As prime tomato-transplanting time arrives this month up north — once the danger of frost has passed — you may be in search of seedlings. Or maybe you started your tomatoes from seed, or have young plants (homegrown or store-bought) already in the ground, or in a big pot on your balcony.

Whatever stage of the tomato timeline you’re at, you may be wondering: What needs to happen next, in the weeks before that first ripe tomato?

I asked Tom Stearns, who founded High Mowing Organic Seeds in 1996 and has grown a lot of tomatoes.

“The first seed I ever saved was a tomato’s,” said Mr. Stearns, who in the decades since has trialed heirlooms and hybrids at his Vermont farm, helping breeders from Cornell, the University of New Hampshire, Oregon State University and elsewhere fine-tune the development of new organic varieties.

Mr. Stearns’s company, which has the largest selection of certified organic seed varieties in North America, has seen a 300 percent increase in home-garden sales since mid-March. With other seed sellers reporting similar upticks, that probably means a lot of people are trying their first vegetable gardens — and their first tomato plants.

He shared his advice.

Buying Transplants? Shop Closer to the Source

Big-box stores typically have limited choices, familiar basic varieties. “Farm stands and farmers’ markets will have much more diversity — more options, and more interesting options,” Mr. Stearns said. Many independent garden centers also buy from local farmers, if there is no stand nearby.

“Plus, the farmer knows the varieties, and not just their flavors, but how they grow in your area,” Mr. Stearns said.

Ask about the disease-resistance and how big each variety gets — whether it’s a determinate or bush type, with less of a vining habit than rangy indeterminates that can grow as high as 10 feet.

Heat Things Up

Tomatoes want a full-sun spot, period. Warming the soil, especially in northern areas, provides these tender perennials of South American ancestry with additional comfort. A mulch of black plastic is an easy solution.

“There are not too many cases where I think the use of a fossil-fuel product is worth it on the home-garden scale,” Mr. Stearns said. “But tomatoes are one case where that has merit.”

Landscape fabric is a more resource-conscious alternative, he said: “A 20-foot strip can mulch a row of 10 plants for 10 years.” Roll it up at the season’s end to store and reuse.

Southern growers get a pass on this. “If you’re in a warm place, where heat is not your limiting factor, go with straw,” Mr. Stearns advised.

Prevent Soil Splashing Up

All mulches suppress weeds and conserve moisture, fostering what Mr. Stearns calls “tomato-hygiene management,” which is critical because tomatoes are susceptible to fungal, bacterial, viral and other diseases.

A clean mulch layer between the soil surface and the lowest leaves — the bottom rung on the ladder on which disease spores can splash up and start to climb the plant — is a key defense.

Bonus tip: Remove the lowest set of leaves before planting, so the first rung is that much harder to reach.

“Especially in the Northeast, mid-Atlantic and upper Midwest, as an example, the fungal diseases Septoria and early blight are in your soil all the time,” Mr. Stearns said. “Rotating where you grow your tomatoes from year to year is not really going to help — you must manage the disease.”

Plant Deep — Really Deep

Tomatoes have the ability to produce roots off their stems, known as adventitious roots. Capitalize on this.

“A lot of tomato seedlings can come to you leggy and weak,” Mr. Stearns said. “So planting the seedling deep, halfway up the stalk, is good.”

Deeper rooting also helps with drought resistance later on.

Feed the Soil, but Don’t Overfeed the Plant

Although tomatoes are always classified as heavy feeders, this can be misleading. In well-prepared, fertile soil, a tomato plant is resourceful.

“Tomato root systems are enormous, and most varieties are pretty good at foraging for nutrients three or four feet away from the main stem,” Mr. Stearns said. At his farm, fish emulsion is added to the irrigation water, providing beneficial micronutrients.

The heavy-feeder reputation can prompt unnecessary fertilization. “There is a risk that concentrated nitrogen fertilizer can promote green growth at the expense of fruit,” he said, “and even make plants vulnerable to disease.”

