What to Do With Tomato Plants in September – 7 Easy Tasks to Get the Most Out of Your 'Maters Before the First Frost

Tomato plants in September sit in a strange spot. Days still feel like summer, but nights have cooled and the light is shorter, and the plant keeps throwing out flowers it has no chance of finishing. Most gardeners let it ride until frost takes the whole thing down. That leaves a lot of fruit on the table.

Most of the advice on how to grow tomatoes covers the first half of the season, back when more of everything was the goal. September flips that. The work now is mostly subtraction – cutting off what will never finish so the plant spends what daylight is left on fruit that can.

Here are the top tasks to do in September to get the most out of your tomato plant before the season ends.

What to Do With Tomatoes in September

How urgent these tasks are come down to your average first frost date. Count backward from that date to decide what to tackle first.

1. Top the Vines

Pruning the growing tip off each main stem of your tomato plants and they will stop growing and ripen the existing fruit instead. The energy will go into swelling and coloring the fruit that's already set. Four to six weeks ahead of the average first frost is the window for this type of tomato pruning, which for much of the country, puts it right about mid-September.

Indeterminate tomatoes are the plants that need this pruning technique. Determinate tomato varieties wrap up on their own and tend to get set back by early topping.

Make the cut just above the highest cluster with a realistic shot, and leave enough leaf cover that the fruit underneath doesn’t scald. Use a sharp, clean pair of pruners. Home Depot carries the Fiskars classic bypass pruning shears that get through green stems without crushing them.

2. Strip Late Flowers & Fruit

It feels wasteful to pull blossoms off a healthy plant. It isn’t. A tomato flower needs roughly six to eight weeks to become ripe fruit, and cool nights stretch that out further. Any flowers opening now are a draw on the plant's energy resources, but produce nothing to show for it. The same goes for fruit that's no bigger than a marble.

Pinch off flowers and small fruit with your fingers as you walk the row. Go back through in a week or two to do another pass. Topping usually prompts a flush of side shoots that also waste energy. Pluck off any tomato suckers that appear as well.

3. Remove Diseased Foliage

Small round spots with pale centers on the lower leaves point to septoria leaf spot (Septoria lycopersici). Bull’s-eye rings inside a dark spot usually mean early blight (Alternaria solani). Both need wet foliage to spread and both overwinter in plant debris, so removing diseased foliage is vital to keep next year's garden healthy.

Late blight (Phytophthora infestans) is the one to move fast to remove. Greasy dark patches creeping in from the leaf margin, sometimes with white fuzz on the underside in damp weather.

Bag infected material instead of composting it and wipe your pruner blades between plants. It’s a containment measure, not a cure. But it's important to clean up diseased plants now to prevent issues next summer.

4. Keep Watering Evenly

September rain often arrives in bursts after dry stretches, and that swing is a common cause of splitting tomatoes. Fruit that's already at full size pulls water in faster than the skin can stretch. Watering tomatoes steadily at the root zone is the fix rather than more water overall.

A soaker hose handles this better than a wand and keeps the foliage dry, which is half the battle with leaf disease. Walmart stocks the Expert Gardener soaker hose that's cheap enough to buy two without putting much of a dent in your wallet. Tuck mulch over the top of it if there isn’t any down already.

5. Get Frost Covers Ready

Frost warnings rarely give more than a day or two of notice, and hunting for a tarp at dusk is how plants get missed and accidentally frozen. A floating row cover buys somewhere between 2 and 8°F (1-4°C) depending on weight, which is often the gap between losing everything and picking fresh tomatoes for another two weeks.

These Agfabric plant covers from Lowe’s are the light-duty option. Heavier grades hold more warmth, but cut more light. Old bedsheets work in a pinch. Run whatever you use down to the ground and weigh the edges with rocks or bricks to prevent heat escaping out the sides.

6. Harvest Mature Green Tomatoes

Green tomatoes that you pick ahead of a freeze ripens well, but only if they have hit maturity – full-sized with the skin shifted from flat dark green to pale whitish or yellow-green. Anything still small and dark won’t color up properly and is better off in a jar of relish.

Chilling injury sets in around 50°F (10°C) out on the vine and wrecks flavor before you ever pick. Clip each fruit with a short stub of stem attached. Sorting by ripeness helps a batch finish together. Hold them between 55 and 70°F (13-21°C). Add a banana to the box to speed up ripening.

7. Save Seeds From the Best Plants

This one’s easy to skip and easy to regret, but saving tomato seeds is a great way to save money next year and grow more of your favorite crop. Squeeze seeds and gel from a fully ripe fruit off the best plant in the row into a jar, add an inch (2.5 cm) of water, and leave it loosely covered on the counter.

Three to five days on, a film forms across the top and the viable seeds have sunk to the bottom of the jar. The fermenting process strips the gel coat, which carries germination inhibitors and can shelter seedborne disease. Rinse seeds clean and spread them to dry, then label and store in an envelope. Open-pollinated heirloom tomato seeds come back true; hybrids don’t.