Leek

The ladder troubleshoots from the gentlest solution to the strongest. Start with the gentlest option and only move on when and if it doesn't work.

1. Garden sanitation

   When the season ends, pull up any onion-family plants still standing in the bed, including ones that came up on their own, and clear out the leftover leaves and trimmings. Thrips pass the winter in that material, so what stays on the ground in fall is where next year's thrips start. Get it out of the garden instead of piling it beside the bed.Thrips tabaci overwinters in allium material left on the soil surface, so the end-of-season clearance of leek debris and any volunteer alliums is the practice with the most weight against it. Rogue out volunteer alliums as well, since they bridge one crop to the next. Done in fall it sets the spring starting population, which is a different job from treating a summer outbreak.

2. Clearing the weeds that host it

   Early in the year, before the leeks are up, dig or hoe the weedy edges around the bed rather than leaving them to grow. Weeds that dry down later in the season push thrips off themselves and onto the crop, so a rough margin left standing sends the pest your way. Do this while there is no crop for the thrips to land on; once the leeks are up, leave the edges alone.Cultivate weedy margins early in the year, before the crop is up, because thrips move off weeds as those dry down and onto the crop. Leave the margins alone once the crop is up: the clearance is worth most before the crop emerges, while there is no allium standing for the displaced thrips to move onto. It is preventive only; it does nothing for thrips already in the leaf folds.

3. Balanced nitrogen feeding

   Feed leeks enough to grow well and no more. Nitrogen is the part of fertilizer that drives leafy growth, and a heavy dose makes thrips worse, so keep the feeding steady rather than generous. A plant that is fed properly but not pushed also holds up better against what the thrips do to it, and on this pest that toughness is a large part of what you can do.Hold nitrogen at adequate rather than heavy. Excess nitrogen promotes thrips, and the aim of the feeding is vigor, which buys tolerance of the rasping rather than fewer insects on the plant. That distinction matters most in a hot, dry spell, when populations build fastest and a stressed leek shows the silvering and the slowed shaft growth first.

4. Water spray

   When you first spot a few silvery streaks, wash the thrips out with a hard stream from the hose, pointed down into the creases where the leaves fold together and into the tight neck of the plant, because that is where they hide. Do this in the morning so the leaves dry before evening. It handles a light infestation; if the numbers keep climbing anyway, ask your local extension office what to use in your area.A forceful jet of water into the leaf folds and the neck of the shaft knocks a light infestation back, and those folds are the place to aim, since they are also where the insects sit out of reach of any spray. Time it for early in the day so the foliage is dry by nightfall. A persistent outbreak that keeps building in hot, dry weather is one to take to local extension for a current recommendation.

5. Beneficial predators

   Minute pirate bugs and lacewings eat thrips, and they will do part of the work for you if you let them. The way to keep them around is to skip broad-spectrum insecticides, the kinds that kill insects across the board, because those take out the helpers along with the pest. That is a real reason to try the hose and the cleanup first and hold off on spraying.Minute pirate bugs and lacewings feed on thrips, and they help where broad-spectrum insecticides are kept off the planting. Conservation is the whole of the practice: nothing is bought or released, and what protects it is keeping broad-spectrum materials off the bed. Predators lag a fast-building population in hot, dry weather, so expect them to hold a moderate one rather than rescue a heavy one.

No spray for this one. Prevention is the control.

The ladder troubleshoots from the gentlest solution to the strongest. Start with the gentlest option and only move on when and if it doesn't work.

1. Crop rotation

   Plant this year's leeks on ground that did not grow onions, leeks, garlic or their relatives last year. The moths pass the winter in old plant debris, and a bed that carried an onion-family crop is where they will come out in spring, so moving the planting puts distance between the new crop and the emerging moths. Decide it when you lay out the garden; it is also what makes a cover safe to use.Rotate off last year's allium ground. Acrolepiopsis assectella overwinters as an adult in sheltered plant debris, so a bed that carried alliums is a source of the first flight in its own right; it is that overwintered generation the move addresses. Rotation is also the precondition for exclusion: a cover over old allium ground shuts the emerging moths in with the crop.

