Starting Cosmos Seeds Indoors: How to Get the Best Results

Cosmos plants are popular garden flowers that can be grown outdoors or indoors. To start growing cosmos plants indoors, it is essential to know when to start the seeds.

In this article, we will provide tips on how to get the best results when starting cosmos seeds indoors.

When to Start Cosmos Seeds Indoors

cosmos seeds indoors (when to start cosmos seeds indoors)

If you want to get a head start on your gardening this year, Cornell University Home Gardening recommends starting your cosmos seeds indoors four to six weeks before the last spring frost.

If you start them before four to six weeks, you risk producing leggy and weak plants. However, if you begin them too soon, the seedlings will not have enough time to develop fully.

The seeds germinate very quickly, so it’s essential to keep an eye on them and transplant them into larger pots as needed for a few weeks before moving them outdoors during the hardening-off process.

How To Start Growing Cosmos Indoors

cosmos seeds indoors (when to start cosmos seeds indoors)

Pick A Tray Or Pot

If you’re interested in starting to grow cosmos indoors, one of the first decisions you’ll need to make is what kind of tray or pot to use. There are a few things to keep in mind when making this decision.

First, consider the size of the tray or pot. Cosmos can grow quite tall, so make sure you choose a size that will give them plenty of room to grow.

Second, think about the material the tray or pot is made of. Some materials, such as plastic, can retain heat more than others. This can be beneficial for cosmoses, as they prefer warm temperatures. However, if your home tends to be cooler, you may want to choose a material that won’t retain heat as much, such as ceramic or terra cotta.

Finally, consider the drainage of the tray or pot. Cosmos need well-drained soil, so ensure good drainage holes in the bottom of the tray or pot.

Fill With Soil

One of the most critical steps in growing cosmos is getting the soil right. Cosmos need well-drained soil that’s rich in nutrients. To create this type of soil, mix equal parts potting mix, sand, and perlite. Once you’ve mixed the ingredients, fill your pots or trays with the soil mixture, and you’re ready to plant!

Plant The Seeds

Now it’s time to plant the seeds! Cosmos seeds are very small, so it can be helpful to use a toothpick or other sharp object to make a small hole in the soil before planting.

Once you’ve made the holes, drop one or two cosmos seeds into each hole and lightly cover them with soil. Be sure to keep the soil moist but not too wet and in a warm location until the seeds germinate.

Light Requirements

As we mentioned earlier, the cosmos prefer warm temperatures. They also need a good amount of light to grow well. If you live in a cooler climate, you may need to supplement the natural light with grow lights.

Water Requirements

Cosmos are drought-tolerant plants, so they don’t need a lot of water. Overwatering can be a problem, as it can lead to fungal diseases.

To water, your cosmos, wait until the top inch or so of soil is dry and give them a good soaking. Allow the excess water to drain away, and don’t water again until the soil is dry.

Fertilizer Requirements

Cosmos are not heavy feeders, so they don’t need a lot of fertilizer. A light application of compost or a balanced fertilizer every few weeks should be sufficient.

Transplant Seedlings

Once the cosmos seeds have germinated, it’s time to transplant them into larger pots. Be careful when handling the seedlings, as they are delicate at this stage.

Fill the new pots with the same soil mixture you used when planting the seeds. Once you’ve transplanted all the seedlings, water them well and place them in a warm location.

Hardening Off

Now that your cosmos plants are big enough to be transplanted outdoors, it’s time to start the hardening-off process. This is a gradual process that helps plants acclimate to the outdoors.

Start by placing the plants in a sheltered location outdoors for a few hours each day. After a week or two, you can begin leaving them out overnight. Finally, after about three weeks, your plants should be ready to be transplanted in to your garden!


Starting cosmos seeds indoors is a great way to get a head start on the growing season. By following the steps outlined above, you’ll be well on your way to having a beautiful crop of cosmos! Thanks for reading, and good luck!