Give the Plants Light and Air

Fungal diseases like nothing better than a humid jungle. So make sure plants in a row are 18 inches apart, and leave at least four feet between rows with pruned plants that you have staked or trellised (or more with unpruned caged plants).

And don’t hesitate to prune, because increased airflow and light help plants stay vigorous.

“A tomato makes about twice as many branches and leaves as it needs to produce fruit,” Mr. Stearns said. “So you can cut off all the suckers” — those small shoots that sprout where the stem and a side branch meet — “and every other leaf, and have no negative impact on yields.”

Just don’t prune before the dew dries or after a rain: Stay out of the tomato row when foliage is wet to minimize spreading trouble.

Offer the Plants Proper Support

Cages, stakes and trellises can get tomatoes up off the ground. If you are not going to keep up with pruning, use a large cage.

But Mr. Stearns recommends trellising: “It promotes good yields; it’s easy to see what’s going on with your plants; and fruit is easy to harvest without damaging anything.”

There are various trellis techniques, including the Florida weave, where twine is woven, figure-eight style, in and out of a series of posts set a foot deep in the tomato row (get the how-to on the High Mowing blog). As plants grow, more weaving is added every eight inches or so up the posts.

“Again: Think air circulation,” he said. “Never do a tomato tepee, with several plants tied up to one support. Inside, it will be like 100 percent humidity — dew will never dry off in there. All the plants need is 24 hours at 100 percent humidity and disease is happening, disease you can help prevent.”

Check for Consistent, Even Moisture

Mulch helps, but in the extreme situation of a fast-draining sandy soil in hot weather, your plants may require twice-weekly watering. Stick a finger into the soil to feel whether it is slightly moist — which it should be at all times.

Watch for Trouble Signs

“Tomato problems can kind of sneak up on you,” Mr. Stearns said. “You can walk out one day and the plants look fine, and a week later they’ve melted.”

One thing to always look for and remove at once: yellowing leaves. Dispose of affected foliage at a distance from the garden. The Missouri Botanical Garden’s fact sheet includes photos of common foliage diseases.

What If You Have Flowers but No Fruit (or Neither)?

While tomatoes are technically self-pollinators, with male and female flower parts in each blossom, wind movement or a bumblebee helps pollen move from anther to stigma (the receptive female part). Sometimes temperature extremes, or even high humidity that makes the pollen too sticky, can interfere, and flowers don’t get pollinated thoroughly, or even drop off.

“Some diseases can affect flower branches and knock back the yield, too,” Mr. Stearns said. “And then there are all those things that steal the fruit before you get to them.”

Sometimes plants are lush with tropical growth, but no flowers or fruit — in which case, you may have over-fertilized, Mr. Stearns said: “Water more to wash out some of the nitrogen. It’s water-soluble, and you can leach it out a bit.”

If There’s Room, Plant a Row of Paste or Plum Types

At harvest time, make sauce to freeze or can, and also freeze whole fruits in freezer bags to substitute for canned tomatoes in soups, stews and sauces. It’s just the kind of plan-ahead ingredient we all wish we had on hand right now, while waiting for tomato time.

The Basics

Tomatoes crave full sun and warm soil. While Southern gardeners can just mulch with straw, in the North, preheat soil before transplanting with black plastic or reusable landscape fabric. And don’t transplant until the danger of frost is past.

Practice good tomato hygiene: Prevent soil splash, potentially laden with disease spores, from getting up onto plants. A mulch layer helps, and so does pruning off the lowest set of leaves before planting.

Open plants up to light and air by spacing 18 inches apart within a row and leaving four feet between rows. Cage if you don’t plan to prune, but better yet: Prune and trellis your plants.

Plant deep — really deep — because tomatoes form roots off buried stem tissue, becoming more drought-resilient.

Don’t overfeed: In a fertile soil, tomatoes don’t need the bagged stuff, which can cause excess green growth at the expense of fruit, and promote disease.

Be vigilant for trouble signs, especially yellowing leaves. Remove and discard.

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