You Say Patatas; I Say Tamatas….
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---Mary in Oakdale, California
A. The first thing we had to do here is look up where she lives; I know Oak-LAND California; but where is Oak-DALE?
Turns out it is due East of San Francisco and about the same distance West of Nevada, which might lead one to believe that its cold and damp on one side and Death Valley on the other, but the yearly averages put it on the warm side of that chasm. It can drop below freezing in the winter, but typically does not. It can get blisteringly hot in the summer—historic highs are in the 110s, but it generally doesn't get that extreme. It is, however, BONE dry in the summer.
The reason we mention the weather is that tomato plants will stop setting fruit when the air temperature reaches around 95 degrees Fahrenheit or above. And, of course, the hotter the clime the more water they need.
Hopefully Mary has lived there long enough to know that she should plant her tomatoes early in the season and provide afternoon shade when summertime temps do hit the triple digits. And it would probably be wise to utilize some water harvesting techniques, like rain barrels, cisterns and grey water from the shower and kitchen sink.
Now let's move on to the actual question: she has volunteer potatoes coming up around her tomato plants; should she move the potatoes?
Answer: No, she should not. Or maybe she should. It depends. Oh; and it also depends on what you mean by 'move'. And….
Volunteer potatoes are VERY common in an established garden. The underground tubers often stray surprisingly far from the above-ground plant, and even an experienced grower can miss a few at harvest time. And the volunteer parents don't have to be big; I've had very productive plants spring up from spuds that were the size of a marble.
These volunteers often show up in inconvenient areas. I've had potato plants sprout in the middle of a bed full of garlic—and they had to be pulled because they were competing with the underground space I needed for my garlic bulbs to get nice and big.
"Pulled"? Not transplanted to another area?
That would be ideal, but I've never been able to do it successfully; the potato plants never take in their new location. Depending on how big the tomatoes are, she should be able to transplant THEM. Tomato plants are fairly easy to move if they're under, say, two feet tall; but she might not have a place to safely move them to, because her mention of rotation implies that she knows not to plant tomatoes in the same spot in multiple seasons because of a 'wilt' that builds up in the soil after two or three years of repeated tomatoes.
But if she's rotating every year, she should be able to go back to some of last year's spots. It takes two or three years for the wilt in the soil to build up enough to cause the distinctive yellowing of the lower leaves that then moves up the plant. And the wilt can't build up without tomato roots around it.
Conversely, if she's worried about overcrowding, she could pull up the potatoes and plant new certified 'seed potatoes' elsewhere. (See below for why certification is important.)
But what if she just leaves things as they are?
It should be fine—with one big exception. And that exception doesn't involve any issues with tomatoes and potatoes growing close to each other. In fact, amateur botanists love to create single plants that produce both crops. They'll graft the tomato variety they want to grow onto the top of a young potato plant and tomatoes will grow up top while spuds grow underground. The plants are so closely related that the grafts generally take hold very easily.
The exception—and it is a serious one—is if there were symptoms of late blight on the tomatoes and/or potatoes last season. The late blight pathogen—which was responsible for the potato crop failure that led to The Great Hunger in Ireland in the 1860s—affects both tomatoes and potatoes and can overwinter on unharvested potato tubers left in the ground over the winter.
Follow these links for articles that describe the symptoms of late blight in great detail:
Late Blight Crisis: Keep an Eye on Your Tomato and Potato Plants
Help Banish Late Blight!
Tomatoes: Is There a Bad Blight on the Rise?
And we would be remiss if we did not remind our listeners/readers that they should always buy certified disease-free 'seed' potatoes for new plantings to help keep other nasty problems at bay.