Why Are My Cosmos Seedlings Leggy? And What To Do About It

By Paul Smart •  Updated: 08/17/21 •  5 min read

Why Are My Cosmos Seedlings Leggy? Cosmos is a popular annual that is widely used in cottage gardens due to its beautiful flowers which come in an array of colours from whites, to orange, red and purple. However, gardeners often complain that their Cosmos seedlings are extremely leggy. So what can be done about it?

Cosmos plants by nature are relatively tall annuals that will often reach a height of 10 inches or so as seedlings with leaves appearing on the top couple of inches of the plant. However, the extent of this is increased when the plants are not exposed to sufficient light. Lack of light tends to encourage Cosmos to grow even taller in search of more light.

To reduce the chances of the seedling becoming exceptionally leggy it is best to place the plant in a sunny location where there is plenty of light. The ideal place is in a greenhouse however, if that is not possible the next best option is the sunniest window ceil you have. However, window ceil with direct late afternoon sun should be avoided because it can be extremely hot and will easily dry out the seedling.

To reduce the ready leggy nature of the Cosmos it is also advisable pinch out the tops of the seedlings just above a set of true leaves. Doing this will encourage the formation of side shoots which over time will create a bushier plant. It is important to note that you can remove large proportions of growth and it will not affect the longer-term survival of the plant provided that you leave a few true leaves on the plant.

How To Grow Cosmos

Most Cosmos varieties are tender annual plants that need to be resown every year though they will also self-seed readily in the right conditions. The best time to sow Cosmos is in early spring which will allow them time to become well established before the summer arrives.

In a cool climate, it is best to start Cosmos indoors in a seed tray though they can be sown direct later in the season. When planting seeds in seed trays it is best to use a seed raising mix as garden soil will contain weed seeds and possible some pathogens. When planting the seed that should be planted at a depth of 0.5 inches (1 cm), the seedling will emerge after 7 to 10 days and will typically need to be in the seed trays for approximately 6 weeks.

As most common varieties are relatively tall plants you can typically expect the plants to reach 10 inches (25cm) in height at this point. Cosmos seedlings should only be planted out in the garden when any risk of frost has passed.

Planting Cosmos Seedlings In The Garden

When planting Cosmos in the garden it is best to select a warm sunny location with rich moist and well-drained soil. If your soil is poor it will benefit from the addition of compost to increase nutrient levels. Plants should be spaced 10 to 12 inches apart and thick mulch should be placed on the soil to improve water retention.

Cosmos are ideal in mixed borders within a cottage garden as they produce masses of flowers. The height of the Cosmos can vary significantly from as little as 1 ft up to 8 ft depending upon the variety. This generally means that they are best positioned to the middle part or rear of the border.

In terms of issues with disease and pests, Cosmos are largely trouble-free plants to grow. The main issue that occurs is when slugs and snails attack the plants when they are young and tender seedlings. As such they require a little protection in the first few weeks they spend in the garden, however after that period they will not be affected by these pests.

To provide protection it is advisable to sprinkle snail pallets around the plants on the same day that they are transplanted into the garden, this measure is usually sufficient to stop any attacks.

Making The Most Of The Flowering Period

Cosmos is a relatively late flowering annual which in most cases will begin to produce its first blooms in early summer and has the potential to continue producing flowers right through until the first frosts in Autumn. These properties make the plant a particularly useful plant as the Cosmos attracts pollinators and also makes a fantastic cut flower.

To maximize the length of the flowering period it is important to deadhead the flower to encourage the plant to continue to produce new buds. To increase the chances of this happening it is important cut the stem just above a leaf node where leaves appear on the stork as this is the point where new growth will occur.

For taller varieties it may also be necessary to support them in order to keep the upright. The traditional method of doing this is to use stakes and tie the plant to the stakes using string. However, a less obstrusive way to support Cosmos is to use wire bent into a horse shoe shape.

The horse shoe shape has its ends bent at a 90 degree to form the spear like structures that is pushed into the ground. The structure is easy to install and works best when put into place before the Cosmos needs the support.

The supports can be easily made in a few minutes if you have the right material. To see how they are made watch the video below.

Paul Smart