2. Timing the planting

   Where your season has room for it, plant leeks a little later, after the first moths of spring have come and gone. The overwintered moths come out once spring warms to around 50°F, which can be as early as March, and lay on whatever leeks are up, so a bed planted after that first wave misses it. Check when the moth turns up where you live before shifting the date, since it changes from place to place.Delaying planting past the first emergence sidesteps the spring generation. Adults emerge when spring temperatures reach about 50°F, sometimes in a March warm spell, and New York records the first flight around mid-April to mid-May, so a transplanting set after that window escapes the first round of egg-laying. The mid-June to mid-July and late July to late August flights still have to be met with the cover, so this supplements exclusion rather than replacing it.

3. Garden sanitation

   At the end of the season, clear every bit of onion-family debris out of the bed: old leaves, trimmings, stems and any plants left standing. The moths spend the winter tucked into exactly that kind of sheltered plant litter, so what you leave behind in fall is what flies in spring. Take it out of the garden rather than stacking it at the edge of the bed.Adults overwinter in sheltered plant debris, so the end-of-season clearance of allium residue is what lowers the overwintering population before the first spring flight. Clear it at the end of the season and remove it from the site rather than leaving it on the surface. Pair it with rotation, since debris missed on last year's ground is exactly what a new planting on that ground sits over.

4. Floating row cover

   Put insect netting or row cover over the bed before the moths come out of winter shelter, which is once spring warms to about 50°F, sometimes as early as March, or on planting day if that comes later. Tuck the edges into the soil and leave it on for the whole season instead of lifting it between flights. Do not put the cover over a bed that grew onions or leeks last year, since the moths winter in that debris and would be sealed in with the crop.Exclusion is the main control at garden scale and its timing decides it: insect netting or row cover goes on before the overwintered adults emerge, at about 50°F in early spring, or on the day of transplanting if that is later. Seal the edges and leave it in place through all two to three generations rather than lifting it between them. Do not lay row cover over ground that carried alliums last season; the adults overwinter in that debris and a cover traps them under it with the crop.

5. Handpicking

   Look over the plants as you walk the row and pick off any caterpillars you find, along with the little net-like cocoons they spin on the leaves once they have finished feeding. Pull a damaged leaf apart to find the slim, creamy yellow caterpillar inside, and check the newest leaves at the heart of the plant, where the damage starts. Drop what you find into soapy water.Remove larvae and cocoons by hand on a regular scouting pass. The larvae feed inside the folded young leaves for two to three weeks, so split a leaf showing the row of holes or the frass to reach one, and the net-like cocoon on the foliage is the stage that sits in plain sight. With injury running from June through September across two to three generations, one cleanup is not enough; scout through the season.

6. Spinosad

   If caterpillars get past the cover, spinosad is the organic spray to use. Time it to the moths. A pheromone trap, a scented lure that catches the moths and shows when a wave peaks, goes out by mid-April; spray 7 to 10 days after that peak. Spray at dusk and keep it off anything in flower; it is hard on bees while wet. Bt, a bacterial spray sold for caterpillars, did not make a real dent in this pest in testing, so do not count on it.Spinosad applied 7 to 10 days after a peak flight is the organic material with trial support against this moth. A pheromone trap set by mid-April marks each peak, and with flights around mid-April to mid-May, mid-June to mid-July and late July to late August there are up to three timings a season. Apply at dusk, not to plants in flower; it is toxic to bees while wet and for about a day after. Bt did not significantly reduce larvae in laboratory tests and is not the substitute.

The bottom rungs are products with label rules: timing, pollinators, and days to wait before harvest. Before you spray →

The ladder troubleshoots from the gentlest solution to the strongest. Start with the gentlest option and only move on when and if it doesn't work.

