Catnip

Rots at the crown in wet winter ground. Grow it lean, sunny and well-drained, or in a raised bed. You can often save a clump by lifting and dividing it, cutting away the rotted portions and replanting only firm pieces.

Wilting in the middle of the day on a plant whose soil is damp - the classic tell, and the reason people water a rotting plant and finish it off. Leaves yellowing from the bottom up, dull off-color foliage, and growth that stalls for no visible reason. Whole shoots, or one side of the plant, collapsing while the rest still looks fine. Soft, dark, or mushy tissue at the base of the stem or at the crown, sometimes with a sour smell. Lift a failing plant and look at the roots: healthy ones are firm and pale, rotted ones are brown or black and slimy, and the outer layer slips off between your fingers leaving a thread of core behind. Seedlings that topple over at soil level are the same problem, called damping-off.

There is no cure for tissue that has already rotted, so act on the conditions rather than looking for a spray. Stop watering at once and fix the drainage - open up compacted soil, or move the plant to a raised, sharply drained position or a container. Lift a badly affected plant, cut away every soft dark root and rotted section with a clean blade back to firm pale tissue, and replant only the healthy parts into fresh free-draining soil or mix. Often the better move is to give up on the plant and save its offspring instead: cuttings, offsets, or divisions taken from firm healthy growth root readily and give you a clean start. Bin the rotted material rather than composting it, and correct the drainage before you replant, because the rot comes straight back in the same wet spot.

Drainage is the whole defense. Match the plant to the site first - something that wants sharp drainage will not survive heavy clay or a low, water-collecting spot however well you tend it. Where the ground is heavy, plant on a raised bed, berm, or slope, or grow in a container. Set plants at the depth they were already growing, since burying the crown or the root flare is a common cause of rot, and keep mulch pulled back a few inches from the stem base instead of piled against it. Water the soil rather than the calendar: check that the top inch or two is dry before watering again, cut back hard in cool weather and winter dormancy, and never leave a pot standing in a full saucer. Do not plant or dig in soggy ground, and go easy on rich amendments for plants that want it lean. Rotate annual crops, and start with clean healthy plants and fresh mix.

Root rot is not one organism but a group of soil fungi and water molds - Pythium, Phytophthora, Fusarium, Rhizoctonia and others - that sit harmlessly in most soils and only turn destructive when the ground stays saturated. Waterlogged soil drives the air out, the roots suffocate, and the weakened tissue is invaded and goes soft, brown, and stringy. Because the damage is underground you usually see it first as something that looks like drought: wilting, yellowing, stunting, a plant that does not respond to being watered. Overwatering, heavy compacted ground, a low spot that collects runoff, a pot with no drainage or a saucer left standing full, and cold wet soil in winter are the usual causes. Once roots have actually rotted there is no cure - this is a problem you prevent rather than treat.

Ironic on a plant reputed to repel them, but catnip gets its own share, heaviest on tender new growth. Established catnip outgrows light damage, and a hard cutback forces clean regrowth.

Dozens of pinhead-sized round holes peppered across the leaves, thickest on the youngest, most tender growth. Tiny beetles that spring away as you brush the plants or lean in for a closer look - that jump is the identification. Seedlings that go lacy and stop growing, and transplants that sit and sulk instead of taking off. Damage heaviest in warm, dry weather and along the edge of a bed nearest weedy ground or last year's crop debris. On root crops, shallow narrow grooves or channels scarring the skin at harvest, from larvae feeding underground.

Row cover and healthy fast growth carry most crops through, and on an established plant the damage is usually cosmetic and needs nothing at all. Where a young planting is genuinely being stripped, kaolin clay coats the leaves and deters feeding, and spinosad will knock the beetles back. Spinosad is acutely toxic to bees while the spray is still wet - apply it at dusk when bees are not flying, never onto plants in flower, and follow the product label. Anything you spray must be labeled for the plant you are treating, and you must observe its pre-harvest interval on a crop you are picking. Yellow sticky traps are useful for telling you when the beetles have arrived. Where the damage falls on the part you eat, harvesting young often beats them outright; where you are growing for a root or a fruit, leaf damage can simply be tolerated. Larvae feeding underground cannot be reached by a foliar spray, so rotation and clean beds are what work against those.

