If you cut them back and are then diligent about pinching the new growth, you'll be able to control the HT. You could always pinch just the apical meristems (growing tips) and see if you get a low break on the stem, but cutting back hard in the summer is the surest way to get low breaks.

Al

North. Are Dieffs taller than current stakes? If so, the thing to do is use taller and semi-thick pieces. Too thin, they'll tip over, especially since foliage is large and heavy.
Stakes come in wood, plastic and bamboo.

Carefully remove old stakes, then insert new, larger/thicker types, preferably, in the same hole old stakes were. Add a little soil then gently kneed closest to the stake for a firmer hold.

What is used to keep stake and stems together? Clips, Ties? If ties, unless they're in good condition, replace those too. Some ties have wire. When ties age/crack, wire can slice plant.
Piece together firmly, but not too tight, otherwise plastic can dig into stems.

That should do it..unless you decide to cut back. Toni

Yes Toni, the plants quickly get taller than the stakes. Then they get so top heavy the whole thing starts to lean. I've used whatever I can find for stakes..old broom handles and bamboo mostly. The bamboo I can find here is too thin to be much use. I use old nylons as ties.

Al, I'm familiar with pinching back on lots of my plants, but never tried it on such a fat fleshy single stemmed plant.

I'm wondering how ugly it would look to stick a tomato cage into a pot and let the plant grow up in it. I could then anchor it from opposite sides.

The world is hugged by the faithful arms of volunteers. ~Terri Guillemets

I suppose you could spray it green to sort of camouflage it.

By pinching, you'll get multiple stems out of the deal, unless you rub off the new buds as they occur. It's often a problem trying to make a compromise between the plant's natural growth habit and our vision for the plant. I think the realistic choices are to A) allow the plant to grow as tall as you can tolerate & then cut it back hard, or B) grow it in a multi-stemmed clump employing the pinch to keep it in bounds, and pruning/thinning as required to keep the clump happy.

Al