturtiums by choosing a sunny spot with poor-to-average, well-draining soil, as rich soil yields fewer flowers. Loosen the soil to a depth of 8 inches, clearing debris. Avoid fertilizer, as too many nutrients produce excess foliage.

When planting, sow seeds directly 1/2 inches deep spacing 8-12" apart. 

Starting Nasturtium Seeds Indoors

Supplies Needed

• Nasturtium Seeds

• Growing media: Coconut coir and commercial potting soils are the two most common growing substrates used in containers, and both are available online or at local retailers. A quality growing medium in your containers is essential; it holds on to moisture and nutrients, provides air space around the roots, and anchors the plant’s roots to keep it upright.  

• Containers: Seed-starting trays are the easiest to use, especially with domes or covers. The trays are wide and shallow, allowing you to plant many seeds in one tray. You can also use individual pots to start your seeds or recycle plastic containers from your kitchen (clean yogurt cups, sour cream containers, etc.). Just ensure they are cleaned and sterilized.

• Plant tags: Use plastic or wooden plant tags to label what seeds you have planted and where. Plastic plant tags are more durable, and the words don’t fade as quickly; wooden tags are biodegradable and more environmentally friendly.

• Supplemental grow light (highly recommended): LED, fluorescent, and compact fluorescent grow lights are good choices for hobby growers. Grow lights emit different wavelengths or “colors” of light crucial for plant growth. 

• Seed starting mat (highly recommended): A seed-starting mat is used to speed up germination. It is similar to a household heating pad in size and shape and goes underneath a seed-starting tray to warm the growing medium.

Reusing Potting Soil

You can reuse potting soil from past gardening ventures, but it should be sterilized to remove pathogens or fungal spores, especially if you’ve had trouble with seedlings damping off in the past.

To sterilize your potting soil, thoroughly moisten it and place it in an oven-safe dish no more than three inches deep. Cover with aluminum foil and place in an oven preheated to 200°F. When the soil's internal temperature reaches 180°F, bake for thirty minutes without opening the oven door. Shut the oven off and keep the soil inside until it reaches room temperature.

Prepping Growing Medium

It’s best to pre-moisten the medium before filling your containers. The potting soil should feel damp but not soggy. It should hold together in a lump without excess water dripping if you squeeze a handful.

Moisten the potting soil by putting some in a large basin or bucket and adding lukewarm water. Use a clean trowel, serving spoon, or your hands to mix it thoroughly. Add more water or potting soil until you reach the desired dampness.

Step-by-Step Planting Instructions

1. Fill containers almost to the top with pre-moistened growing medium.
2. Plant seeds at a depth of about 1 inch, then cover with growing medium. 
3. Mist the potting soil with room-temperature water.
4. Cover the container by placing a lid or a plastic storage bag over it to increase the humidity.
5. Set containers on top of the seed-starting mat or somewhere slightly warmer than the air temperature in your home, such as on top of the refrigerator.
6. Periodically mist potting soil to moisten the medium without disturbing the seeds.
7. If heavy condensation collects on the inside of the lid or bag, remove it for a bit or prop it open to allow air movement.
8. Once seeds germinate and sprout, take the cover off to prevent damping off and move the container(s) to a spot with plenty of light.

Hardening Nasturtium Seedlings Before Transplanting Outdoors

About a week before you hope to move seedlings outside to the garden, start acclimating them to outdoor conditions. This process, called “hardening off," helps minimize transplant shock from severe temperature variation and light-exposure differences. Start by setting the planting trays or containers outside in a sunny spot protected from the wind for a few hours. Gradually increase the length of time the plants are outside every day, bringing them in at night, until it’s time to transplant.

Transplanting Seedlings Outdoors

When you transplant your nasturtiums outside, disturb or manipulate the root system as little as possible. Dig a hole slightly larger than the seedling’s root ball, gently remove the seedling from its container, and place it in the hole, keeping the soil level consistent with how it grew indoors. Backfill with soil and lightly press down to eliminate air pockets, then water thoroughly to help settle the roots.

