Growing a Garden: Managing Seedlings

How Much Light Should Seedlings Get

Once your seeds have made it through the germination stage, lighting conditions may change depending on the plant variety. At the seedling stage, window light may not be enough, so consider providing supplemental lighting. If you're switching from windowsill growing to fluorescent light, be sure to do so gradually. Too much of a change in light and temperature could harm the plants.

Lack of proper light produces leggy plants, which could mean weak stems and the inability to hold the weight of fruits and vegetables. Compared to High-Intensity Discharge Lamps, the cooler light of fluorescent bulbs can often rest 1-2" above the tender seedlings, providing adequate light without damaging them.

When Do I Put My Seedlings Under Light

The University of Maryland Extension recommends seedling growth to use a 14-16 hour light cycle. This schedule ensures your plants have plenty of "daylight hours" to gather energy. When the night cycle comes, plants consume this energy to develop shape and structure.

A great thing to consider at this stage is ways to raise and lower your lights. You're going to want to have something sturdy, like a chain or wire, to hang your fixture above the seedlings. Having this allows you to adjust the light to accommodate plant growth by maintaining that distance of a few inches of space.

What Is the Best Temperature for Seedlings?

When examining the temperature preferences of plants, you're bound to see some variability. Just from a bit of research, you'll find recommendations of daytime temperatures, ranging from 65-80F (18-26C). The University of Maryland Extension and the University of Missouri Extension recommend daytime temperatures between 65-75F (18-24C).

Why Is Temperature Important for Seedlings? 

Proper temperature is vital at every stage of a plant's life as they don't have the same internal cooling systems humans do. Too much or too little heat can create multiple problems ranging from stunted growth, inability to produce flowers or fruit, and plant death. When examining nighttime conditions, vegetable crop plants generally prefer temperatures of 60-65F (15-18C). Below that, you begin to see the plant's growth suppressed and even halt in heat-loving plants. Aim to keep your plants in a safe area by creating your indoor zones and sectors. Opening the door to the outside, no matter how careful, can produce a quick draft of cold air that could shock nearby tender plants.

Let's check in on our tomato seedlings. You've been watching them daily as they slowly grow. The day comes that they sprout their first true leaves. At this point, you want to make sure you are keeping the surrounding area at a constant temperature. So you invest in a small indoor thermometer and place it close to your growing plants. Watching for a few days, you determine that the temperature fluctuates between 75F when the sun pushes through the nearby window and drops to 65F in the evening after the sun sets and the ambient temperature drops. This temperature range is what you were hoping for. It was a good thing you first took the time to research where to grow your seedlings in your house.

How Often Do You Water Seedlings

It's essential when watering your seedlings to do so evenly. I've witnessed times when one tray of seedlings is healthy and thriving, while the one beside it is wilted and stressed. But how much water is enough and when is it too much? The truth is, it's hard to know the optimal moisture level from looking at your plants. The aim is for moist soil, not wet or dry. You don't want your growing medium so wet that it's running out of your hand. Instead, aim for growing medium moist enough that when you squeeze it in your fist, you can see moisture accumulating between your fingers but never dripping. The roots of your plants need saturated soil to grow, but they also need air pockets to absorb oxygen. Too much water drowns the roots and kills the plant.

A useful metric is to examine the top of your growing medium. If it is dry, give your plants a little water. If it is still wet, wait a little longer. You do want your seedlings medium to dry slightly, so it never becomes soggy, but it will kill the plant if it dries too much. Be sure to check them daily and ensure your seedlings aren't ever wilting. With watering also comes the risk of damping-off.

What Is Damping Off?

Damping-off is a result of pythium (mold) or Rhizoctonia (fungus). It occurs when soil is left coolish and wet for a prolonged period, allowing these culprits to rot away the roots. As we discussed, in ways to start plants from seeds, contaminated soils, and containers are one way this disease spreads from planter to planter.

We know mold can be harmful to the home. Similarly, if a plant affected by rot is left unchecked, it will eventually wither and die.

