Q: When is the proper time to dig up sweet potatoes? This is the first year that I planted them.

A: As you know, sweet potatoes are a warm-weather vegetable, taking 100-110 days to mature, so depending on when they were planted, it’s about time to harvest. Our area gets about 194 growing days each year if we’re lucky, so you really have to make the most of every warm day to plant and mature this crop and not have it spoil. You may have a few weeks left — you’ll be harvesting just as the rest of us are planting spring bulbs as the ground temperatures drop to 50 degrees.

Dig tubers as the vines begin to yellow and shrivel and dry — the strength they gained from the beautiful green vines all summer will have ended. Try to dig tubers on a dry day.

Dig them carefully with a fork, being careful not to bruise the tubers or they won’t store well. Bruised tubers can be refrigerated and used immediately — many gardeners do the digging by hand.

After curing in warm, humid conditions, tubers can be stored at cool, near-room temperature — don’t refrigerate for long-term storage. And don’t wash your tubers until you’re ready to use or they might rot. Just brush them gently.

Q: I haven’t composted in years but am getting back into it. I remember the basic “no meat scraps,” but what about seeds, i.e., grass seed, tomato scrap seed, any seed — are they OK to add? Also, I know no tomato plant refuse (due to disease), but what about the dirt from their containers? Is it OK to add any dirt to a compost? I do some of my veggies and all of my tomatoes in containers, so if it’s not OK to add to compost, what’s a good use for all of the used dirt I’m going to have from all those pots?

A: Good for you for returning to composting. Yes, the same rules still apply: no meat, bones or grease; no eggs, but eggshells are OK; no salad greens if they have dressing on them; no animal feces and no cat box litter — really just common sense to prevent attracting rodents and other animals and spreading disease.

Your excess seeds can go in the compost pile if it’s well-managed and “cooks” to a temperature of at least 140 degrees — check with a soil thermometer. These are the minimum temperatures that will kill the seed’s ability to germinate. A bonus: Next spring as you turn the pile, you may see some tiny tomato or pepper or pumpkin seedlings that have survived — either save them or turn them over.

The soil in plant containers doesn’t have to be completely replaced every year — every two to three years is often enough — but you will want to do some soil amending every year. Do it in the spring or fall: Remember that you don’t change all of your garden soil every year, but you do add supplements and soil additives like compost and fertilizers after a soil test.

In your containers and raised beds, scrape off the top few inches of soil and replace it with good composted soil, either bagged or homemade, every year. If you use bagged potting soil that has a fertilizer already added, do your soil replacing in the spring because the fertilizer will only last about six months once it gets even slightly damp and containers will be in storage those months.

Any soil you remove from containers can be added to the compost, a 1- to 2-inch layer at a time. It will add beneficial microbes as it combines with other materials. Extra soil can simply be used in the yard to fill in or level areas or wherever you need extra soil — or can be used as mulch in the garden and spread around — then it will combine with chopped leaves that you’re going to compost this fall and winter.

If you have large amounts of diseased contaminated soil, I would compost it in a separate area, or spread it on the ground in a thin area and allow it to freeze this winter. Don’t use it for any planting for a few years — it takes that long to kill the little buggers.

Q: I found my first hornworm. I only found one, on a “done” plant, no less, and it had what looked like white egg cases or something all over it. I cut the branch off, put it into a bucket and pitched it into the drink, all the while wearing a horrible grimace on my face! Does this mean I can now expect them next and/or every year? Where do they come from and what do they turn into, assuming they pupate?

A: Aren’t you lucky! No — I’m serious! You found a hornworm that has had eggs laid on its body. The eggs, probably that of a particular wasp, will hatch and burrow into the hornworm’s body and will eat and kill the hornworm. Had you known, you could have left the hornworm to die on its own.

Hornworms are the pupa of the sphinx moth, sometimes called a hummingbird moth, a large brown and black moth that you see in your garden all summer. Yes, you might have more next year, and the best way to remove the large insect is by hand-picking. If hand-picking isn’t your thing, any good insect spray with caterpillar listed on the kill label will do, but be persistent.

This week’s dirt

The planning season has begun and will last at least until late April or early May. It’s the gardener’s season to start evaluating what worked and what didn’t. In the meantime, keep weeding — weeding isn’t over until the ground freezes. All those little weed seeds are busy getting into their survival mode — and preparing for next spring in your garden. Ever wonder why there are so many weeds that pop up in April before you even get finished cleaning out? It’s because they get there first. They slept there all winter and begin infiltrating your garden and using your water and fertilizers intended for next summer’s crops. The more of those seed carriers and root spreading weeds you get rid of now, the more you will save yourself from scores and scores of weeds next spring.

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North Shore Gardener by Barbara Barger of Beverly is a feature of Friday’s Lifestyles section. Reach Barbara by email at nsgardener@comcast.net or write to her c/o The Salem News, 32 Dunham Road, Beverly, MA 01915. Previous North Shore Gardener columns can be found at www.nsgardener.com.