Caring For Your Nasturtium Seedlings

Sunlight 

Indoors, seedlings need a spot that gets twelve to sixteen hours of bright daylight. The best places are close to south- or west-facing windows. 

• South-facing windows get the most sun; they typically have light streaming through them for most if not all of the day. 
• West-facing windows stream direct sunlight for a good portion of the day and are often shaded from the intense sun in the late afternoon. 

There is a high likelihood the light inside your house isn’t adequate for good growth, especially in the winter when there is less daylight. To compensate for low light and prevent spindly, leggy plants, supplement the natural light using a grow light.

Temperature Requirements

Temperatures around 55–70°F are ideal for nasturtium germination and early growth.

Watering Needs

Once seedlings have sprouted, keep the soil or growing medium moist without overwatering. 

About four to six weeks after germination (after you transplant indoor seedlings outdoors), you can scale back on watering. Instead of keeping the soil slightly moist, allow the soil to dry out slightly and give plants about an inch of water once or twice a week. 

• For outdoor seedlings, water them lightly every 2-3 days, soaking the soil 3 to 4 inches out from around the plants well. Avoid spraying the plants directly with the hose stream and dislodging them from the soil.

• For indoor seedlings, place the container(s) in a shallow pan of water to let them wick up the water they need. Bottom watering helps prevent overwatering and won’t dislodge the seeds or seedlings like overhead watering may.

Indoor Watering Concerns

Tap water is usually acceptable for indoor plants, but there are a few things to keep in mind:

• If your home has hard water, you will see a white buildup on the soil surface over time. This buildup comprises calcium and magnesium salts from the limestone in the water. Periodically flush the soil with filtered or tap water to remove.
• If you have softened water, collect water from your pipes before it goes through the softener or water your plants distilled or bottled water.
• If the local municipality treats your water, it may have a reasonably high chlorine content, which can be problematic in sensitive plants. Before watering your plants, fill a watering container and let it sit for 24 hours to allow the chlorine to dissipate.

Fertilizer

Nasturtiums are low-maintenance plants that thrive in poor to average soil, and one of their unique qualities is that too much fertilizer can reduce flowering. Over-fertilizing encourages lush foliage growth at the expense of the bright, abundant blooms that make nasturtiums so popular.

• Before Planting: If your garden soil is very poor, mix in a small amount of compost to provide gentle nutrients. Avoid using heavy or high-nitrogen fertilizers, as these will encourage leafy growth rather than flowers.

• During Growth: For most nasturtiums, fertilization is not necessary. If you do choose to feed them, use a balanced, low-strength liquid fertilizer sparingly, and only once or twice during the growing season. Dilute to half the recommended strength to avoid overstimulating foliage growth.

• Flowering Stage: Avoid fertilizing once the plants begin blooming. Maintaining moderate soil fertility encourages continued flower production and helps nasturtiums remain compact and vigorous.

Pro Tip: For nasturtiums, less is more. These flowers perform best in lean soil with minimal fertilization, producing long-lasting, vibrant blooms and thriving in containers, garden beds, or trailing over fences without the need for constant feeding.

Troubleshooting Common Problems with Nasturtiums 

Seedling Failure or Slow Germination

This can happen if seeds are planted too deeply or in cold soil. Nasturtium seeds have a hard outer coating, so presoaking seeds in warm water for 12–24 hours before sowing can improve germination rates. Plant seeds about ½ inch deep in well-draining soil and keep them evenly moist until sprouts emerge.

Poor Flowering

This can be caused by excessive nitrogen or very rich soil, which encourages leafy growth instead of blooms. To encourage strong flowering, plant hollyhocks in moderately fertile soil and limit heavy fertilization during the growing season.

Pests

Nasturtiums can also be affected by pests, such as aphids, white flies, or caterpillars, which may feed on leaves and young shoots. Regularly inspect plants and treat infestations with insecticidal soap, neem oil, or gentle water sprays to control pests without harming the vine.

Diseases

Diseases can occur, especially in damp, poorly ventilated conditions. Issues like fungal leaf spots or powdery mildew can develop if vines are overcrowded or watered overhead. Ensuring proper spacing, improving air circulation, and watering at the base of the plant can prevent most fungal problems.