1. Crop rotation

   Do not plant leeks where onions, garlic, leeks or any of their relatives grew last year, and set the new bed as far from that ground as the garden allows. The fly spends the winter in the soil right where last year's onion-family crop stood, so a bed with that history is where it comes up in spring. Pick the spot when you plan the garden, and pick it before you decide about a cover.Delia antiqua pupae overwinter in the soil around last season's alliums, so rotating off that ground and siting the new planting away from it removes the crop from the spring emergence. Distance matters as well as the bed itself. Rotation is also what makes exclusion safe, since a cover over last year's allium ground shuts the emerging flies in with the transplants.

2. Timing the planting

   If your season gives you the room, plant leeks a bit later in spring, once the first wave of flies has finished. Those first flies lay their eggs at the base of young plants, so a planting that goes in after that wave has passed misses the worst of it. Ask your extension office when the fly shows up in your area before moving the date.Planting later in spring, after the first flight has passed, sidesteps the first round of egg-laying, the round that falls on young transplants. The window is local, so confirm the flight timing with extension for your area rather than working from a calendar date. It trades a later start against stand loss, and it suits a season long enough to absorb the delay.

3. Garden sanitation

   In fall, lift the finished plants with their roots, and take out any small or damaged leeks you culled, rather than leaving any of it in the bed. The fly breeds in leftover onion-family material and old bulbs, so what stays behind feeds it. In spring, keep fresh manure and green manure, meaning a cover crop turned in to rot, out of the leek bed, because the fly is drawn to lay its eggs where things are rotting.Remove culls and the lifted plants with their roots in fall, and get them off the site, since cull piles and volunteer onions keep the fly going between crops and the pupae sit in the soil around the old plants. Keep spring manure and green manure out of the bed: rotting organic matter draws egg-laying, and a cool, wet spring makes the larvae worse on top of it.

4. Floating row cover

   Cover the transplants with row cover, a light fabric that lets in light and water but keeps insects off, on the day you plant them, so it is in place before the spring flies come looking for somewhere to lay. Seal the edges down along the soil, because the eggs go in at the base of the plant. Do not put the cover on a bed that grew onions or leeks last year, since the flies coming up out of that soil would be sealed in with the crop.Row cover goes on transplants from planting day, ahead of the spring flight, with the edges sealed to the soil, since the fly lays at the base of the plant and an unsealed edge is an open door. Keep it in place until the spring flight's egg-laying has passed, since that is what it blocks. It belongs only on rotated ground: a bed that carried alliums last year holds the overwintering pupae, and a cover over it traps the emerging flies under it with the crop.

No spray for this one. Prevention is the control.

The ladder troubleshoots from the gentlest solution to the strongest. Start with the gentlest option and only move on when and if it doesn't work.

1. Crop rotation

   Give leeks a fresh spot each year, as far from last year's onion-family beds as your garden allows. The flies come from pupae, the small brown resting cases, that wait in the old plant tissue or in the soil right around it, so a bed that had leeks or onions last year is where the next flight starts. Moving the planting is also what makes a cover safe to use on it.Rotate off last season's allium ground and put as much distance as you can between the new planting and it. Phytomyza gymnostoma pupae overwinter in plant tissue or the surrounding soil, so old allium ground is an emergence site rather than neutral ground, and with a spring flight from late March and a fall flight from about September the same bed is exposed twice. Rotation is also the precondition for netting.

2. Timing the planting

   Time the crop so it misses both waves of flies: set transplants out after the middle of May, once the spring flight is over, and harvest by early September, before the fall flight begins. Fall leeks take the worst of this pest because by then they are often the only onion-family crop still in the ground, so a crop that is out before September is out of the line of fire. Confirm the flight dates for your area first.Transplanting after mid-May and harvesting by early September can dodge both flights, since adults emerge from late March through April, sometimes into May, and the second flight runs from about September into October. The fall flight is the damaging one on leeks, which are frequently the only allium standing by then. Emergence shifts with region and season, so check the local dates before building a planting plan on them.

