Growing Concerns 578: Slugs on hostas, sunflowers are killers

Summer!

Slug control now is pointless revenge -- put efforts into prevention next spring

 

Dear Janet,

I have trouble with my hostas. The leaves have holes. I have been told it is slugs or grubs. I have tried everything to no avail. One year I cut off the leaves but still the same.

Also I can't get rid of this weed. It's flat to the ground with tiny oval leaves that each have a dot in their center. It's not the flat one with the succulent leaves, it's more wiry and drier than that. It has a woody straight-down root. I've tried various products but these guys just won't die.

C.G.

Dear C.G.,

Grubs eat roots. Slugs make holes in leaves. Trouble is, most holes you see now were made months ago as slugs were just starting their season and growing rapidly. Small holes in young leaves enlarged with the leaf. Now the slugs are full grown and dormant much of the time. They're not eating much and when they do, they have a lush garden to hide in.

That's why spring is the time to deal with slugs. Next April when hostas are just spears breaking the ground, remove the mulch from slug-troubled areas. Slug eggs and overwintering slugs are in that mulch, so hot-compost it to kill them.

Then, while the slugs are breaking dormancy and the plants are so small that there are no hiding places for slugs in a mulch-free bed, put out traps. The simplest are sections of newspaper. Wet them and spread them flat on the ground, intact as if that day's edition just dropped onto a table ready to read. Put them down in the morning, flip them over in the evening and you'll find slugs on the moist underside. Peel off the slug-coated page and throw it away in a very tightly sealed container. Flip the paper again the next evening. And the next, for a few weeks or until you stop finding slugs.

Finally, put clean mulch in the bed and prepare to enjoy a year with much less damage.

Think about changing the environment to make it less slug friendly, too. Reduce their food by digging out the worst-chewed types of hostas and replacing them with slug-resistant 'Sugar and Cream,' 'Golden Tiara,' 'Blue Cadet',  'Krossa Regal' or any of the puckered-leaf types such as 'Sum and Substance' or 'Elegans.' Eliminate some hiding places by getting rid of big-chunk mulches. Make the atmosphere less friendly by letting the bed dry out between waterings.

You can trap them or put out poison slug baits today but listen as you do it and you can almost hear them laughing at you from their myriad hiding and dining places. They are not going to rise so readily to your bait. Also, holes won't kill a hosta leaf, so plants don't shed them and grow new ones. So, even if you do catch or kill a few thousand of the slug descendants of this spring's hundreds, today's trapping won't fix the problem. You'll still have the aggravation of looking at Swiss cheese hostas.

Now, about that weed. You're describing prostrate spurge, Euphorbia supina. It's tough to get hold of, even if it's growing in the open rather than the paving cracks it excels at filling, and breaks when pulled. The good news is it's an annual. It isn't going to survive winter and keep growing a bigger root next year. It starts over from seed each spring so you have a fresh run at it each year.

Hoe-ing, pre-emergent herbicide and mulching are all effective, but no single action provides complete control. That comes with persistence. In a year like this, when moist times great for seed germination just won't end, it has been a trial to keep up.

Short report

Yes, those sunflowers are killers!

You're not imagining it. The growing is tough under a bird feeder amid sunflower hulls and spilled seed. Sunflowers wage chemical warfare, inhibiting plant growth in their area. Their tactics are similar to a black walnut's but the chemicals differ. Agricultural scientists are studying sunflowers and other allelopathic plants to determine what's affected and how. They aim to advise farmers on which crop rotations are best and perhaps how to use sunflowers or derivatives to reduce the growth of weeds like purslane that are very sensitive to  sunflower's presence.

Researchers will sort out which food crops will and won't grow with sunflowers, but ornamental gardeners must compile their own lists. On my school's Web site (address below) in the "Ins and Outs of Plants" folder on the discussion forum, you can read what others have learned and add what you know. So far the word is that Sedum Autumn Joy has performed well in sunflower hull mulch for two years, while ribbon grass (Phalaris arundinacea 'Picta') survived for over a decade before succumbing.

 

Green thumbs up

to a temporary truce in the turf weed war. These days it's too hot for hand pulling and most lawn weeds are toughened and semi-dormant, resistant to weed killers. Drink some lemonade. Bide your time. Resume the battle in late September when violets, chickweed, ground ivy, creeping charlie and weed grasses become tender and vulnerable.

 

Green thumbs down

to letting those houseplants take over your home. It's a good time now to cut them back, root prune, or start cuttings as replacements.

 Originally published 8/7/04