What Are True Leaves on Seedlings?

When starting plants from seed, one of the most important things to understand is the difference between seed leaves and true leaves. Seed leaves, also called cotyledons, are the first leaves that emerge after a seed germinates. True leaves develop a bit later and signal that the seedling is established and ready for the next stage of growth. This article will explain in detail what true leaves are, how to identify them, and why they are such a vital milestone in seedling development.

What Are True Leaves?

True leaves are the second set of leaves that form on a seedling, after the initial cotyledons (seed leaves). They emerge from the growth tip (apical meristem) of the plant after the seedling has become established post-germination.

The true leaves come after the seed leaves. While there will only ever be one set of seed leaves that the plant keeps its whole life new true leaves will continuously grow as long as the plant itself is growing. The first few sets of true leaves will look like miniature versions of the mature plant’s foliage, just baby-sized.

Unlike the cotyledons, which were already formed inside the seed itself, true leaves are produced by the young seedling through photosynthesis. True leaves take on the characteristic form and function that subsequent leaves on the maturing plant will have. This transition allows the seedling to switch from dependence on the food reserves stored in the seed to making its own food via photosynthesis in the true leaves.

Identifying True Leaves

It’s easy to distinguish true leaves from the cotyledons based on their appearance and order of emergence:

• Cotyledons are the first leaves to pop out, right after the seed germinates. They often look very simple, rounded, or nondescript.

• True leaves emerge next from the apical meristem, after the cotyledons have become established. True leaves take on the definitive shape and features of the foliage of that particular plant variety.

• Additional sets of true leaves continue forming as the seedling matures and grows larger.

For example, tomato seedlings first put out a pair of oval cotyledons. The next leaves that emerge are divided into leaflets and closely resemble tiny versions of mature tomato plant leaves. These are the first true leaves.

Many common vegetables and herbs have noticeably different cotyledons versus true leaves like this. Comparing the seedling leaves to the foliage of a mature plant makes it much easier to distinguish between the two types.

Why Are True Leaves So Important?

The emergence of true leaves is a major developmental milestone for seedlings. Here are some of the key reasons they matter so much:

• They mark the transition to independence. With true leaves, the seedling can begin photosynthesis and produce its own food instead of relying on the seed reserves.

• They indicate the seedling is established. True leaves signal that the roots are developed enough to support leaf growth and the seedling is ready for transplanting or thinning.

• They show the plant is strong enough for fertilizer. Once true leaves appear, the seedling requires more nutrients. Now is when soluble fertilizer can be applied.

• They allow identification of the plant variety. The true leaf shape reveals exactly what plant species or cultivar the seedling is. Their appearance verifies seeds sprouted correctly.

• They enable early pest/disease diagnosis. Issues like nutrient deficiencies, fungal diseases, and pests can be caught early by close inspection of the true leaves.

Noticing those first true leaves is the cue for the gardener to take action. This includes transplanting or thinning, starting fertilization, and monitoring for potential problems. Careful attention at this vital stage helps ensure healthy, thriving seedlings.

Frequently Asked Questions

Here are answers to some common questions about true leaves:

How many sets of true leaves will a plant produce?

Plants will keep generating true leaves continuously as long as growing conditions enable new growth. A tomato seedling may put out 5-7 sets before transplanting. True leaf production depends on the plant species, cultivar, and individual growth rate.

How soon after cotyledons do true leaves appear?

The timing varies widely between plant species. Fast growers like lettuce may form true leaves within days of the cotyledons emerging. Slower vegetables like parsley can take several weeks after the cotyledons appear. Check seed packets for estimates on timing.

Can true leaves replace cotyledons if they become damaged?

No, unfortunately cotyledons and true leaves arise from different plant tissues so cannot compensate for each other. While true leaves can make up for cotyledon loss, the reverse is not possible. Protect those cotyledons to ensure robust seedling establishment!

Is fertilizer required for true leaves to grow?

Yes, the emergence of true foliage creates a need for supplemental nutrients. Apply soluble fertilizer formulated for seedlings at half strength once the first set of true leaves appears.

Can true leaves be used to thin out crowded seedlings?

Yes! Select the strongest seedling and use sharp scissors to snip off the extras at soil level. The tender thinnings can be enjoyed in salads. Just leave the strongest seedling alone and undisturbed.

Watching seeds progress into fledgling seedlings with their very first true leaves is an exciting and rewarding process for gardeners. Proper identification and care of true leaves leads to thriving vegetable, herb and flower starts. Noticing this important developmental milestone helps ensure robust, healthy plants!