3. Garden sanitation

   After harvest, pull up the old plants and get rid of any that show tunnels or the rows of white dots, rather than leaving them in the bed or on the compost pile. The pupae, the small brown resting cases, wait in that old tissue for the next flight, so clearing it takes them out of the garden with it.Destroy infested debris after harvest to remove overwintering pupae, which lodge in the sheath and base of the plant and in the soil around it. Leaving mined leaves or lifted plants on the surface leaves the next flight in place; take the material off the site. It is the fall half of the practice, and rotation is the spring half.

4. Floating row cover

   Get insect netting or row cover on before each flight starts, by late March for the spring flies and by early September for the fall ones, bury the edges so nothing can crawl under, and leave it on for about eight weeks. Late does not work: leeks covered two weeks into the fall flight ended up with more maggots, not fewer. Do not put the cover on a bed that grew onions or leeks last year, or you seal the flies coming up from the soil in with the crop.Netting goes on before a flight begins, by late March for the spring flight and by early September for the fall one, with the edges buried and the cover kept on through the roughly eight-week egg-laying period. Covering two weeks after the fall flight has started has been shown to leave more larvae and pupae in the plants, not fewer. Do not lay row cover over ground that held infested alliums last season; the emerging flies are trapped under it with the crop, and the bed should be rotated instead.

5. Spinosad

   When netting was not possible and you are seeing the rows of white dots, spinosad is the organic spray that has worked on this fly. Spray twice in the two to four weeks after the flies first show up, at dusk, and keep it off anything in flower, because it is hard on bees while wet. It reaches the maggots inside the leaf, which a spray that sits on the surface does not.Spinosad is the organic material with trial support here, applied twice in the two to four weeks after the adults first appear, timed to their arrival rather than to the mines, which show only once larvae are inside the leaf. It penetrates leaf tissue, so it reaches larvae a surface contact spray misses. Apply at dusk, not on plants in flower; it is toxic to bees while wet and for about a day after.

The bottom rungs are products with label rules: timing, pollinators, and days to wait before harvest. Before you spray →

The ladder troubleshoots from the gentlest solution to the strongest. Start with the gentlest option and only move on when and if it doesn't work.

1. Airflow and spacing

   Give leeks enough room that air moves between the plants and the leaves dry after rain or dew. This rust needs cool, damp weather and leaves that stay wet for hours to get started, and crowded plants hold that dampness, so spacing set at planting is the first thing that works against it. On leeks the cultural steps are usually enough, since they carry rust better than garlic does.Space for airflow so foliage dries, because Puccinia porri infects on cool conditions, roughly 57 to 75°F with the optimum near 59°F, and several hours of leaf wetness; dense stands extend that wetness. The disease tracks cool, damp spring and fall weather rather than midsummer, so the layout matters most for the stretches leeks spend in cool conditions. Leek is a comparatively resistant host, and spacing plus the other cultural steps usually carry it.

2. Balanced nitrogen feeding

   Go easy on nitrogen, the part of fertilizer that pushes leafy growth. Leeks pushed hard with it get more rust, and heavy feeding also thickens the stand and holds the leaves wet longer, which undoes the spacing you gave them. Feed enough to keep the plants growing steadily and no more.Hold nitrogen back. Heavy nitrogen increases rust on alliums, and part of the route is the canopy: lush, dense growth keeps leaves wet for the hours the fungus needs. Feeding to adequacy rather than excess is the point, since underfeeding costs shaft size for a different reason.

3. Water at the base

   Water at the soil line instead of spraying over the tops of the plants. Wet leaves are what this fungus needs, and watering over the leaves gives it hours of wetness the weather did not. A hose laid at the base or a soaker line keeps the foliage dry, and watering early in the day helps for the same reason.Irrigate at the soil line rather than over the leaves. Infection depends on several hours of leaf wetness at cool temperatures, and overhead watering supplies that wetness on days the weather would not; spores travel on air, and it is the leaf wetness, not splash, that the watering choice controls here. In a cool, wet spring or fall the weather does the wetting regardless, which is where spacing and rotation take over.

