When Is It Time to Start Seedlings Indoors?


Submitted 2 years ago
Created by
Henry Homeyer

Answering a Reader's Questions

A reader named Vicki wrote recently asking if it’s time to start Brussels sprouts indoors, and if so, how? No, Vicki, it’s not time for them yet. But I will start a few things soon. Here are some tips.

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First, know that most vegetables and flowers only need to be planted 8 to 12 weeks before you put them out. For me, frost sensitive vegetable seedlings go outdoors in early to mid-June. Counting back 8 weeks means planting in April, not January. But look at your seed catalogs – or go online – to see how long a particular plant takes to germinate and get ready to plant outdoors.

Brussels sprouts seeds only need to be planted indoors 4 to 6 weeks before being put outdoors

I find that the Johnny’s Selected Seeds catalog is one of the best source of growing tips. The link provided will give you the chance to click on a vegetable and get 2 pages of good information – most of which is also in their print catalog.

For another good source of information click on this link to go to the Gardeners Supply Company’s vegetable encyclopedia and you will be able to get great information for about 40 kinds of vegetables. I helped write that years ago.

Professional growers don’t want to spend a lot of time babying seedlings – time is money – and recognize that smaller seedlings generally transplant better than large ones. Roots of small plants are not bound up, even in a small cell, so they transplant better.

Some gardeners like to start tomatoes 16 weeks ahead of planting, and then transplant seedlings from starter 6-packs to individual 4-inch pots, allowing them to plant large tomatoes in the ground. But that’s a lot of time tending and watering.

These tomato seedlings are ready for bigger pots

Those same gardeners often try to get seedlings into the ground in May – even if the ground is chilly. I like to put mine out when the soil is 60 degrees or warmer. Then the plants take right off and are not stressed. Warm weather plants like tomatoes and peppers tend to sulk when put in cold, wet soil.

Vicki asked if I thought she needed an electric heat mat for her Brussels sprouts. Not necessarily. Heat mats are good for impatient gardeners who want to get things growing fast - or for heat-loving plants that are hard to germinate - like the cutflower, lisisanthus. Lisianthus requires 18 days at 72 degrees or more to germinate.

Soil temperature is a safety feature for plants: cold soil means more cold weather is still coming. If the soil is truly warm, it’s safe for tender plants to germinate. So it may take 2 weeks or more for Brussel sprouts to germinate at 50 degrees, but only 5 to 8 days at 75 to 85 degrees. Soil mats should be turned off once the plants have germinated.

Vicki was planning on growing her Brussels sprouts on a south-facing windowsill. Don’t do it, Vicki. Invest in some lights. Seedlings need intense light in order to develop strong bodies and thick stems. If there is not enough light, plants get tall fast and lean toward the source of light. They are flimsy and more prone to disease, especially if you start them too early.

I tell wanna-be gardeners that they need to have 6 hours of direct sunshine outdoors to grow good tomatoes and other fruits, but that lettuces and kale – leafy greens- can get by with 4 hours of direct sun. It’s hard to get 6 hours of sun indoors at this time of year, especially since the sunshine is so often filtered through clouds. And in modern windows, much of the sun’s strength is filtered out so that it won’t bleach out your rugs and curtains. So you might be able to grow some lettuce in a window sill, but not tomatoes or Brussels sprouts. 

What kind of lights are best? I recommend fluorescent lights using T-8 bulbs. Those are tubes that are narrower in diameter than the old T-12 bulbs, but more energy efficient. Shop lights are relatively inexpensive, though special “Gro-Light” fluorescent tubes can be pricey. I use ordinary shop lights with no-frill tubes.

I built an A-frame plant stand that is perfect for shop lights and flats of seedlings. You can see the directions to build it by clicking on this link. at It will hold 6 flats of seedling, and for around $50 you can get everything you need (not including the light fixtures).

My plant stand

The height of the lights above the flats of seedlings needs to be adjustable so that the lights are always about 6 inches above the tops of the plants. I use jack chain, a lightweight chain available at hardware stores, and a hook at each end. As the plants grow, I shorten the chain.

Vicki asked me about fertilizing her Brussels sprouts seedlings. It depends on the soil mix you use. If you buy a commercial seed-starting mix, it is light and fluffy and good for developing roots, but it has little in the way of nutrients, and those present will wash away in a couple of weeks.

Me? I make a 50-50 mix of seed starting mix and good compost. The minerals in the compost will not be so quickly depleted. In either case, a regular light dose of an organic liquid fertilizer such as Neptune’s Harvest fish and seaweed will keep your plants from suffering from lack of minerals. But don’t overdo it. More is not better. And fish fertilizer can be stinky if used in a strong concentration!

So, Vicki, don’t start your Brussels sprouts until April, or even May. Onions and peppers and artichokes are things I start in late January, or even as late as mid-February. That keeps me busy and thinking of Spring, but doesn’t overwhelm me.

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I'm the author of 4 gardening books.  Visit my personal website by clicking here.  

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