Hayfever exacerbates my asthma, far more so than cold. It's worse in early summer (I'm allergic to both tree and grass pollens). In addition to oral antihistamines, eye drops and steroid nasal spray to manage the allergy, I use steroid inhalers for asthma, but I'm not sure they'll achieve much if it's just postnasal phlegm going to the lungs.
I find xylometazoline nasal spray is effective as a short-term remedy for being able to breathe at night. Sudafed type decongestants might also be worth trying.
Talk to your pharmacist and/or GP, anyway. If they're feeling helpful, they may have useful ideas.
Other than that, do more evening/night riding: It's not as hot and there's less pollen (and indeed traffic) about.
