How to grow tomatoes from seed indoors on a windowsill

You can grow tomatoes from tomato seeds right on a sunny windowsill with a few cheap supplies. Small plastic cups, a bag of seed starting mix and a south-facing window with strong direct sun can raise good seedlings. A plain window sometimes gives too little light, and then a basic grow light can help.

Count backward from your planting date

Timing matters. If you start too early, the plants get tall and weak looking for light. Start too late, and you delay your summer harvest.

Find the average last frost date for your town. You can look this up on a local weather website or ask at a garden center. Plan to plant your tomatoes outside only once the danger of frost has passed. The soil needs time to warm up.

University of Maryland Extension says to sow tomatoes 5 to 6 weeks before you expect to plant them outside. Mark that weekend on your calendar. That is your sowing day.

Pick the right tomato seeds for your space

Read the seed packet carefully before you buy. Look at the days to maturity. This tells you how long the plant takes to yield fruit.

Tomato plants grow in a couple of main ways. Determinate tomatoes grow into a compact bush. They stop growing at a certain height and ripen all their fruit in a short window. These are great if you want to grow tomatoes in pots on a patio.

Indeterminate tomatoes grow like vines. They keep getting taller and produce fruit until frost kills them. They need tall stakes or large cages to hold them up.

Pick a variety that fits your yard and your weather.

Fill small cups with seed starting mix

Why not use dirt from the yard? Garden soil is too heavy for tiny roots. It can carry weed seeds, pests or diseases. Buy a fresh bag of seed starting mix instead.

It is light, drains well and holds just enough moisture. Poke a small drainage hole in the bottom of small yogurt cups or small plastic cups. Fill each cup almost to the top.

Water the mix in a bucket until it is thoroughly damp before you fill the cups. Dry seed mix repels water at first. It should feel like a wrung-out sponge.

Drop a couple of seeds into each cup. Cover them with a thin layer of the damp mix. Press lightly so the seed touches the soil.

Keep the soil warm for germination

Seeds need warmth to wake up. Put your cups in a shallow plastic tray to catch water drips. Set the tray in a warm spot.

A potting mix temperature of 65 to 75 degrees Fahrenheit is good for germinating most vegetable seeds. A sunny windowsill may reach this range during the day, so check it with a thermometer.

If your windows are drafty, move the tray to a warm interior room until sprouts appear. Check the cups every day. Keep the top of the soil damp.

You can drape a clear plastic bag loosely over the tray to hold moisture in. Take the plastic off as soon as you see green shoots.

Move the sprouts to a sunny window

Tomato seedlings crave light. Move them to your brightest south-facing window immediately. They need strong direct light to grow thick stems, and a plain window may not give enough.

Without enough light, they stretch toward the glass and fall over. A seedling with a pale, thin stem is struggling for light.

Turn the tray around every afternoon. The plants will bend toward the sun. Rotating the tray keeps them growing straight. Keep the soil moist but never muddy.

Water from the bottom to protect fragile roots

Watering from the top can wash tiny seeds out of the soil. It can also knock fragile sprouts over flat. Pour water directly into the plastic tray instead. The dry soil will pull the moisture up through the holes in the cups.

Check the tray a short while later. Dump out any standing water so the roots do not rot. Check the cups daily by touching the top of the mix. Add water to the tray when the surface feels dry.

Thin the seedlings down to one

You planted a couple of seeds in every cup as an insurance policy. Sometimes all the seeds sprout. If you leave both, they fight for water and light until both are stunted.

Snip the smaller, weaker sprout off with scissors right at the soil line. Pulling it out can damage the roots of the one you want to keep. This leaves a single strong plant with plenty of room to grow.

Repot the plants into larger containers

The seedlings will soon outgrow their first small cups. Tomatoes are often moved into larger containers 2 to 3 weeks after germination. Look for the first set of true leaves.

The very first leaves look like simple slivers. The true leaves have the jagged shape of a real tomato leaf. Get large plastic drinking cups ready. Poke drainage holes in the bottoms.

Bury the stems deep

Fill the new cups partway with potting mix.

Gently squeeze the sides of the small cup and slide the seedling out. Hold it by the leaves, not the fragile stem. Set the root ball deep into the new cup.

Tomatoes can grow new roots along a buried stem.

Bury the plant right up to its bottom leaves. Fill the rest of the cup with soil and press it down. Soak the soil with water and put the cups back in the window.

Feed the growing plants

Potting mix usually holds some plant food for the first weeks. If you add a water-soluble fertilizer, follow the label dose for indoor plants.

Never mix it stronger than the label says. Young roots can burn if the fertilizer is too strong.

Keep turning the cups every day so the plants grow straight. Brush your hand lightly over the tops of the plants when you walk by. The movement helps the stems thicken and grow sturdy.

Keep a close eye on the watering. Large plants in cups dry out fast on a sunny afternoon.

Get the plants used to the outdoors

Windowsill plants are soft. Indoor air is still, and window glass blocks the harsh sun. Moving them straight to the garden can set them back or kill them.

You have to introduce them to the outside weather slowly. This process takes a while, so check with your local extension office for timing.

Start by placing the cups outside in complete shade for a short afternoon. Bring them back inside before evening.

Increase sun and wind exposure

Add a little more outside time each day. Move the cups into dappled sunlight a few days later. Morning sun is gentler than afternoon sun.

Let them feel a light breeze to strengthen their stems. Watch the soil moisture closely.

Wind and sun dry out small pots fast. Water the cups deeply whenever they feel light. Move the plants into full sun for the entire day by the end of the process. Leave them outside overnight if the temperature stays warm enough.

Move the tomatoes into the garden

Check the weather forecast before you plant. The danger of frost must be completely gone. Night temperatures should sit reliably in the warm range.

Dig a hole in your garden bed. Make it deep enough to bury the stem again.

Pluck off the bottom few leaves. Plant the tomato deep so only the top of the plant shows above the soil. This helps the plant root along the buried stem.

Water the new plants in

Fill the hole with soil and firm it up. Soak the ground thoroughly. This removes air pockets around the roots.

If you want to start a vegetable garden off right, put your plant supports in now. Drive a tall wooden stake or place a metal cage over the plant. Doing this on planting day prevents root damage later.

If the plant is already tall, tie it gently to the stake with a soft rag. Spread a thick layer of straw or shredded leaves over the dirt. Mulch stops soil from splashing onto the lower leaves when it rains. Keep the ground evenly moist for a couple of weeks while the plants settle into their permanent home.