Floating row cover or fine insect mesh, laid over the bed from the day of sowing or transplanting and sealed at the edges so nothing can walk in underneath, is the single best defense - and on the crops they favor it is close to essential. Leave it on until plants are large and growing strongly, or until flowering on anything that needs pollinating. Push growth: a well-watered, unchecked plant outgrows the damage, so the whole game is getting seedlings through their vulnerable weeks fast. Keep beds weed-free, since related weeds host the beetles alongside your crop, and clear crop debris at the end of the season where the adults overwinter. Rotate away from a bed that grew a related crop last year. Where you can, shift sowing dates - cooler-season and fall plantings often miss the peak of adult feeding entirely.

Flea beetles are tiny dark beetles - often shiny black or bronze, rarely more than a couple of millimeters long - with enlarged back legs that let them spring away like fleas the moment you lean in. The adults chew countless small round pits and shot-holes through leaves, giving the foliage a peppered, buckshot-riddled look. They overwinter as adults in leaf litter, hedgerows, and crop debris, emerge hungry in spring, and are worst in warm dry spells. The damage is mostly a seedling and transplant problem: a small plant with a few leaves can be checked, stunted, or killed outright, while the same plant shrugs them off once it is large and growing strongly. Different species specialize on different plant families, so the beetles hammering your brassicas are not the ones on your nightshades. Some species also have larvae that feed on roots and tubers underground.

Common on heat-stressed and indoor catnip. Shearing the clump back hard is the quickest fix - it regrows clean and the plant benefits anyway.

Fine pale stippling or speckling across the upper leaf surface, as though the color had been sandblasted out of it, and later a dull bronze or rusty cast over the whole leaf. Delicate webbing across leaf joints, shoot tips, and undersides - that comes with a heavy infestation and means you caught it late. Leaves that dry, curl, and drop early while the plant looks thirsty no matter how much you water it. Hold a leaf over a sheet of white paper and tap it: the specks that fall and then start walking are mites. With a hand lens, look on the undersides for the mites themselves, pale or reddish, and their round translucent eggs. Damage that shows up first on the hottest, dustiest, most sheltered side of a plant, or on a pot sitting near a radiator indoors.

Start with water. A strong jet aimed at the leaf undersides, repeated every few days for a couple of weeks, knocks populations down and breaks the cycle - on many plants that alone ends it. On anything that regrows quickly, shearing back badly infested growth resets the plant to clean foliage. If it keeps building, spray insecticidal soap or horticultural oil directly onto the undersides where the mites actually live, and repeat every five to seven days for three rounds: contact sprays do not kill the eggs, so a single application always fails. Anything you spray must be labeled for the plant you are treating - follow the label and observe its pre-harvest interval on anything you eat. Predatory mites are an effective release in a greenhouse or on an indoor plant. Above all, correct the heat, dust, and drought stress that caused it, or the mites simply come back.

Do not let plants get drought-stressed - that is the single strongest trigger, and a plant watered steadily through heat is far less likely to have a mite problem at all. Hose the foliage down, undersides included, during hot dry spells: it knocks mites off, washes away the dust they thrive in, and raises the humidity around the plant for a while. Indoors and under cover, keep the air from going hot, dry, and stagnant, wipe or rinse the leaves occasionally, and keep pots away from radiators and heating vents. Inspect any plant carefully before you bring it indoors for winter or set it among others, since mites almost always arrive on something else. Avoid broad-spectrum insecticides: they wipe out the predatory mites, lacewings, and lady beetles that do most of this work for you, and reliably leave you with a worse outbreak than you started with.

Spider mites are not insects but tiny sap-feeding arachnids, smaller than a grain of salt, and you almost always notice the damage before you notice them. They live on the undersides of leaves and drain the cells one at a time, which is why the leaf goes finely stippled with pale dots before it bronzes and dries. They breed explosively in hot, dry, dusty, still air - a generation can pass in a week at high temperature - so a barely visible population becomes a webbed, bronzed plant within a fortnight. Drought-stressed plants and plants wintered indoors in dry heated air are the classic hosts. Broad-spectrum insecticides make matters worse rather than better, because they kill the predatory mites and lady beetles that normally hold mites in check.