4. Garden sanitation

   Pull out any volunteer onions or garlic near the leeks, meaning the plants that come up on their own from bulbs left behind, since they carry the rust from one season to the next. After harvest, clear the old leaves and other debris out of the bed rather than leaving them lying there.Destroy volunteer alliums, which carry the fungus between crops, and clear crop debris at the end of the season. Spores travel on air, so a volunteer inside or beside the planting is a live source through the cool, damp weather the disease runs on, and it sits outside any rotation you plan. On leek this and the other cultural steps are usually what carries it.

5. Crop rotation

   Where rust has shown up, keep onions, garlic and leeks off that ground for two to three years and plant the new ones away from any planting that has it. That gives the fungus a gap with no onion-family plant to live on, and distance from an infected patch keeps its airborne spores from landing straight on the new one.Where rust has been a problem, keep new allium plantings two to three years off that ground and separated from an infected planting. Spores are airborne, so the separation matters as well as the years: a fresh bed beside a rusted one is within reach of its spores.

No spray for this one. Prevention is the control.

The ladder troubleshoots from the gentlest solution to the strongest. Start with the gentlest option and only move on when and if it doesn't work.

1. Clean planting stock

   Start with clean transplants from a source you trust, because this fungus can arrive on the plants themselves and there is no cure once it is in the ground. Its tiny black resting bodies live in the soil for more than 20 years, and it takes very little to start: a few of them in a sample of soil is enough. Moving to a new bed later does not undo that, so the clean start is the decision that counts.Plant only clean stock; it is the decision made before anything touches the soil, and on this disease that is where the weight sits, since there is no cure once plants are infected. Sclerotia persist in soil for over 20 years and germinate in response to allium roots, and even a few per soil sample can start the disease, which is why rotation alone will not control it. Keeping the fungus out of clean ground is what the practice comes down to.

2. Garden sanitation

   Pull infected plants as soon as you see them and destroy them; do not compost them. Then keep the soil itself from traveling: knock the dirt off boots, tools and pots before moving from that bed to clean ground, because the fungus moves in soil and a very small amount is enough to start a new patch. Do not count on resting the bed to clear it, since the fungus lasts in the soil for decades.There is no cure once plants are infected; remove and destroy them, and then treat infested soil as the thing that moves. Strict sanitation means no soil carried on tools, boots or containers from an affected bed into clean ground, since sclerotia persist for over 20 years and germinate in response to allium roots. Rotation alone will not control it for the same reason, so keeping the inoculum from spreading is what the practice rests on.

No spray for this one. Prevention is the control.

The ladder troubleshoots from the gentlest solution to the strongest. Start with the gentlest option and only move on when and if it doesn't work.

1. Crop rotation

   Give leeks a bed that has not grown onions, garlic, leeks or their relatives for three to six years. This fungus builds up a little more with every onion-family crop on the same ground, especially in warm soil, so a long gap is the point. It helps rather than fixes: the fungus lasts in the soil for years and lives on other crops too, so expect less pink root after a rotation, not none. Leeks usually get off lighter than onions anyway.Rotate off allium ground for three to six years. Setophoma terrestris accumulates with each allium crop on the same ground and is most active in warm soil, so the length of the break is what matters, and it reduces the disease rather than clearing it, since the fungus persists in soil for years and lives on other crops. Leek is a lesser host than onion, so the pressure this has to hold is lower here to begin with.

2. Improve drainage

   Where the soil sits wet, grow leeks in a raised bed or on a better-drained spot. This fungus is worse where the ground stays wet, and a raised or well-drained bed takes that away; it cannot help a plant whose roots have already turned pink. It works alongside the long break from onion-family crops, not instead of it.Use raised or well-drained beds where soil sits wet; the disease runs harder in ground that stays wet, and drainage does nothing for a plant whose roots are already pink. On heavy or low-lying ground it is settled before planting, since there is no rescue once the roots have gone. Pair it with the long rotation; neither replaces the other.

No spray for this one. Prevention is the control.