How to Stop Damping Off

If you find a seedling that shows signs of mold or rot, quickly dispose of the affected plants and soil and sterilize all containers and equipment that came in contact with mold or fungus.

Now, let's imagine our tomato seedlings are growing well in their new light, and the thermometer shows that temperatures are ideal. Every day you check to make sure the top of your medium is moist but not soggy.

One morning you find that the soil is dry to the touch and decide to water. You remember from working with your seeds how well watering from the bottom worked, and you fill a tray, resting your seedling containers on it. Observing it, you notice when the medium is moist and remove it from the remaining liquid.

When Do You Fertilize Seedlings?

Before you begin this process, let's consider a few things. If you purchased store-bought soil or soilless mix for your seedlings, check the nutrient content of it. If there are already nutrients present, you may not need to feed your young plants for multiple weeks. Consult the information on your store-bought mix to determine if you need to add fertilizer.

If there are no nutrients in your mixture, then it may be time to feed your future food producers. Most nutrients you purchase from the store will be for mature plants. Meaning their calculations for nutrient-to-water ratio are based on a developed plant's ability to absorb nutrients. At the seedling stage, plants cannot consume a lot of nutrients, and high nutrient amounts could result in the burning of the tender seedling leaves.

When feeding your young plants, consider diluting your fertilizer to ¼ strength. This dilution is a standard metric for seedlings to reduce the likelihood of overfeeding and reduce the chance of nutrient burns. If you decide to topwater your seedlings, be sure to wash away any fertilizer that comes in contact with the foliage. Watering from the base is preferable and is also a way to protect your plant leaves.

Let's consider our tomato seedling. The plant has begun to show signs of vigorous growth, and you've noticed that new leaves are starting to reveal themselves. Unlike the first leaves, these aren't as smooth and have the distinct appearance of a tomato leaf. You think this is a great time to start feeding your plants. You aim to avoid damaging the leaves by watering from the base and protect the roots by diluting the solution at this stage. Feeding them once, you watch them over the new few weeks of growth to ensure they don't show any signs of nutrient deficiency or overabundance (which can be just as bad).

When Do You Transplant Seedlings? 

Transplanting can take place over time and for various reasons. To begin, let's consider multiple small and recently germinated plants in a single container. You're going to want to consider transplanting into larger containers while they are still young. Transplanting, at this stage, gives the seedling time to deal with the transplant shock to its roots. The University of Maryland extension found that this time could be right after the first true leaves appear. The seedling is typically between 1-2" (3cm-5cm) when I transplant, but I also aim to germinate in small, individual cells.

How Do You Separate Plants Into Containers?

If separating small plants in a single container, use something to ease the roots apart carefully. My best advice here is to be patient, as quickly ripping two entangled plants apart can be traumatic to the roots and potentially irreversible. Damaging the plant during this stage can happen, and learning how to handle the plants properly to protect both the roots and the stem is fundamental.

How Do You Prepare a Container For Transplanting Plants?

When transplanting into a new pot, ensure the container is clean to avoid soil-borne diseases. Fill the transplant pot with moist soil and poke a hole in the center large enough for the seedling roots. To extract your seedling, lift the plant using the topmost leaves and not the stem. Using a spoon, separate the roots from surrounding plants or, if you have a tray with individual cells, pinch the bottom of a single cell to release the soil or soilless medium.

Place the plant into the new container. If possible, spread the roots without breaking them and add soil around them in the hole. You want the roots and the potting soil to make contact easily as you "water-in" your plants after planting. Lay moist soil over the hole and gently press down around the base of the plant. Now, in their new home, it's time to water them immediately.

At the end of the transplanting, your plants have gone through a fairly traumatic ordeal and will need time to bounce back.

How Do You Move Transplanted Seedlings?

Before moving transplanted plants, give them a few days in the light and temperature it's used to before it perks up. During this time, you will want to protect your seedlings, so no significant changes to the environment in they were growing.