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Grow plants under grow lights through winter. Start seeds too.

Ask Gardenerd: What’s Wrong with My Cucumber Sprouts?

A great seasonal question about cucumber sprouts came in to Ask Gardenerd this week from Rosemary Stankey. She asks: “I started cucumbers from seed and they sprouted quickly. The question is they have two leaves on top but the stems are at least 3-4 inches long with no indication more leaves. Is this usual?” -Rosemary Stankey

Rosemary, you’re experiencing a very common problem with your sprouts. The problem is not with the sprouts, though. It’s the lighting you’re using to grow them. Let’s break it down.

Long stems with big spaces between leaf sets is an indicator of legginess. Photo courtesy of Sarah The Gardener in NZ.

Leggy Cucumber Sprouts

The problem you describe, Rosemary, has a name – leggy. Leggy sprouts occur when there isn’t enough light on those sprouts. “Not enough light” can mean a couple things:

  1. You have less than 12-16 hours of direct sunlight on those sprouts
  2. Your light is too far away from your sprouts
  3. It’s the wrong kind of light

First let’s talk about duration. Once seeds germinate, they need 12-16 hours of direct light. If you are setting your seedlings outside in dappled shade or in a kitchen window, chances are you’re not getting enough direct sunlight. Grow lights provide even, direct light for as long as your plants need. We used the Sunlite Tabletop grow light when we got started, then moved up to a 3-tiered system. We set ours on a vacation light timer to go on in the morning and off at night. That ensures 16 hours of light and stocky sprouts.

Tomato seedlings wordless wednesday
Tomato seedlings under grow lights. They’re nearly ready to go in the ground.

Next, let’s address #2: the distance of light from your seedlings. Your light should be no more than 3″ above the leaf surface to avoid triggering a plants natural response to reach (grow) toward the light. That long stem on your cucumbers seedlings, Rosemary, tells me they didn’t get enough light during a crucial part of their development. Long stems make plants weak. Weak plants tend not to develop more leaves, and possibly die (breaking along the stem) when planted out. Your light source works best when it can be lowered and raised as plants grow (or you can use this old-school trick: stack phone books to raise your seedling tray up to the light).

If you are ready to plant out your cucumber seedlings, you can try burying them a little deeper and see if they send out new leaves. But in all honesty, it’s probably best to start over with adequate lighting. Which brings me to our next point…

22 different tomato seedlings under grow lights.

Finally, #3: What kind of light are you using? Regular compact florescent lights don’t have the full spectrum of light rays needed for growing plants. Full-spectrum lights are important. We use T-5 full spectrum lights in our grow light system. There are full-spectrum lights available in many sizes to suit whatever light fixture you have.

One More Thing

Just remember that all plants need to rest, which means they want darkness for a few hours at least. Make sure your lights go off at some point at night, so the sprouts can rest for the next day. They will grow stronger when given that natural light/dark cycle.

Thanks for writing in, Rosemary. I hope this helps you grow wonderful plants moving forward.

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