When to Start Planting In Your Greenhouse This Spring

Rick cornered Helpful Husband on the 19^th hole at the golf course. Rick keeps trying to grow a perfect spring tomato but once again his tomatoes are tall, thin and floppy. He hoped asking Helpful Husband would get the message across to me and it did.

I get this question every spring. Even greenhouse gardeners aren’t immune to leggy tomato starts and weak stems. If your plants are looking more spindly than sturdy, here’s what’s likely going wrong—and how to fix it.

1. You're Starting Too Early

Unless you are a commercial grower, there is no point in starting plants indoors before early to mid-March. Many growers wait until late March or early April. Starting too early means your plants grow too tall before it's warm enough to plant them outside.

2. Your Plants Need More Light

Rick mentioned that he is using fabric to warm his seedlings, but that's also blocking the light. Fleece can reduce light by 10-50%! I use a clear plastic cover until seeds sprout—then remove it immediately. And remember: the tops of your plants should be within 15 cm (6 inches) of a grow light to prevent stretching.

3. It's Too Warm—Without Wind

Indoor setups and sunny spring greenhouses can get hot. That heat causes rapid, tall growth. If you can't cool things down, simulate nature by adding a fan. Air movement strengths stems and keeps growth compact.

4. You're Over-Fertilizing

Seedlings don’t need feeding right away. Rick has been fertilizing from day one, which encourages weak, fast growth. Wait to fertilize until you pot up your seedlings or move them into greenhouse beds.

5. You Can Still Fix Leggy Tomatoes

Got tall tomatoes? No problem. Remove the lower leaves and plant the stem deeper—either in a trench or a deep pot. Tomatoes will root along their stems, giving you a stronger plant.

Recap: 5 Ways to Prevent Leggy Seedlings

1. Start tomatoes mid-to-late March (or later if your plant out in May)
2. Keep sprouted plants uncovered and close to the light
3. Maintain cool temps—no lower than 41°F / 5°C
4. Use a fan to mimic natural wind
5. Plant leggy tomatoes deep when transplanting

P.S. to Rick—you're always welcome to grab one of my short, sturdy tomato starts. They've been stressed on purpose and are ready to take off in warm, moist greenhouse soil 😄